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* blk-mq: get rid of unused BLK_MQ_F_SHOULD_SORT flagJens Axboe2014-08-151-4/+3
| | | | | | | We used to use this for determining whether to sort the dispatch list, but it's unused now. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: bitmap tag: fix races on shared ::wake_index fieldsAlexander Gordeev2014-06-171-1/+1
| | | | | | | | | Fix racy updates of shared blk_mq_bitmap_tags::wake_index and blk_mq_hw_ctx::wake_index fields. Cc: Ming Lei <tom.leiming@gmail.com> Signed-off-by: Alexander Gordeev <agordeev@redhat.com> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: bump max tag depth to 10K tagsJens Axboe2014-06-061-1/+1
| | | | | | | | | | | For some scsi-mq cases, the tag map can be huge. So increase the max number of tags we support. Additionally, don't fail with EINVAL if a user requests too many tags. Warn that the tag depth has been adjusted down, and store the new value inside the tag_set passed in. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: let blk_mq_tag_to_rq() take blk_mq_tags as the main parameterJens Axboe2014-06-041-1/+1
| | | | | | | | | | We currently pass in the hardware queue, and get the tags from there. But from scsi-mq, with a shared tag space, it's a lot more convenient to pass in the blk_mq_tags instead as the hardware queue isn't always directly available. So instead of having to re-map to a given hardware queue from rq->mq_ctx, just pass in the tags structure. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: make the sysfs mq/ layout reflect current mappingsJens Axboe2014-05-301-0/+1
| | | | | | | | | | | | | Currently blk-mq registers all the hardware queues in sysfs, regardless of whether it uses them (e.g. they have CPU mappings) or not. The unused hardware queues lack the cpux/ directories, and the other sysfs entries (like active, pending, etc) are all zeroes. Change this so that sysfs correctly reflects the current mappings of the hardware queues. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: blk_mq_tag_to_rq should handle flush requestShaohua Li2014-05-301-1/+1
| | | | | | | | | flush request is special, which borrows the tag from the parent request. Hence blk_mq_tag_to_rq needs special handling to return the flush request from the tag. Signed-off-by: Shaohua Li <shli@fusionio.com> Signed-off-by: Jens Axboe <axboe@fb.com>
* block: add queue flag for disabling SG mergingJens Axboe2014-05-291-0/+1
| | | | | | | | | | | | | If devices are not SG starved, we waste a lot of time potentially collapsing SG segments. Enough that 1.5% of the CPU time goes to this, at only 400K IOPS. Add a queue flag, QUEUE_FLAG_NO_SG_MERGE, which just returns the number of vectors in a bio instead of looping over all segments and checking for collapsible ones. Add a BLK_MQ_F_SG_MERGE flag so that drivers can opt-in on the sg merging, if they so desire. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: remove alloc_hctx and free_hctx methodsChristoph Hellwig2014-05-281-10/+0
| | | | | | | | There is no need for drivers to control hardware context allocation now that we do the context to node mapping in common code. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: merge blk_mq_alloc_reserved_request into blk_mq_alloc_requestChristoph Hellwig2014-05-281-2/+2
| | | | | | | | Instead of having two almost identical copies of the same code just let the callers pass in the reserved flag directly. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add helper to insert requests from irq contextChristoph Hellwig2014-05-281-0/+2
| | | | | | | | | | | | | | | Both the cache flush state machine and the SCSI midlayer want to submit requests from irq context, and the current per-request requeue_work unfortunately causes corruption due to sharing with the csd field for flushes. Replace them with a per-request_queue list of requests to be requeued. Based on an earlier test by Ming Lei. Signed-off-by: Christoph Hellwig <hch@lst.de> Reported-by: Ming Lei <tom.leiming@gmail.com> Tested-by: Ming Lei <tom.leiming@gmail.com> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: remove stale comment for blk_mq_complete_request()Jens Axboe2014-05-281-5/+0
| | | | | | | It works for both IPI and local completions as of commit 95f096849932. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: allow non-softirq completionsJens Axboe2014-05-271-0/+4
| | | | | | | | | | | | | | | | | | | Right now we export two ways of completing a request: 1) blk_mq_complete_request(). This uses an IPI (if needed) and completes through q->softirq_done_fn(). It also works with timeouts. 2) blk_mq_end_io(). This completes inline, and ignores any timeout state of the request. Let blk_mq_complete_request() handle non-softirq_done_fn completions as well, by just completing inline. If a driver has enough completion ports to place completions correctly, it need not define a mq_ops->complete() and we can avoid an indirect function call by doing the completion inline. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: pass in suggested NUMA node to ->alloc_hctx()Jens Axboe2014-05-271-2/+2
| | | | | | | | | | | Drivers currently have to figure this out on their own, and they are missing information to do it properly. The ones that did attempt to do it, do it wrong. So just pass in the suggested node directly to the alloc function. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: export blk_mq_tag_busy_iterSam Bradshaw2014-05-231-0/+1
| | | | | | | | | | | Export the blk-mq in-flight tag iterator for driver consumption. This is particularly useful in exception paths or SRSI where in-flight IOs need to be cancelled and/or reissued. The NVMe driver conversion will use this. Signed-off-by: Sam Bradshaw <sbradshaw@micron.com> Signed-off-by: Matias Bjørling <m@bjorling.me> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: allow the hctx cpu hotplug notifier to return errorsJens Axboe2014-05-211-1/+1
| | | | | | | Prepare this for the next patch which adds more smarts in the plugging logic, so that we can save some memory. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: allow changing of queue depth through sysfsJens Axboe2014-05-201-1/+1
| | | | | | | | | | | For request_fn based devices, the block layer exports a 'nr_requests' file through sysfs to allow adjusting of queue depth on the fly. Currently this returns -EINVAL for blk-mq, since it's not wired up. Wire this up for blk-mq, so that it now also always dynamic adjustments of the allowed queue depth for any given block device managed by blk-mq. Signed-off-by: Jens Axboe <axboe@fb.com>
* Merge branch 'for-3.16/blk-mq-tagging' into for-3.16/coreJens Axboe2014-05-191-0/+7
|\ | | | | | | | | | | | | Signed-off-by: Jens Axboe <axboe@fb.com> Conflicts: block/blk-mq-tag.c
| * blk-mq: improve support for shared tags mapsJens Axboe2014-05-131-0/+7
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | This adds support for active queue tracking, meaning that the blk-mq tagging maintains a count of active users of a tag set. This allows us to maintain a notion of fairness between users, so that we can distribute the tag depth evenly without starving some users while allowing others to try unfair deep queues. If sharing of a tag set is detected, each hardware queue will track the depth of its own queue. And if this exceeds the total depth divided by the number of active queues, the user is actively throttled down. The active queue count is done lazily to avoid bouncing that data between submitter and completer. Each hardware queue gets marked active when it allocates its first tag, and gets marked inactive when 1) the last tag is cleared, and 2) the queue timeout grace period has passed. Signed-off-by: Jens Axboe <axboe@fb.com>
* | blk-mq: switch ctx pending map to the sparser blk_align_bitmapJens Axboe2014-05-191-2/+8
|/ | | | | | | | | | | | | | | Each hardware queue has a bitmap of software queues with pending requests. When new IO is queued on a software queue, the bit is set, and when IO is pruned on a hardware queue run, the bit is cleared. This causes a lot of traffic. Switch this from the regular BITS_PER_LONG bitmap to a sparser layout, similarly to what was done for blk-mq tagging. 20% performance increase was observed for single threaded IO, and about 15% performanc increase on multiple threads driving the same device. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: implement new and more efficient tagging schemeJens Axboe2014-05-091-2/+4
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | blk-mq currently uses percpu_ida for tag allocation. But that only works well if the ratio between tag space and number of CPUs is sufficiently high. For most devices and systems, that is not the case. The end result if that we either only utilize the tag space partially, or we end up attempting to fully exhaust it and run into lots of lock contention with stealing between CPUs. This is not optimal. This new tagging scheme is a hybrid bitmap allocator. It uses two tricks to both be SMP friendly and allow full exhaustion of the space: 1) We cache the last allocated (or freed) tag on a per blk-mq software context basis. This allows us to limit the space we have to search. The key element here is not caching it in the shared tag structure, otherwise we end up dirtying more shared cache lines on each allocate/free operation. 2) The tag space is split into cache line sized groups, and each context will start off randomly in that space. Even up to full utilization of the space, this divides the tag users efficiently into cache line groups, avoiding dirtying the same one both between allocators and between allocator and freeer. This scheme shows drastically better behaviour, both on small tag spaces but on large ones as well. It has been tested extensively to show better performance for all the cases blk-mq cares about. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add basic round-robin of what CPU to queue workqueue work onJens Axboe2014-05-071-0/+4
| | | | | | | | Right now we just pick the first CPU in the mask, but that can easily overload that one. Add some basic batching and round-robin all the entries in the mask instead. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: respect rq_affinityChristoph Hellwig2014-04-251-1/+0
| | | | | | | | | | | | | | | | | | | | | | | The blk-mq code is using it's own version of the I/O completion affinity tunables, which causes a few issues: - the rq_affinity sysfs file doesn't work for blk-mq devices, even if it still is present, thus breaking existing tuning setups. - the rq_affinity = 1 mode, which is the defauly for legacy request based drivers isn't implemented at all. - blk-mq drivers don't implement any completion affinity with the default flag settings. This patches removes the blk-mq ipi_redirect flag and sysfs file, as well as the internal BLK_MQ_F_SHOULD_IPI flag and replaces it with code that respects the queue-wide rq_affinity flags and also implements the rq_affinity = 1 mode. This means I/O completion affinity can now only be tuned block-queue wide instead of per context, which seems more sensible to me anyway. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add blk_mq_requeue_requestChristoph Hellwig2014-04-161-0/+2
| | | | | | | | | | | | This allows to requeue a request that has been accepted by ->queue_rq earlier. This is needed by the SCSI layer in various error conditions. The existing internal blk_mq_requeue_request is renamed to __blk_mq_requeue_request as it is a lower level building block for this funtionality. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add blk_mq_start_hw_queuesChristoph Hellwig2014-04-161-0/+1
| | | | | | | | | Add a helper to unconditionally kick contexts of a queue. This will be needed by the SCSI layer to provide fair queueing between multiple devices on a single host. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add blk_mq_delay_queueChristoph Hellwig2014-04-161-1/+3
| | | | | | | | | | | | Add a blk-mq equivalent to blk_delay_queue so that the scsi layer can ask to be kicked again after a delay. Signed-off-by: Christoph Hellwig <hch@lst.de> Modified by me to kill the unnecessary preempt disable/enable in the delayed workqueue handler. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add async parameter to blk_mq_start_stopped_hw_queuesChristoph Hellwig2014-04-161-1/+1
| | | | | Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: allow drivers to hook into I/O completionChristoph Hellwig2014-04-161-7/+2
| | | | | | | | | | Split out the bottom half of blk_mq_end_io so that drivers can perform work when they know a request has been completed, but before it has been freed. This also obsoletes blk_mq_end_io_partial as drivers can now pass any value to blk_update_request directly. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: split out tag initialization, support shared tagsChristoph Hellwig2014-04-151-18/+16
| | | | | | | | | | | | Add a new blk_mq_tag_set structure that gets set up before we initialize the queue. A single blk_mq_tag_set structure can be shared by multiple queues. Signed-off-by: Christoph Hellwig <hch@lst.de> Modular export of blk_mq_{alloc,free}_tagset added by me. Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: add ->init_request and ->exit_request methodsChristoph Hellwig2014-04-151-2/+12
| | | | | | | | | | | | | | | | | | | | | | | | The current blk_mq_init_commands/blk_mq_free_commands interface has a two problems: 1) Because only the constructor is passed to blk_mq_init_commands there is no easy way to clean up when a comman initialization failed. The current code simply leaks the allocations done in the constructor. 2) There is no good place to call blk_mq_free_commands: before blk_cleanup_queue there is no guarantee that all outstanding commands have completed, so we can't free them yet. After blk_cleanup_queue the queue has usually been freed. This can be worked around by grabbing an unconditional reference before calling blk_cleanup_queue and dropping it after blk_mq_free_commands is done, although that's not exatly pretty and driver writers are guaranteed to get it wrong sooner or later. Both issues are easily fixed by making the request constructor and destructor normal blk_mq_ops methods. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: ensure that hardware queues are always run on the mapped CPUsJens Axboe2014-04-091-0/+1
| | | | | | | | | Instead of providing soft mappings with no guarantees on hardware queues always being run on the right CPU, switch to a hard mapping guarantee that ensure that we always run the hardware queue on (one of, if more) the mapped CPU. Signed-off-by: Jens Axboe <axboe@fb.com>
* Merge branch 'for-3.15/core' of git://git.kernel.dk/linux-blockLinus Torvalds2014-04-011-2/+3
|\ | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Pull core block layer updates from Jens Axboe: "This is the pull request for the core block IO bits for the 3.15 kernel. It's a smaller round this time, it contains: - Various little blk-mq fixes and additions from Christoph and myself. - Cleanup of the IPI usage from the block layer, and associated helper code. From Frederic Weisbecker and Jan Kara. - Duplicate code cleanup in bio-integrity from Gu Zheng. This will give you a merge conflict, but that should be easy to resolve. - blk-mq notify spinlock fix for RT from Mike Galbraith. - A blktrace partial accounting bug fix from Roman Pen. - Missing REQ_SYNC detection fix for blk-mq from Shaohua Li" * 'for-3.15/core' of git://git.kernel.dk/linux-block: (25 commits) blk-mq: add REQ_SYNC early rt,blk,mq: Make blk_mq_cpu_notify_lock a raw spinlock blk-mq: support partial I/O completions blk-mq: merge blk_mq_insert_request and blk_mq_run_request blk-mq: remove blk_mq_alloc_rq blk-mq: don't dump CPU -> hw queue map on driver load blk-mq: fix wrong usage of hctx->state vs hctx->flags blk-mq: allow blk_mq_init_commands() to return failure block: remove old blk_iopoll_enabled variable blktrace: fix accounting of partially completed requests smp: Rename __smp_call_function_single() to smp_call_function_single_async() smp: Remove wait argument from __smp_call_function_single() watchdog: Simplify a little the IPI call smp: Move __smp_call_function_single() below its safe version smp: Consolidate the various smp_call_function_single() declensions smp: Teach __smp_call_function_single() to check for offline cpus smp: Remove unused list_head from csd smp: Iterate functions through llist_for_each_entry_safe() block: Stop abusing rq->csd.list in blk-softirq block: Remove useless IPI struct initialization ...
| * blk-mq: merge blk_mq_insert_request and blk_mq_run_requestChristoph Hellwig2014-03-211-2/+1
| | | | | | | | | | | | | | | | | | It's almost identical to blk_mq_insert_request, so fold the two into one slightly more generic function by making the flush special case a bit smarted. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
| * blk-mq: fix wrong usage of hctx->state vs hctx->flagsJens Axboe2014-03-191-1/+1
| | | | | | | | | | | | | | | | | | | | | | | | BLK_MQ_F_* flags are for hctx->flags, and are non-atomic and set at registration time. BLK_MQ_S_* flags are dynamic and atomic, and are accessed through hctx->state. Some of the BLK_MQ_S_STOPPED uses were wrong. Additionally, the header file should not use a bit shift for the _S_ flags, as they are done through the set/test_bit functions. Signed-off-by: Jens Axboe <axboe@fb.com>
| * blk-mq: allow blk_mq_init_commands() to return failureJens Axboe2014-03-141-1/+2
| | | | | | | | | | | | | | | | | | | | | | | | | | If drivers do dynamic allocation in the hardware command init path, then we need to be able to handle and return failures. And if they do allocations or mappings in the init command path, then we need a cleanup function to free up that space at exit time. So add blk_mq_free_commands() as the cleanup function. This is required for the mtip32xx driver conversion to blk-mq. Signed-off-by: Jens Axboe <axboe@fb.com>
* | blk-mq: support partial I/O completionsChristoph Hellwig2014-02-211-1/+7
| | | | | | | | | | | | | | | | | | | | Add a new blk_mq_end_io_partial function to partially complete requests as needed by the SCSI layer. We do this by reusing blk_update_request to advance the bio instead of having a simplified version of it in the blk-mq code. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* | blk-mq: merge blk_mq_insert_request and blk_mq_run_requestChristoph Hellwig2014-02-211-2/+1
|/ | | | | | | | | It's almost identical to blk_mq_insert_request, so fold the two into one slightly more generic function by making the flush special case a bit smarted. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: rework flush sequencing logicChristoph Hellwig2014-02-101-4/+1
| | | | | | | | | | | | | | | | | | | Witch to using a preallocated flush_rq for blk-mq similar to what's done with the old request path. This allows us to set up the request properly with a tag from the actually allowed range and ->rq_disk as needed by some drivers. To make life easier we also switch to dynamic allocation of ->flush_rq for the old path. This effectively reverts most of "blk-mq: fix for flush deadlock" and "blk-mq: Don't reserve a tag for flush request" Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: rework I/O completionsChristoph Hellwig2014-02-101-0/+4
| | | | | | | | | | | | Rework I/O completions to work more like the old code path. blk_mq_end_io now stays out of the business of deferring completions to others CPUs and calling blk_mark_rq_complete. The latter is very important to allow completing requests that have timed out and thus are already marked completed, the former allows using the IPI callout even for driver specific completions instead of having to reimplement them. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: support at_head inserations for blk_execute_rqChristoph Hellwig2014-02-071-1/+2
| | | | | | | | This is neede for proper SG_IO operation as well as various uses of blk_execute_rq from the SCSI midlayer. Signed-off-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@fb.com>
* blk-mq: Don't reserve a tag for flush requestShaohua Li2014-01-301-0/+3
| | | | | | | | | | | | | | | | Reserving a tag (request) for flush to avoid dead lock is a overkill. A tag is valuable resource. We can track the number of flush requests and disallow having too many pending flush requests allocated. With this patch, blk_mq_alloc_request_pinned() could do a busy nop (but not a dead loop) if too many pending requests are allocated and new flush request is allocated. But this should not be a problem, too many pending flush requests are very rare case. I verified this can fix the deadlock caused by too many pending flush requests. Signed-off-by: Shaohua Li <shli@fusionio.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
* blk-mq: for_each_* macro correctnessJose Alonso2014-01-281-6/+6
| | | | | | | | | | | | | | I observed that there are for_each macros that do an extra memory access beyond the defined area. Normally this does not cause problems. But, this can cause exceptions. For example: if the area is allocated at the end of a page and the next page is not accessible. For correctness, I suggest changing the arguments of the 'for loop' like others 'for_each' do in the kernel. Signed-off-by: Jose Alonso <joalonsof@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
* block: blk-mq: don't export blk_mq_free_queue()Ming Lei2013-12-311-1/+0
| | | | | | | | | blk_mq_free_queue() is called from release handler of queue kobject, so it needn't be called from drivers. Cc: Jens Axboe <axboe@kernel.dk> Signed-off-by: Ming Lei <tom.leiming@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
* blk-mq: fix for flush deadlockChristoph Hellwig2013-10-281-1/+1
| | | | | | | | | | | | | | The flush state machine takes in a struct request, which then is submitted multiple times to the underling driver. The old block code requeses the same request for each of those, so it does not have an issue with tapping into the request pool. The new one on the other hand allocates a new request for each of the actualy steps of the flush sequence. If have already allocated all of the tags for IO, we will fail allocating the flush request. Set aside a reserved request just for flushes. Signed-off-by: Jens Axboe <axboe@kernel.dk>
* blk-mq: add blk_mq_stop_hw_queuesChristoph Hellwig2013-10-251-0/+1
| | | | | | | | | | | Add a helper to iterate over all hw queues and stop them. This is useful for driver that implement PM suspend functionality. Signed-off-by: Christoph Hellwig <hch@lst.de> Modified to just call blk_mq_stop_hw_queue() by Jens. Signed-off-by: Jens Axboe <axboe@kernel.dk>
* blk-mq: new multi-queue block IO queueing mechanismJens Axboe2013-10-251-0/+182
Linux currently has two models for block devices: - The classic request_fn based approach, where drivers use struct request units for IO. The block layer provides various helper functionalities to let drivers share code, things like tag management, timeout handling, queueing, etc. - The "stacked" approach, where a driver squeezes in between the block layer and IO submitter. Since this bypasses the IO stack, driver generally have to manage everything themselves. With drivers being written for new high IOPS devices, the classic request_fn based driver doesn't work well enough. The design dates back to when both SMP and high IOPS was rare. It has problems with scaling to bigger machines, and runs into scaling issues even on smaller machines when you have IOPS in the hundreds of thousands per device. The stacked approach is then most often selected as the model for the driver. But this means that everybody has to re-invent everything, and along with that we get all the problems again that the shared approach solved. This commit introduces blk-mq, block multi queue support. The design is centered around per-cpu queues for queueing IO, which then funnel down into x number of hardware submission queues. We might have a 1:1 mapping between the two, or it might be an N:M mapping. That all depends on what the hardware supports. blk-mq provides various helper functions, which include: - Scalable support for request tagging. Most devices need to be able to uniquely identify a request both in the driver and to the hardware. The tagging uses per-cpu caches for freed tags, to enable cache hot reuse. - Timeout handling without tracking request on a per-device basis. Basically the driver should be able to get a notification, if a request happens to fail. - Optional support for non 1:1 mappings between issue and submission queues. blk-mq can redirect IO completions to the desired location. - Support for per-request payloads. Drivers almost always need to associate a request structure with some driver private command structure. Drivers can tell blk-mq this at init time, and then any request handed to the driver will have the required size of memory associated with it. - Support for merging of IO, and plugging. The stacked model gets neither of these. Even for high IOPS devices, merging sequential IO reduces per-command overhead and thus increases bandwidth. For now, this is provided as a potential 3rd queueing model, with the hope being that, as it matures, it can replace both the classic and stacked model. That would get us back to having just 1 real model for block devices, leaving the stacked approach to dm/md devices (as it was originally intended). Contributions in this patch from the following people: Shaohua Li <shli@fusionio.com> Alexander Gordeev <agordeev@redhat.com> Christoph Hellwig <hch@infradead.org> Mike Christie <michaelc@cs.wisc.edu> Matias Bjorling <m@bjorling.me> Jeff Moyer <jmoyer@redhat.com> Acked-by: Christoph Hellwig <hch@lst.de> Signed-off-by: Jens Axboe <axboe@kernel.dk>
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