SCSI restarts its queue in scsi_end_request() automatically, so we don't
need to handle this case in blk-mq.

Especailly any request won't be dequeued in this case, we needn't to
worry about IO hang caused by restart vs. dispatch.
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Now restart is used in the following cases, and TAG_SHARED is for
SCSI only.

1) .get_budget() returns BLK_STS_RESOURCE
- if resource in target/host level isn't satisfied, this SCSI device
will be added in shost->starved_list, and the whole queue will be rerun
(via SCSI's built-in RESTART) in scsi_end_request() after any request
initiated from this host/targe is completed. Forget to mention, host level
resource can't be an issue for blk-mq at all.

- the same is true if resource in the queue level isn't satisfied.

- if there isn't outstanding request on this queue, then SCSI's RESTART
can't work(blk-mq's can't work too), and the queue will be run after
SCSI_QUEUE_DELAY, and finally all starved sdevs will be handled by SCSI's
RESTART when this request is finished

2) scsi_dispatch_cmd() returns BLK_STS_RESOURCE
- if there isn't onprogressing request on this queue, the queue
will be run after SCSI_QUEUE_DELAY

- otherwise, SCSI's RESTART covers the rerun.

3) blk_mq_get_driver_tag() failed
- BLK_MQ_S_TAG_WAITING covers the cross-queue RESTART for driver
allocation.

In one word, SCSI's built-in RESTART is enough to cover the queue
rerun, and we don't need to pay special attention to TAG_SHARED wrt. restart.

In my test on scsi_debug(8 luns), this patch improves IOPS by 20% ~ 30% when
running I/O on these 8 luns concurrently.

Aslo Roman Pen reported the current RESTART is very expensive especialy
when there are lots of LUNs attached in one host, such as in his
test, RESTART causes half of IOPS be cut.

Fixes: https://marc.info/?l=linux-kernel&m=150832216727524&w=2
Fixes: 6d8c6c0f ("blk-mq: Restart a single queue if tag sets are shared")
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

SCSI devices use host-wide tagset, and the shared driver tag space is
often quite big. However, there is also a queue depth for each lun(
.cmd_per_lun), which is often small, for example, on both lpfc and
qla2xxx, .cmd_per_lun is just 3.

So lots of requests may stay in sw queue, and we always flush all
belonging to same hw queue and dispatch them all to driver.
Unfortunately it is easy to cause queue busy because of the small
.cmd_per_lun.  Once these requests are flushed out, they have to stay in
hctx->dispatch, and no bio merge can happen on these requests, and
sequential IO performance is harmed.

This patch introduces blk_mq_dequeue_from_ctx for dequeuing a request
from a sw queue, so that we can dispatch them in scheduler's way. We can
then avoid dequeueing too many requests from sw queue, since we don't
flush ->dispatch completely.

This patch improves dispatching from sw queue by using the .get_budget
and .put_budget callbacks.
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

For SCSI devices, there is often a per-request-queue depth, which needs
to be respected before queuing one request.

Currently blk-mq always dequeues the request first, then calls
.queue_rq() to dispatch the request to lld. One obvious issue with this
approach is that I/O merging may not be successful, because when the
per-request-queue depth can't be respected, .queue_rq() has to return
BLK_STS_RESOURCE, and then this request has to stay in hctx->dispatch
list. This means it never gets a chance to be merged with other IO.

This patch introduces .get_budget and .put_budget callback in blk_mq_ops,
then we can try to get reserved budget first before dequeuing request.
If the budget for queueing I/O can't be satisfied, we don't need to
dequeue request at all. Hence the request can be left in the IO
scheduler queue, for more merging opportunities.
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

So that it becomes easy to support to dispatch from sw queue in the
following patch.

No functional change.
Reviewed-by: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Suggested-by: Christoph Hellwig <hch@lst.de> # for simplifying dispatch logic
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

When the hw queue is busy, we shouldn't take requests from the scheduler
queue any more, otherwise it is difficult to do IO merge.

This patch fixes the awful IO performance on some SCSI devices(lpfc,
qla2xxx, ...) when mq-deadline/kyber is used by not taking requests if
hw queue is busy.
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NBart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

When mq-deadline is taken, IOPS of sequential read and
seqential write is observed more than 20% drop on sata(scsi-mq)
devices, compared with using 'none' scheduler.

The reason is that the default nr_requests for scheduler is
too big for small queuedepth devices, and latency is increased
much.

Since the principle of taking 256 requests for mq scheduler
is based on 128 queue depth, this patch changes into
double size of min(hw queue_depth, 128).
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

If we have shared tags enabled, then every IO completion will trigger
a full loop of every queue belonging to a tag set, and every hardware
queue for each of those queues, even if nothing needs to be done.
This causes a massive performance regression if you have a lot of
shared devices.

Instead of doing this huge full scan on every IO, add an atomic
counter to the main queue that tracks how many hardware queues have
been marked as needing a restart. With that, we can avoid looking for
restartable queues, if we don't have to.

Max reports that this restores performance. Before this patch, 4K
IOPS was limited to 22-23K IOPS. With the patch, we are running at
950-970K IOPS.

Fixes: 6d8c6c0f ("blk-mq: Restart a single queue if tag sets are shared")
Reported-by: NMax Gurtovoy <maxg@mellanox.com>
Tested-by: NMax Gurtovoy <maxg@mellanox.com>
Reviewed-by: NBart Van Assche <bart.vanassche@sandisk.com>
Tested-by: NBart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Document the locking assumptions in functions that modify
blk_mq_ctx.rq_list to make it easier for humans to verify
this code.
Signed-off-by: NBart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Ming Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

It is required that no dispatch can happen any more once
blk_mq_quiesce_queue() returns, and we don't have such requirement
on APIs of stopping queue.

But blk_mq_quiesce_queue() still may not block/drain dispatch in the
the case of BLK_MQ_S_START_ON_RUN, so use the new introduced flag of
QUEUE_FLAG_QUIESCED and evaluate it inside RCU read-side critical
sections for fixing this issue.

Also blk_mq_quiesce_queue() is implemented via stopping queue, which
limits its uses, and easy to cause race, because any queue restart in
other paths may break blk_mq_quiesce_queue(). With the introduced
flag of QUEUE_FLAG_QUIESCED, we don't need to depend on stopping queue
for quiescing any more.
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Reviewed-by: NBart Van Assche <Bart.VanAssche@sandisk.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

blk_mq_sched_assign_ioc now only handles the assigned of the ioc if
the schedule needs it (bfq only at the moment).  The caller to the
per-request initializer is moved out so that it can be merged with
a similar call for the kyber I/O scheduler.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Having these as separate helpers in a header really does not help
readability, or my chances to refactor this code sanely.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Having them out of line in blk-mq-sched.c just makes the code flow
unnecessarily complicated.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

Because what the per-sw-queue bio merge does is basically same with
scheduler's .bio_merge(), this patch makes per-sw-queue bio merge
as the default .bio_merge if no scheduler is used or io scheduler
doesn't provide .bio_merge().
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

This provides the infrastructure for schedulers to expose their internal
state through debugfs. We add a list of queue attributes and a list of
hctx attributes to struct elevator_type and wire them up when switching
schedulers.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Reviewed-by: NHannes Reinecke <hare@suse.com>

Add missing seq_file.h header in blk-mq-debugfs.h
Signed-off-by: NJens Axboe <axboe@fb.com>

We have update the troublesome driver (mtip32xx) to deal with this
appropriately. So kill the hack that bypassed scheduler allocation
and insertion for reserved requests.
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Tested-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

At least one driver, mtip32xx, has a hard coded dependency on
the value of the reserved tag used for internal commands. While
that should really be fixed up, for now let's ensure that we just
bypass the scheduler tags an allocation marked as reserved. They
are used for house keeping or error handling, so we can safely
ignore them in the scheduler.
Tested-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

commit c13660a0 ("blk-mq-sched: change ->dispatch_requests()
to ->dispatch_request()") removed the last user of this function.
Hence also remove the function itself.
Signed-off-by: NBart Van Assche <bart.vanassche@sandisk.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

Schedulers need to be informed when a hardware queue is added or removed
at runtime so they can allocate/free per-hardware queue data. So,
replace the blk_mq_sched_init_hctx_data() helper, which only makes sense
at init time, with .init_hctx() and .exit_hctx() hooks.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

To improve scalability, if hardware queues are shared, restart
a single hardware queue in round-robin fashion. Rename
blk_mq_sched_restart_queues() to reflect the new semantics.
Remove blk_mq_sched_mark_restart_queue() because this function
has no callers. Remove flag QUEUE_FLAG_RESTART because this
patch removes the code that uses this flag.
Signed-off-by: NBart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

In elevator_switch(), if blk_mq_init_sched() fails, we attempt to fall
back to the original scheduler. However, at this point, we've already
torn down the original scheduler's tags, so this causes a crash. Doing
the fallback like the legacy elevator path is much harder for mq, so fix
it by just falling back to none, instead.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

If a new hardware queue is added at runtime, we don't allocate scheduler
tags for it, leading to a crash. This hooks up the scheduler framework
to blk_mq_{init,exit}_hctx() to make sure everything gets properly
initialized/freed.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

Preparation cleanup for the next couple of fixes, push
blk_mq_sched_setup() and e->ops.mq.init_sched() into a helper.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

While dispatching requests, if we fail to get a driver tag, we mark the
hardware queue as waiting for a tag and put the requests on a
hctx->dispatch list to be run later when a driver tag is freed. However,
blk_mq_dispatch_rq_list() may dispatch requests from multiple hardware
queues if using a single-queue scheduler with a multiqueue device. If
blk_mq_get_driver_tag() fails, it doesn't update the hardware queue we
are processing. This means we end up using the hardware queue of the
previous request, which may or may not be the same as that of the
current request. If it isn't, the wrong hardware queue will end up
waiting for a tag, and the requests will be on the wrong dispatch list,
leading to a hang.

The fix is twofold:

1. Make sure we save which hardware queue we were trying to get a
   request for in blk_mq_get_driver_tag() regardless of whether it
   succeeds or not.
2. Make blk_mq_dispatch_rq_list() take a request_queue instead of a
   blk_mq_hw_queue to make it clear that it must handle multiple
   hardware queues, since I've already messed this up on a couple of
   occasions.

This didn't appear in testing with nvme and mq-deadline because nvme has
more driver tags than the default number of scheduler tags. However,
with the blk_mq_update_nr_hw_queues() fix, it showed up with nbd.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

No functional difference, it just makes a little more sense to update
the tag map where we actually allocate the tag.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>
Tested-by: NSagi Grimberg <sagi@grimberg.me>

blk_mq_alloc_request_hctx() allocates a driver request directly, unlike
its blk_mq_alloc_request() counterpart. It also crashes because it
doesn't update the tags->rqs map.

Fix it by making it allocate a scheduler request.
Reported-by: NSagi Grimberg <sagi@grimberg.me>
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>
Tested-by: NSagi Grimberg <sagi@grimberg.me>

Signed-off-by: NSagi Grimberg <sagi@grimberg.me>

Modified by me to also check at driver tag allocation time if the
original request was reserved, so we can be sure to allocate a
properly reserved tag at that point in time, too.
Signed-off-by: NJens Axboe <axboe@fb.com>

In blk_mq_sched_dispatch_requests(), we call blk_mq_sched_mark_restart()
after we dispatch requests left over on our hardware queue dispatch
list. This is so we'll go back and dispatch requests from the scheduler.
In this case, it's only necessary to restart the hardware queue that we
are running; there's no reason to run other hardware queues just because
we are using shared tags.

So, split out blk_mq_sched_mark_restart() into two operations, one for
just the hardware queue and one for the whole request queue. The core
code only needs the hctx variant, but I/O schedulers will want to use
both.

This also requires adjusting blk_mq_sched_restart_queues() to always
check the queue restart flag, not just when using shared tags.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

The wording in the entries were poor and not understandable
by even deities. Kill the selection for default block scheduler,
and impose a policy with sane defaults.
Architected-by: NLinus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

Usually we don't ask the scheduler for work, if we already have
leftovers on the dispatch list. This is done to leave work on
the scheduler side for as long as possible, for proper merging.
But if we do have work leftover but didn't dispatch anything,
then we should ask the scheduler since we could potentially
issue requests from that.
Signed-off-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>

If we are currently out of driver tags, we don't want to add a
new flush (without a tag) to the head of the requeue list. We
want to add it to the back, behind the others that are
potentially also waiting for a tag.
Signed-off-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>

bio is used in bfq-mq's get_rq_priv, to get the request group. We could
pass directly the group here, but I thought that passing the bio was
more general, giving the possibility to get other pieces of information
if needed.
Signed-off-by: NPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: NJens Axboe <axboe@fb.com>

Switch these constants to an enum, and make let the compiler ensure that
all callers of blk_try_merge and elv_merge handle all potential values.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@fb.com>

If we end up doing a request-to-request merge when we have completed
a bio-to-request merge, we free the request from deep down in that
path. For blk-mq-sched, the merge path has to hold the appropriate
lock, but we don't need it for freeing the request. And in fact
holding the lock is problematic, since we are now calling the
mq sched put_rq_private() hook with the lock held. Other call paths
do not hold this lock.

Fix this inconsistency by ensuring that the caller frees a merged
request. Then we can do it outside of the lock, making it both more
efficient and fixing the blk-mq-sched problem of invoking parts of
the scheduler with an unknown lock state.
Reported-by: NPaolo Valente <paolo.valente@linaro.org>
Signed-off-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>

There's a weird inconsistency that flushes are mostly hidden from the
scheduler, but it needs to be aware of them in ->insert_requests().
Instead of having every scheduler call blk_mq_sched_bypass_insert(),
let's do it in the common framework.
Signed-off-by: NOmar Sandoval <osandov@fb.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

Use op_is_flush() where applicable.
Signed-off-by: NJens Axboe <axboe@fb.com>

Instead of letting the caller check this and handle the details
of inserting a flush request, put the logic in the scheduler
insertion function. This fixes direct flush insertion outside
of the usual make_request_fn calls, like from dm via
blk_insert_cloned_request().
Signed-off-by: NJens Axboe <axboe@fb.com>

This centralizes the checks for bios that needs to be go into the flush
state machine.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NMartin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: NBart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: NJens Axboe <axboe@fb.com>

When we invoke dispatch_requests(), the scheduler empties everything
into the passed in list. This isn't always a good thing, since it
means that we remove items that we could have potentially merged
with.

Change the function to dispatch single requests at the time. If
we do that, we can backoff exactly at the point where the device
can't consume more IO, and leave the rest with the scheduler for
better merging and future dispatch decision making.
Signed-off-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Tested-by: NHannes Reinecke <hare@suse.com>

If we have both multiple hardware queues and shared tag map between
devices, we need to ensure that we propagate the hardware queue
restart bit higher up. This is because we can get into a situation
where we don't have any IO pending on a hardware queue, yet we fail
getting a tag to start new IO. If that happens, it's not enough to
mark the hardware queue as needing a restart, we need to bubble
that up to the higher level queue as well.
Signed-off-by: NJens Axboe <axboe@fb.com>
Reviewed-by: NOmar Sandoval <osandov@fb.com>
Tested-by: NHannes Reinecke <hare@suse.com>