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By design LNet is a lossy connectionless network: there are cases where messages can be dropped without the sender being notified. Here we explore the possibilities of making LNet more resilient, including having it retransmit messages over alternate routes. What can be done in this area is constrained by the design of LNet.
In the following discussion, node will often be the shorthand for local node, while peer will be shorthand for peer node or remote node.
Within LNet there are three cases of interest: PUT, PUT+ACK, and GET+REPLY.
The protocols used to implement LNet tend to be connection-oriented, and implement some kind of handshake or ack protocol that tells the sender that a message has been received. As long as the LND actually reports errors to LNet (not a given, alas) this means that in practice the sender of a message can reliably determine whether the message was successfully sent to another node. When the destination node is on the same LNet network, this is sufficient to enable LNet itself to detect failures even in the bare Put case. But in a routed configuration this only guarantees that the LNet router received the message, and if the LNet router then fails to forward it, a bare Put will be lost without trace.
Users of LNet that send bare Put messages must implement their own methods to detect whether a message was lost. The general rule is simple: the recipient of a Put is supposed react somehow, and if the reaction doesn't happen within a set amount of time, the sender assumes that either the PUT was lost, or the recipient is in some other kind of trouble.
For our purposes PtlRPC is of interest. PtlRPC messages can be classified as Request+Response pairs. Both a Request and a Response are built from one or more Get or PUT messages. A node that sends a PtlRPC Request requires the receiver to send a Response within a set amount of time, and failing this the Request times out and PtlRPC takes corrective action.
Adaptive timeouts add an interesting wrinkle to this mechanism: they allow the recipient of a Request to tell the sender to "please wait", informing it that the recipient is alive and working but not able to send the Response before the normal timeout. For LNet the interesting implication is that while this is going on, there will be some traffic between the sender and recipient. However, this traffic may also be in the form of out-of-band information invisible to LNet.
The interfaces that LNet provides to the upper layers should work as follows. Set up an MD (Memory Descriptor) to send data from (for a PUT) or receive data into (for a Get). An event handler is associated with the MD. Then call LNetGet() or LNetPut() as appropriate.
If all goes well, the event handler sees two events: LNET_EVENT_SEND to indicate the GET message was sent, and LNET_EVENT_REPLY to indicate the REPLY message was received. Note that the send event can happen after the reply event (this is actually the typical case).
If sending the GET message failed, LNET_EVENT_SEND will include an error status, no LNET_EVENT_REPLY will happen, and clean up must be done accordingly. If the return value of LNetGet() indicates an error then sending the message certainly failed, but a 0 return does not imply success, only that no failure has yet been encountered.
A damaged REPLY message will be dropped, and does not result in an LNET_EVENT_REPLY. Effectively the only way for LNET_EVENT_REPLY to have an error status is if LNet detects a timeout before the REPLY is received.
The caller of LNetPut() requests an ACK by using LNET_ACK_REQ as the value of the ack parameter.
A PUT with an ACK is similar to a GET + REPLY pair. The events in this case are LNET_EVENT_SEND and LNET_EVENT_ACK.
For a PUT, the LNET_EVENT_SEND indicates that the MD is no longer used by the LNet code and the caller is free do discard or re-use it.
As with send, LNET_EVENT_ACK is expected to only carry an error indication if there was a timeout before the ACK was received.
The caller of LNetPut() requests no ACK by using LNET_NOACK_REQ as the value of the ack parameter.
A PUT without an ACK will only generate an LNET_EVENT_SEND, which indicates that the MD can now be re-used or discarded.
There are a number of failures we can encounter, only some of which LNet may address.
In a routed LNet configuration these scenarios apply to each hop.
These failures will show up in a number of ways:
Let's take a look at what LNet on the node can do in each of these cases.
This is the easiest case to work with. The LND can report such a failure to LNet, and LNet then refrains from using this interface for any traffic.
LNet can mark the interface down, and depending on the capabilities of the LND either recheck periodically or wait for the LND to mark the interface up.
The peer interface cannot be reached from the node interface, but the node interface can talk to other peers. If the peer interface can be reached from other node interfaces then we're dealing with some path failure. Otherwise the peer interface may be bad. If there is only a single node interface that can talk to the peer interface, then the node cannot distinguish between these cases.
LNet can mark this particular node/peer interface combination as something to be avoided.
When there are paths from more than one node interface to the peer interface, and none of these work, but other peer interfaces do work, then LNet can mark the peer interface as bad. Recovery could be done by periodically probing the peer interface, maybe using LNet Ping as a poor-man's equivalent of an LNet Control Packet.
Several peer interfaces on a net cannot be reached from a node interface, but the same node interface can talk to other peers. This is a more severe variant of the previous case.
All remote interfaces on a net cannot be reached from a local interface. If there are other, working, interfaces connected to the same net then the balance of probability shifts to the local interface being bad, or there is a severe problem with the fabric.
In practice LNet will not detect "all" remote interfaces being down. But it can detect that for a period of time, no traffic was successfully sent from a local interface, and therefore start avoiding that interface as a whole. Recovery would involve periodically probing the interface, maybe using LNet Ping.
The node is likely down. There is little LNet can do here, this is a problem to be handled by upper layers. This includes indicating when LNet should attempt to reconnect.
LNet might treat this as the "remote interface not reachable" case for all the interfaces of the remote node. That is, without much difference due to apparently all interfaces of the remote node being down, except for a log message indicating this.
This is the case where the LND does not signal any problem, so the ACK for a PUT or REPLY for a GET should arrive promptly, with the only delays due to credit-based throttling, and yet it does not do so. Note that this assumes that where possible the LND layer already implements reasonably tight timeouts, so that LNet can assume the problem is somewhere else.
LNet will timeout after the configured or passed in transaction timeout and will send an event to the ULP indicating that the PUT/GET has timed out without receiving the expected ACK/REPLY respectively.
No problem was signaled by the LND, and there is no ACK that we could time out waiting for. LNet does not have enough information to do anything, so the ULP must do so instead.
If this case must be made tractable, LNet can be changed to make the Ack non-optional.
When there are multiple paths available for a message, it makes sense to try and resend it on failure. But where should the resending logic be implemented?
The easiest path is to tell upper layers to resend. For example, PtlRPC has some related logic already. Except that when PtlRPC detects a failure, it disconnects, reconnects, and triggers a recovery operation. This is a fairly heavy-weight process, while the type of resending logic desired is to "just try another path" which differs from what exists today and needs to be implemented for each user.
The LNet Resiliency feature shall ensure that failures are delt with at the LNet level by resending the message on the available local and remote interfaces with in a timeout provided.
LNet shall use a trickle down approach for managing timeouts. The ULP (pltrpc or other upper layer protocol) shall provide a timeout value in the call to LNetPut() or LNetGet(). LNet shall use that as the transaction timeout value to wait for an ACK or REPLY. LNet shall further provide a configuration parameter for the number of retries. The number of retries shall allow the user to specify a maximum number of times LNet shall attempt to resend an unsuccessful message. LNet shall then calculate the message timeout by dividing the transaction timeout with the number of retries. LNet shall pass the calculated message timeout to the LND, which will use it to ensure that the LND protocol completes an LNet message within the message timeout. If the LND is not able to complete the message within the provided timeout it will close the connection and drop all messages on that connection. It will afterword proceed to call into LNet via lnet_finailze() to notify it of the error encountered.
For PUT that doesn't require an ACK the timeout will be used to provide the transaction timeout to the LND. In that case LNet will resend the PUT if the LND detects an issue with the transmit. LNet shall be able to send a TIMEOUT event to the ULP if the PUT was not successfully transmited. However, if the PUT is successfully transmited, there is no way for LNet to determine if it has been processed properly on the receiving end.
lnet_msg is a structure used to keep information on the data that will be transmitted over the wire. It does not itself go over the wire. lnet_msg is passed to the LND for transmission.
Before it's passed to the LND it is placed on an active list, msc_active. The diagram below describes the datastructures
The CPT is determined by the lnet_cpt_of_nid_locked() function. lnet_send() running in the context of the calling threads place a message on msc_active just before sending it to the LND. lnet_parse() places messages on msc_active when it receives it from the LND.
msc_active represent all messages which are currently being processed by LNet.
lnet_finalize(), running in the context of the calling threads, likely the LND scheduler threads, will determine if a message needs to be resent and place it on the resend list. The resend list is a list of all messages which are currently awaiting a resend.
A monitor thread monitors and ensures that messages which have expired are finalized. This processing is detailed in later sections.
There are three types of failures that LNet needs to deal with:
Timeouts will be provided by the ULP in the LNetPut() and LNetGet() APIs.
When there is a message send failure due to the reasons outlined above. The behavior should be as follows:
Two new fiels will be added to lnet_msg:
struct lnet_msg {
...
»·······__u32»··»·······»·······msg_status;
»·······cfs_time_t»·····»·······msg_deadline;
...
} |
When a message encounters one of the errors above, the LND will update the msg_status field appropriately and call lnet_finalize()
lnet_finalize() will check if the message has timed out or if it needs to be resent and will take action on it. lnet_finalize() currently calls lnet_complete_msg_locked() to continue the processing. If the message has not been sent, then lnet_finalize() should call another function to resend, lnet_resend_msg_locked().
lnet_resend_msg_locked()shall queue the message on a resend queue and wake up a thread responsible for resending messages.
The router checker thread, which is always started, will be refactored to handle resending messages.
When a message is initially sent it's tagged with a deadline for this message. The message will be placed on the active queue. If the message is not completed within that timeout it will be finalized and removed from the active queue. A timeout event will be passed to the ULP.
The monitor thread will wake up ever second and check the top of the active queue, IE the oldest message on the list. If that message has expired it updates its status to TIMEDOUT and finalizes it. It then moves on to the next message on the list and stops once its find a message that has not expired.
If the LND times out and LNet attemps to resend, it'll place it on the resend queue. A message can be on the both the active and resend queue.
As shown in the diagram above both lnet_send() and lnet_parse() put messages on the active queue. lnet_finalize() consumes messages off the active queue when it's time to decommit them.
When the LND calls lnet_finalize() on a timed out message, lnet_finalize() will put the message on the resend queue and wake up the monitor thread, which will go through the resend queue in FIFO order, pop the message and call lnet_send() on it.
When the monitor thread wakes up every second it'll perform the following ordered operations:
The assumption is that under normal circumstances the number of re-sends should be low, so the thread will not add any logic to pace out the resend rate, such as what lnet_finalize() does.
It is possible that a message can be on the resend queue when it either completes or times out. In both of these case it will be removed from the resend queue as well as the active queue and finalized.
The message will continue to be protected by the LNet net CPT lock to ensure mutual access.
When the message is committed, lnet_msg_commit(), the message cpt is assigned. This cpt value is then used to protect the message in subsequent usages. Relevant to this discussion is when the message is examined in lnet_finalize() and either removed from the active queue or placed on the resend queue.
The ULP will provide the transaction timeout value on which LNet will base its own timeout values. In the absence of that LNet will fall back on a configurable transaction timeout value.
This trickle down approach will simplify the configuration of the LNet Resiliency feature, as well as make the timeout consistent through out the system, instead of configuring the LND timeout to be much larger than the pltrpc timeout as it is now. Furthermore, the ptlrpc uses a backoff algorithm, which allows it to wait longer for responses. With this trickle down approach, LNet will be able to cope with that timeout backoff algorithm.
The LNetGet() and LNetPut() APIs will be changed to reflect that.
/** * Initiate an asynchronous GET operation. * * On the initiator node, an LNET_EVENT_SEND is logged when the GET request * is sent, and an LNET_EVENT_REPLY is logged when the data returned from * the target node in the REPLY has been written to local MD. * LNET_EVENT_REPLY will have a timeout flag set if the REPLY has not arrived * with in the timeout provided. * * On the target node, an LNET_EVENT_GET is logged when the GET request * arrives and is accepted into a MD. * * \param self,target,portal,match_bits,offset See the discussion in LNetPut(). * \param mdh A handle for the MD that describes the memory into which the * requested data will be received. The MD must be "free floating" (See LNetMDBind()). * * \retval 0»·· Success, and only in this case events will be generated * and logged to EQ (if it exists) of the MD. * \retval -EIO Simulated failure. * \retval -ENOMEM Memory allocation failure. * \retval -ENOENT Invalid MD object. */ int LNetGet(lnet_nid_t self, struct lnet_handle_md mdh, »·······struct lnet_process_id target, unsigned int portal, »·······__u64 match_bits, unsigned int offset, int timeout) /** * Initiate an asynchronous PUT operation. * * There are several events associated with a PUT: completion of the send on * the initiator node (LNET_EVENT_SEND), and when the send completes * successfully, the receipt of an acknowledgment (LNET_EVENT_ACK) indicating * that the operation was accepted by the target. LNET_EVENT_ACK will have * the timeout flag set if an ACK is not received within the timeout provided * * The event LNET_EVENT_PUT is used at the target node to indicate the * comple tion of incoming datadelivery. * * The local events will be logged in the EQ associated with the MD pointed to * by \a mdh handle. Using a MD without an associated EQ results in these * events being discarded. In this case, the caller must have another * mechanism (e.g., a higher level protocol) for determining when it is safe * to modify the memory region associated with the MD. * * Note that LNet does not guarantee the order of LNET_EVENT_SEND and * LNET_EVENT_ACK, though intuitively ACK should happen after SEND. * * \param self Indicates the NID of a local interface through which to send * the PUT request. Use LNET_NID_ANY to let LNet choose one by itself. * \param mdh A handle for the MD that describes the memory to be sent. The MD * must be "free floating" (See LNetMDBind()). * \param ack Controls whether an acknowledgment is requested. * Acknowledgments are only sent when they are requested by the initiating * process and the target MD enables them. * \param target A process identifier for the target process. * \param portal The index in the \a target's portal table. * \param match_bits The match bits to use for MD selection at the target * process. * \param offset The offset into the target MD (only used when the target * MD has the LNET_MD_MANAGE_REMOTE option set). * \param timeout The timeout to wait for an ACK if one is expected. * \param hdr_data 64 bits of user data that can be included in the message * header. This data is written to an event queue entry at the target if an * EQ is present on the matching MD. * * \retval 0»·· Success, and only in this case events will be generated * and logged to EQ (if it exists). * \retval -EIO Simulated failure. * \retval -ENOMEM Memory allocation failure. * \retval -ENOENT Invalid MD object. * * \see struct lnet_event::hdr_data and lnet_event_kind_t. */ int LNetPut(lnet_nid_t self, struct lnet_handle_md mdh, enum lnet_ack_req ack, »·······struct lnet_process_id target, unsigned int portal, »·······__u64 match_bits, unsigned int offset, int timeout »·······__u64 hdr_data) |
It is possible that a send can fail immediately. In this case we need to take active measures to ensure that we do not enter a tight loop resending until the timeout expires. This could peak the CPU consumption unexpectedly.
To do that the last sent time will be kept in the message. If the message is not sent successfully on any of the existing interfaces, then it will be placed on a queue and will be resent after a specific deadline expires. This will be termed a "(re)send procedure". An interval must expire between each (re)send procedure. A (re)send procedure will iterate through all local and remote peers, depending on the source of the send failure.
The router pinger thread will be refactored to handle resending messages. The router pinger thread is started irregardless and only used to ping gateways if any are configured. Its operation will be expanded to check the pending message queue and re-send messages.
To keep the code simple when resending the health value of the interface that had the problem will be updated. If we are sending to a non-MR peer, then we will use the same src_nid, otherwise the peer will be confused. The selection algorithm will then consider the health value when it is selecting the NI or peer NI.
There are two aspects to consider when sending a message
The selection algorithm will take an average of these two values and will determine the best interface to select. To make the comparison proper, the health value of the interface will be set to the same value as the credits on initialization. It will be decremented on failure to send and incremented on successful send.
A health_value_range module parameter will be added to control the sensitiveness of the selection. If it is set to 0, then the best interface will be selected. A value higher than 0 will give a range in which to select the interface. If the value is large enough it will in effect be equivalent to turning off health comparison.
Local interface failures will be detected in one of two ways
lnd_send()lnet_select_pathway() can fail for the following reasons:
-EINVAL-HOSTUNREACHInvalid information given
1, 2, 5, 6 and 10 are resource errors and it does not make sense to resend the message as any resend will likely run into the same problem.
LNet should resend the message:
LNet shall calculate the message timeout as follows:
message timeout = transaction timeout / retry count
The message timeout will be stored in the lnet_msg structure and passed down to the LND via lnd_send().
An LNet message can be represented by a sequence of LND message. In the o2iblnd, the PUT and GET are described in the following sequence diagrams.
A third type of message that the LND sends is the IBLND_MSG_IMMEDIATE. The data is embedded in the message and posted. There is no handshake in this case.
For the PUT case described in the sequence diagram, the initiator sends two messages:
Both of these messages are sent using the same tx structure. The tx is allocated and placed on a waiting queue. When the IBLND_MSG_PUT_ACK is received the waiting tx is looked up and used to send the IBLND_MSG_PUT_DONE.
When kiblnd_queue_tx_locked() is called for IBLND_MSG_PUT_REQ it sets the tx_deadline as follows:
timeout_ns = *kiblnd_tunables.kib_timeout * NSEC_PER_SEC; tx->tx_deadline = ktime_add_ns(ktime_get(), timeout_ns); |
When kiblnd_queu_tx_locked() is called for IBLND_MSG_PUT_DONE it reset the tx_deadline again.
This presents an obstacle for the LNet Resiliency feature. LNet provides a timeout for the LND as described above. From LNet's perspective this deadline is for the LNet PUT message. However, if we simply use that value for the timeout_ns calculation, then in essence will will be waing for 2 * LND timeout for the completion of the LNet PUT message. This will mean less re-transmits.
Therefore, the LND, since it has knowledge of its own protocols will need to divide the timeout provided by LNet by the number of transmits it needs to do to complete the LNet level message:
It's possible that the local interface might get into a hard failure scenario by receiving one of these events from the o2iblnd. socklnd needs to be investigated to determine if there are similar cases:
In these cases the local interface can not be used any longer. So it can not be selected as part of the selection algorithm. If there are no other interface available, then no messages can be sent out of the node.
A corresponding event can be received to indicate that the interface is operational again.
A new LNet/LND Api will be created to pass these events from the LND to LNet.
Upper layers request from LNet to send a GET or a PUT via LNetGet() and LNetPut() APIs. LNet then calls into the LND to complete the operation. The LND encapsulates the LNet message into an LND specific message with its own message type. For example in the o2iblnd it is kib_msg_t.
When the LND transmits the LND message it sets a tx_deadline for that particular transmit. This tx_deadline remains active until the remote has confirmed receipt of the message. Receipt of the message at the remote is when LNet is informed that a message has been received by the LND, done via lnet_parse(), then LNet calls back into the LND layer to receive the message.
Therefore if a tx_deadline is hit, it is safe to assume that the remote end has not received the message. The reasons are described further below.
By handling the tx_deadline properly we are able to account for almost all next-hop failures. LNet would've done its best to ensure that a message has arrived at the immediate next hop.
The tx_deadline is LND-specific, and derived from the timeout (or sock_timeout) module parameter of the LND.
As mentioned above at the LNet layer LNET_MSG_PUT can be told to expect LNET_MSG_ACK to confirm that the LNET_MSG_PUT has been processed by the destination. Similarly LNET_MSG_GET expects an LNET_MSG_REPLY to confirm that the LNET_MSG_GET has been successfully processed by the destination.
The pair LNET_MSG_PUT+LNET_MSG_ACK and LNET_MSG_GET+LNET_MSG_REPLY is not covered by the tx_deadline in the LND. If the upper layer does not take precautions it could wait forever on the LNET_MSG_ACK or LNET_MSG_REPLY. Therefore it is reasonable to expect that LNET provides a TIMEOUT event if either of these messages are not received within the expected timeout.
The question is whether LNet should resend the LNET_MSG_PUT or LNET_MSG_GET if it doesn't receive the corresponding response.
Consider the case where there are multiple LNet routers between two nodes, N1 and N2. These routers can possibly be routing between different Hardware, example OPA and MLX. N1 via the LND can reliably determine the health of the next-hop's interfaces. It can not however reliably determine the health of further hops in the chain. Each node can determine the health of the immediate next-hops. Therefore, each node in the path can be trusted to ensure that the message has arrived at the immediate next hop.
If there is a failure along the path and N1 does not receive the expected LNET_MSG_ACK or LNET_MSG_REPLY, and it knows that the message has been received by its next-hop, it has no way to determine where the failure happened. If it decides to resend the message, then there is no way to reliably select a reasonable peer_ni. Especially considering that the message has in fact been received properly by the next-hop. We can then say that we will simply try all the peer_nis of the destination. But in fact this will already be done by the node in the chain which is encountering a problem completing the message with its next-hop. So the net effect is the same. If both are implemented, then duplication of messages is a certainty.
Furthermore the responsibility of end-to-end reliability falls on the shoulder of layers using LNet. Ptlrpc's design clearly takes the end-to-end reliability of RPCs in consideration. By adding an LNET_ACK_TIMEOUT and LNET_REPLY_TIMEOUT (or add an error status in the current events), then ptlrpc can react to the error status appropriately.
The argument against this approach is mixed clusters, where not all nodes are MR capable. In this case we can not rely on intermediary nodes to try all the interfaces of its next-hop. However, as is assumed in the Multi-Rail design if not all nodes are MR capable, then not all Multi-Rail features are expected to work.
This appraoch would add the LNet resiliency required and avoid the many corner cases that will need to be addressed when receiving message which have already been processed.
There are multiple timeouts kept at different layers of the code. It is important to set the timeout defaults such that it works best, and to give guidance on how the different timeouts interact together.
Looking at timeouts from a bottom up approach:
lnet_finalize() is called with an appropriate error code. This will result in a resend.INITIAL_CONNECT_TIMEOUT is set to 5 secLDLM_TIMEOUT_DEFAULT and OBD_TIMEOUT_DEFAULT.IB/TCP/GNI re-send timeout < LND transmit timeout < LNet message timeout < LNet transaction timeout < RPC timeout.
A retry count can be specified. That's the number of times to resend after the LND transmit timeout expires.
The timeout value before failing an LNET_MSG_[PUT | GET] will be:
message timeout = (retry count * LND transmit timeout) + (resend interval * retry count)
where
retry count = min(retry count, 5)
message timeout <= transaction timeout
It has been observed that mount could hang for a long time if discovery ping is not responded to. This could happen if an OST is down while a client mounts the File System. In this case it does not make sense to hold up the mount procedure while discovery is taking place. For some cases like discovery the algorithm would specify a different timeout other than what's configured.
Other cases where a timeout can be specified which overrides the configured timeout is router ping and manual ping.
One issue to consider is currently the LND transmit timeout defaults to 50s. So if we do retry up to five times we could be held up for 2500s, which would be unacceptable.
The question to answer is, does it make sense for the LND transmit timeout to be set to 50s? Even though the IB/TCP/GNI timeout can be long, it might make more sense to pre-empt that communication stack and attempt to resend the message from the LNet layer on a different interface, or even reuse the same interface if only on is available.
There are two concepts that need to stay separate. Reliability of RPC messages and LNet Resiliency. This feature attempts to add LNet Resiliency against local and immediate next hop interface failure. End-to-end reliability is to ensure that upper layer messages, namely RPC messages, are received and processed by the final destination, and take appropriate action in case this does not happen. End-to-end reliability is the responsibility of the application that uses LNet, in this case ptlrpc. Ptlrpc already has a mechanism to ensure this.
To clarify the terminology further, LNET MESSAGE should be used to describe one of the following messages:
LNET TRANSACTION should be used to describe
NEXT-HOP should describe a peer that is exactly one hop away.
The role of LNet is to ensure that an LNET MESSAGE arrives at the NEXT-HOP, and to flag when a transaction fails to complete.
Upper layers should ensure that the transaction it requests to initiate completes successfully, and take appropriate action otherwise.
The discussion here refers to the LND Transmit timeout.
Timeouts could occur due to several reasons:
Each of these scenarios can be handled differently
The desired behavior is listed for each of the above scenarios:
Note, that the behavior outlined is consistent with the explcit error cases identified in previous section. Only Scenario 2, diverges as a different path is selected all together, but still the same code structure is used.
All of these cases should end up calling lnet_finalize() API with the proper return code. lnet_finalize() will be the funnel where all these events shall be processed in a consistent manner. When the message is completed via lnet_complete_msg_locked(), the error is checked and the proper behavior as described above is executed.
In the cases when a GET or a PUT transaction is initiated an associated deadline needs to be tagged to the corresponding transaction. This deadline indicates how long LNet should wait for a REPLY or an ACK before it times out the entire transaction.
A new thread is required to check if a transaction deadline has expired. OW: Can a timer do this? Or is one timer per message too resource-intensive? If a queue is used, then ideally new messages can simply be added to the tail, with their deadline always >= the current tail. With the queue sorted by deadline the checker thread can look at the deadline of the message at the head of the tail to determine how long it sleeps.
When a transaction deadline expires an appropriate event is generated towards PTLRPC.
When a the REPLY or the ACK is received the message is removed from the check queue of the thread and success event is generated towards PTLRPC.
Within a transaction deadline, if there is a determination that the GET or PUT message failed to be sent to the next-hop then the GET or PUT can be resent.
OW: How is this deadline determined? Naming this section peer_timeout suggests you want to use that? Conceptually we can distinguish between an LNet transaction timeout and an LNet peer timeout.
Resends are terminated when the peer_timeout for a message expires.
Resends should also terminate if all local_nis and/or peer_nis are in bad health. New messages can still use paths that have less than optimal health.
A message is resent after the LND transmit deadline expires, or on failure return code. Both these paths are handled in the same manner, since a transmit deadline triggers a call to lnet_finalize(). Both inline and asynchronous errors also endup in lnet_finalize().
Therefore the least number of transmits = peer_timeout / LND transmit deadline.
Depending on the frequency of errors, LNet may do more re-transmits. LNet will stop re-transmitting and declare a peer dead, if the peer_timeout expires or all the different paths have been tried with no success.
In the default case where LND transmit timeout is set to 50 seconds and the peer_timeout is set to 180 seconds, then LNet will re-transmit 3 times before it declares the peer dead.
peer_timeout can be increased to fit in more re-transmits or LND transmit timeout can be decreased.
Alexey Lyashkov made a presentation at LAD 16 that outlines the best values for all Lustre timeouts. It can be accessed here.
MD is always protected by the lnet_res_lock, which is CPT specific.
Other data structures such as the_lnet.ln_msg_containers, peer_ni, local ni, etc are protected by the lnet_net_lock.
The MD should be kept intact during the resend procedure. If there is a failure to resend then the MD should be released and message memory freed.
| Parameter | Values | |
| SRC NID | Specified (A) | Not specified (B) |
| DST NID | local (1) | not local (2) |
| DST NID | MR ( C ) | NMR (D) |
Note that when communicating with an NMR peer we need to ensure that the source NI is always the same: there are a few places where the upper layers use the src nid from the message header to determine its originating node, as opposed to using something like a UUID embedded in the message. This means when sending to an NMR node we need to pick a NI and then stick with that going forward.
Note: When sending to a router that scenario boils down to considering the router as the next-hop peer. The final destination peer NIs are no longer considered in the selection. The next-hop can then be MR or non-MR and the code will deal with it accordingly.
peer_ni using dst_nid (non-MR, so this is the only peer_ni candidate)peer_ni is healthypeer_ni even if it is unhealthy if this is the 1st attempt to send this messagepeer_nipeer_ni if setpeer_nifind route to dst_nid
find peer_ni of router
no issue if peer_ni is healthy
try this peer_ni even if it is unhealthy if this is the 1st attempt to send this message
fail if resending to an unhealthy peer_ni
pick the preferred local_NI for the dst_nid if set
If the preferred local_NI is not healthy, fail sending the message and let the upper layers deal with recovery.
otherwise if preferred local_NI is not set, then pick a healthy local NI and make it the preferred NI for this peer_ni
send over this path
LNet will keep attempting to resend a message across different local/remote NIs as long as the interfaces are only in "soft" failure state. Interfaces are demerited when we fail to send over them due to a timeout. This is opposed to a hard failure which is reported by the underlying HW indicating that this interface can no longer be used for sending and receiving.
LNet will terminate resends of a message in one of the following conditions
For hard failures there needs to be a method to recover these interfaces. This can be done through a ping of the interface whether it is local or remote, since that ping will tell us if an interface is up or down.
The router checker infrastructure currently does this exact job for routers. This infrastructure can be expanded to also query the local or remote NIs which are down.
Selection of the local_ni or peer_ni will be dependent on the following criteria:
The interfaces which have soft failures will be demerited so it will naturally be selected as a last option.
LNet Health Refactor lnet_select_pathway() - DONE add health value per ni - DONE add lnet_health_range - DONE handle local timeouts - DONE When re-sending a message we don't need to ensure we send to the same peer_ni as the original send. There are two cases to consider: MR peer: we can just use the current selection algorithm to resend a message Non-MR peer: there will only be on peer_ni anyway (or preferred NI will be set) and we'll need to use the same local NI when sending to a Non-MR. Modify the LNDs to set the appropriate error code on timeout handle tx timeout due being stuck on the queues for too long Due to local problem. At this point we should be able to handle trying different interfaces if there is an interface timeout o2iblnd socklnd Introduce retry_count Only resend up to the retry_count This should be user configurable Should have a max value of 5 retries Rate limit resend rate Introduce resend_interval Make sure to pace out the resends by that interval We need to guard against situations where there is an immediate failure which triggers an immediate resend, causing a resend tight loop Refactor the router pinger thread to handle resending. lnet_finalize() queues those messages on a queue and wakes up the router pinger thread router pinger wakes up every second (or if woken up manually) goes through the queue, timesout and fails any messages that have passed their deadline. Checks if a message to be resent is not being resent before its resend interval. Resends any messages that need to be resent. Introduce an LND API to read the retransmit timeout. Calculate the message timeout as follows: message timeout = (retry count * LND transmit timeout) + (resend interval * retry count) Message timeout is the timeout by which LNet abandons retransmits This implies that LNet has detected some sort of a failure while sending a message use the message timeout instead of the peer timeout as the deadline for the message If the message timesout a failure event is propagated to the top layer. o2iblnd socklnd handle local NIs down events from the LND. NIs are flagged as down and are not considered as part of the selection process. Can only come up by another event from the LND. o2iblnd socklnd Move the peer timeout from the LND to the LNet. It should still be per NI. Add userspace support for setting retry count Add userspace support for setting retransmit interval Add peer_ni_healthvalue This value will reflect the health of the peer_ni and should be initially set the peer credits. Modify the selection algorithm to select the peer_ni based on the average of the health value and the credits Adjust the peer_ni health value due to failure/successs On Success the health value should be incremented if it's not at its maximum value. On Failure the health value should be decremented (stays >= 0) Failures will either be due to remote tx timeout or network error Modify the LNDs to set the appropriate error code on tx timeout o2iblnd socklnd Handle transaction timeout Transaction timeout is the deadline by which LNet knows that a PUT or a GET did not receive the ACK or REPLY respectively. When a PUT or a GET is sent successfully. It is then put on a queue if it expects and ACK or a REPLY router pinger will wake up every second and will check if these messages have not received the expected response within the timeout specified. If not then we'll need to time it out. Provide a mechanism to over ride the transaction timeout. When sending a message the caller of LNetGet()/LNetPut() should specify a timeout for the transaction. If not provided then it defaults to the global transaction timeout. Add a transaction timeout even to be send to the upper layer. Handle transaction timeout in the upper layer (ptlrpc) Add userspace support for maximum transaction timeout This was added in 2.11 to solve the blocked mount Add the following statistics The number of resends due to local tx timeout per local NI The number of resends due to the remote tx timeout per peer NI The number of resends due to a network timeout per local and peer NI The number of local tx timeouts The number of remote tx timeouts The number of network timeouts The number of local interface down events The number of local interface up events. The average time it takes to successfully send a message per peer NI The average time it takes to successfully complete a transaction per peer NI |
There are two types of events to account for:
Both events should be monitored because they provide information on the health of the device and connection respectively.
ib_register_event_handler() can be used to register a handler to handle events of type 1.
a cm_callback can be register with the cm_id to handle RMDA_CM events.
There is a group of events which indicate a fatal error
Below are the events that could occur on the RDMA device. Highlighted in BOLD RED are the events that should be handled for health purposes.
Below are the events that could occur on a connection. Highlighted in BOLD RED are the events that should be handled for health purposes.
RDMA_CM_EVENT_ADDR_RESOLVED: Address resolution (rdma_resolve_addr) completed successfully.
RDMA_CM_EVENT_ADDR_ERROR: Address resolution (rdma_resolve_addr) failed.
RDMA_CM_EVENT_ROUTE_RESOLVED: Route resolution (rdma_resolve_route) completed successfully.
RDMA_CM_EVENT_ROUTE_ERROR: Route resolution (rdma_resolve_route) failed.
RDMA_CM_EVENT_CONNECT_REQUEST: Generated on the passive side to notify the user of a new connection request.
RDMA_CM_EVENT_CONNECT_RESPONSE: Generated on the active side to notify the user of a successful response to a connection request. It is only generated on rdma_cm_id's that do not have a QP associated with them.
RDMA_CM_EVENT_CONNECT_ERROR: Indicates that an error has occurred trying to establish or a connection. May be generated on the active or passive side of a connection.
RDMA_CM_EVENT_UNREACHABLE: Generated on the active side to notify the user that the remote server is not reachable or unable to respond to a connection request. If this event is generated in response to a UD QP resolution request over InfiniBand, the event status field will contain an errno, if negative, or the status result carried in the IB CM SIDR REP message.
RDMA_CM_EVENT_REJECTED: Indicates that a connection request or response was rejected by the remote end point. The event status field will contain the transport specific reject reason if available. Under InfiniBand, this is the reject reason carried in the IB CM REJ message.
RDMA_CM_EVENT_ESTABLISHED: Indicates that a connection has been established with the remote end point.
RDMA_CM_EVENT_DISCONNECTED: The connection has been disconnected.
RDMA_CM_EVENT_DEVICE_REMOVAL: The local RDMA device associated with the rdma_cm_id has been removed. Upon receiving this event, the user must destroy the related rdma_cm_id.
RDMA_CM_EVENT_MULTICAST_JOIN: The multicast join operation (rdma_join_multicast) completed successfully.
RDMA_CM_EVENT_MULTICAST_ERROR: An error either occurred joining a multicast group, or, if the group had already been joined, on an existing group. The specified multicast group is no longer accessible and should be rejoined, if desired.
RDMA_CM_EVENT_ADDR_CHANGE: The network device associated with this ID through address resolution changed its HW address, eg following of bonding failover. This event can serve as a hint for applications who want the links used for their RDMA sessions to align with the network stack.
RDMA_CM_EVENT_TIMEWAIT_EXIT: The QP associated with a connection has exited its timewait state and is now ready to be re-used. After a QP has been disconnected, it is maintained in a timewait state to allow any in flight packets to exit the network. After the timewait state has completed, the rdma_cm will report this event.
This is probably the trickiest situation. Timeout could occur because of network congestion, or because the remote side is too busy, or because it's dead, or hung, etc.
Timeouts are being kept in the LND (o2iblnd) on the transmits. Every transmit which is queued is assigned a deadline. If it expires then the connection on which this transmit is queued, is closed.
peer_timout can be set in routed and non-routed scenario, which provides information on the peer.
Timeouts are also being kept at ptlrpc. These are rpc timeouts.
Refer to section 32.5 in the manual for a description of how RPC timeouts work.
Also refer to section 27.3.7 for LNet Peer Health information.
Given the presence of various timeouts, adding yet another timeout on the message, will further complicate the configuration, and possibly cause further hard to debug issues.
One option to consider is to use the peer_timout feature to recognize when peer_nis are down, and update the peer_ni health information via this mechanism. And let the LND and RPC timeouts take care of further resends.
[Olaf: bear in mind that currently the LND already reports status to LNet through lnet_finalize()]
enum lnet_error_type {
LNET_LOCAL_NI_DOWN, /* don't use this NI until you get an UP */
LNET_LOCAL_NI_UP, /* start using this NI */
LNET_LOCAL_NI_SEND_TIMOUT, /* demerit this NI so it's not selected immediately, provided there are other healthy interfaces */
LNET_PEER_NI_ADDR_ERROR, /* The address for the peer_ni is wrong. Don't use this peer_NI */
LNET_PEER_NI_UNREACHABLE, /* temporarily don't use the peer NI */
LNET_PEER_NI_CONNECT_ERROR, /* temporarily don't use the peer NI */
LNET_PEER_NI_CONNECTION_REJECTED /* temporarily don't use the peer NI */
}; |
As shown in the above diagram whenever a tx is queued to be sent or is posted but haven't received confirmation yet, the tx_deadline is still active. The scheduler thread checks the active connections for any transmits which has passed their deadline, and then it closes those connections and notifies LNet via lnet_notify().
The tx timeout is cancelled when in the call kiblnd_tx_done(). This function checks 3 flags: tx_sending, tx_waiting and tx_queued. If all of them are 0 then the tx is closed as completed. The key flag to note is tx_waiting. That flag indicates that the tx is waiting for a reply. It is set to 1 in kiblnd_send, when sending the PUT_REQ or GET_REQ. It is also set when sending the PUT_ACK. All of these messages expect a reply back. When the expected reply is received then tx_waiting is set to 0 and kiblnd_tx_done() is called, which eventually cancels the tx_timeout, by basically removing the tx from the queues being checked for the timeout.
The notification in the LNet layer that the connection has been closed can be used by MR to attempt to resend the message on a different peer_ni.
<TBD: I don't think that LND attempts to automatically reconnect to the peer if the connection gets torn down because of a tx_timeout.>
TX timeout is exactly what we need to determine if the message has been transmitted successfully to the remote side. If it has not been transmitted successfully we can attempt to resend it on different peer_nis until we're either successful or we've exhausted all of the peer_nis.
The reason for the TX timeout is also important:
After the completion of an o2iblnd tx ib_post_send(), a completion event is added to the completion queue. This triggers kiblnd_complete to be called. If this is an IBLND_WID_TX then kiblnd_tx_complete() is called, which calls kiblnd_tx_done() if the tx is not sending, waiting or queued. In this case the tx_timeout is closed.
In summary, the tx_timeout serves to ensure that messages which do not require an explicit response from the peer are completed on the tx event added by M|OFED to the completion queue. And it also serves to ensure that any messages which require an explicit reply to be completed receive that reply within the tx_timout.
When a node receives a PUT request, the O2IBLND calls lnet_parse() to deal with it. lnet_parse() calls lnet_parse_put(), which matches the MD and initiates a receive. This ends up calling into the LND, kiblnd_recv(), which would send an IBLND_MSG_PUT_[ACK|NAK]. This allows the sending peer LND to know that the PUT has been received, and let go of it's TX, as shown below. On receipt of the ACK|NAK, the peer sends a IBLND_MSG_PUT_DONE, and initates the RDMA operation. Once the tx completes, kiblnd_tx_done() is called which will then call lnet_finalize(). For the PUT, LNet will end sending an LNET_MSG_ACK, if it needs to (look at lnet_parse_put() for the condition on which LNET_MSG_ACK is sent).
In the case of a GET, on receipt of IBLND_MSG_GET_REQ, lnet_parse() -> lnet_parse_get() -> kiblnd_recv(). If a there is data to be sent back, then the LND sends and RDMA operation with IBLND_MSG_GET_DONE, or just the DONE.
The point I'm trying to illustrate here is that there are two levels of messages. There are the LND messages which confirm that a single LNET message has been received by the peer. And there there are the LNet level messages, such as LNET_MSG_ACK and LNET_MSG_REPLY. These two in particular are in response to the LNET_MSG_PUT and LNET_MSG_GET respectively. At the LND level IBLND_MSG_IMMEDIATE is used.
In a routed configuration, the entire LND handshake between the peer and the router is completed. However the LNET level messages like LNET_MSG_ACK and LNET_MSG_REPLY are sent by the final destination, and not by the router. The router simply forwards on the message it receives.
The question that the design needs to answer is this: Should LNet be concerned with resending messages if LNET_MSG_ACK or LNET_MSG_REPLY are not received for LNET_MSG_PUT and LNET_MSG_GET respectively?
At this point (pending further discussion) it is my opinion that it should not. I argue that the decision to get LNET to send the LNET_MSG_ACK or LNET_MSG_REPLY implicitly is actually a poor one. These messages are in direct respons to direct requests by upper layers like RPC. What should've been happening is that when LNET receives an LNET_MSG_[PUT|GET], an event should be generated to the requesting layer, and the requesting layer should be doing another call to LNet, to send the LNET_MSG_[ACK|REPLY]. Maybe it was done that way in order no to hold on resources more than it should, but symantically these messages should belong to the upper layer. Furthermore, the events generated by these messages are used by the upper layer to determine when to do the resends of the PUT/GET. For these reasons I believe that it is a sound decision to only task LNet with attempting to send an LNet message over a different local_ni/peer_ni only if this message is not received by the remote end. This situation is caught by the tx_timeout.
In order to understand fully how the LND transmit timeout can be used for resends, we need to have an understanding of the transmit life cycle shown above.
This shows that the timeout depends on the type of request being sent. If the request expects a response back then the tx_timeout covers the entire transaction lifetime. Otherwise it covers up until the transmit complete event is queued on the completion queue.
Currently, if the transmit timeout is triggered the connection is closed to ensure that all RDMA operations have ceased. LNet is notified on error and if the modprobe parameter auto_down is set (which it is by default) the peer is marked down. In lnet_select_pathway() lnet_post_send_locked() is called. One of the checks it does is to make sure that the peer we're trying to send to is alive. If not, message is dropped and -EHOSTUNREACH is returned up the call chain.
In lnet_select_pathway() if lnet_post_send_locked() fails, then we ought to marke the health of the peer and attempt to select a different peer_ni to send to.
NOTE, currently we don't know why the peer_ni is marked down. As mentioned above the tx_timeout could be triggered for several reasons. Some reasons indicate a problem on the peer side, IE not receiving a response or a transmit complete. Other reasons could indicate local problems, for example the tx never leaves the queued state. Depending on the reason for the tx_timeout LNet should react differently in it's next round of interface selection.
#define lnet_peer_aliveness_enabled(lp) (the_lnet.ln_routing != 0 && \ ((lp)->lpni_net) && \ (lp)->lpni_net->net_tunables.lct_peer_time_out > 0) |
In effect, the aliveness of the peer is not considered at all if the node is not a router.
There are different scenarios to consider with Health:
Communication with a router adheres to the above details. Once the current hop is sure that the message has made it to the next hop, LNet shouldn't worry about resends. Resends are only to ensure that the message LNet is tasked to send makes it to the next hop. The upper layer RPC protocol makes sure that RPC messages are retried if necessary.
Each hop's LNet will do a best effort in getting the message to the following hop. Unfortunately, there is no feedback mechanism from a router to the originator to inform the originator that a message has failed to send, but I believe this is unnecessary and will probably increase the complexity of the code and the system in general. Rule of thumb should be that each hop only worries about the immediate next hop.
TBD
Events that are triggered asynchronously, without initiating a message, such as port down, port up, rdma device removed, shall be handled via a new LNet/LND API that shall be added.
In the other cases, lnet_ni_send() calls into the LND via the lnd_send() callback provided. If the return code is failure lnet_finalize() is called to finalize the message.
lnet_finalize() takes the return code as an input parameter. The above behavior should be implemented in lnet_finalize() since this is the main entry into the LNet module via the LNDs as well.
lnet_finalize() detaches the MD in preparation of completing the message. Once the MD is detached it can be re-used. Therefore, if we are to re-send the message then the MD shouldn't be detached at this point.
lnet_complete_msg_locked() should be modified to manage the local interface health, and decide whether the message should be resent or not. If the message can not be resent due to no available local interfaces then the MD can be detached and the message can be freed.
Currently lnet_select_pathway() iterates through all the local interfaces on a particular peer identified by the NID to send to. In this case we would want to restrict the resend to go to the same peer_ni, but on a different local interface.
This approach lends itself to breaking out the selection of the local interface from lnet_select_pathway(), leading to the following logic:
lnet_ni lnet_get_best_ni(local_net, cur_ni, md_cpt)
{
local_net = get_local_net(peer_net)
for each ni in local_net {
health_value = lnet_local_ni_health(ni)
/* select the best health value */
if (health_value < best_health_value)
continue
distance = get_distance(md_cpt, dev_cpt)
/* select the shortest distance to the MD */
if (distance < lnet_numa_range)
distance = lnet_numa_range
if (distance > shortest_distance)
continue
else if distance < shortest_distance
distance = shortest_distance
/* select based on the most available credits */
else if ni_credits < best_credits
continue
/* if all is equal select based on round robin */
else if ni_credits == best_credits
if best_ni->ni_seq <= ni->ni_seq
continue
}
}
/*
* lnet_select_pathway() will be modified to add a peer_nid parameter.
* This parameter indicates that the peer_ni is predetermined, and is
* identified by the NID provided. The peer_nid parameter is the
* next-hop NID, which can be the final destination or the next-hop
* router. If that peer_NID is not healthy then another peer_NID is
* selected as per the current algorithm. This will force the
* algorithm to prefer the peer_ni which was selected in the initial
* message sending. The peer_ni NID is stored in the message. This
* new parameter extends the concept of the src_nid, which is provided
* to lnet_select_pathway() to inform it that the local NI is
* predetermined.
*/
/* on resend */
enum lnet_error_type {
LNET_LOCAL_NI_DOWN, /* don't use this NI until you get an UP */
LNET_LOCAL_NI_UP, /* start using this NI */
LNET_LOCAL_NI_SEND_TIMEOUT, /* demerit this NI so it's not selected immediately, provided there are other healthy interfaces */
LNET_PEER_NI_NO_LISTENER, /* there is no remote listener. demerit the peer_ni and try another NI */
LNET_PEER_NI_ADDR_ERROR, /* The address for the peer_ni is wrong. Don't use this peer_NI */
LNET_PEER_NI_UNREACHABLE, /* temporarily don't use the peer NI */
LNET_PEER_NI_CONNECT_ERROR, /* temporarily don't use the peer NI */
LNET_PEER_NI_CONNECTION_REJECTED /* temporarily don't use the peer NI */
};
static int lnet_handle_send_failure_locked(msg, local_nid, status)
{
switch (status)
/*
* LNET_LOCAL_NI_DOWN can be received without a message being sent.
* In this case msg == NULL and it is sufficient to update the health
* of the local NI
*/
case LNET_LOCAL_NI_DOWN:
LASSERT(!msg);
local_ni = lnet_get_local_ni(msg->local_nid)
if (!local_ni)
return
/* flag local NI down */
lnet_set_local_ni_health(DOWN)
break;
case LNET_LOCAL_NI_UP:
LASSERT(!msg);
local_ni = lnet_get_local_ni(msg->local_nid)
if (!local_ni)
return
/* flag local NI down */
lnet_set_local_ni_health(UP)
/* This NI will be a candidate for selection in the next message send */
break;
...
}
static int lnet_complete_msg_locked(msg, cpt)
{
status = msg->msg_ev.status
if (status != 0)
rc = lnet_handle_send_failure_locked(msg, status)
if rc == 0
return
/* continue as currently done */
}
|
A remote interface can be considered problematic under multiple scenarios:
When a remote interface fails the following actions take place:
There are several ways a remote interface can recover:
In all these cases a different peer_ni should be tried if one exists. lnet_select_pathway() already takes src_nid as a parameter. When resending due to one of these failures src_nid will be set to the src_nid in the message that is being resent.
static int lnet_handle_send_failure_locked(msg, local_nid, status)
{
switch (status)
...
case LNET_PEER_NI_ADDR_ERROR:
lpni->stats.stat_addr_err++
goto peer_ni_resend
case LNET_PEER_NI_UNREACHABLE:
lpni->stats.stat_unreacheable++
goto peer_ni_resend
case LNET_PEER_NI_CONNECT_ERROR:
lpni->stats.stat_connect_err++
goto peer_ni_resend
case LNET_PEER_NI_CONNECTION_REJECTED:
lpni->stats.stat_connect_rej++
goto peer_ni_resend
default:
/* unexpected failure. failing message */
return
peer_ni_resend
lnet_send(msg, src_nid)
} |