diff options
author | Mike Snitzer <snitzer@redhat.com> | 2016-02-05 08:49:01 -0500 |
---|---|---|
committer | Mike Snitzer <snitzer@redhat.com> | 2016-02-22 11:04:40 -0500 |
commit | 6acfe68bac7e6f16dc312157b1fa6e2368985013 (patch) | |
tree | 76021143405230f21cf73dcc29b06a99c4b122f0 | |
parent | 956a4025808df4abfe2fe25a11feb4c8f33fc336 (diff) |
dm: fix excessive dm-mq context switching
Request-based DM's blk-mq support (dm-mq) was reported to be 50% slower
than if an underlying null_blk device were used directly. One of the
reasons for this drop in performance is that blk_insert_clone_request()
was calling blk_mq_insert_request() with @async=true. This forced the
use of kblockd_schedule_delayed_work_on() to run the blk-mq hw queues
which ushered in ping-ponging between process context (fio in this case)
and kblockd's kworker to submit the cloned request. The ftrace
function_graph tracer showed:
kworker-2013 => fio-12190
fio-12190 => kworker-2013
...
kworker-2013 => fio-12190
fio-12190 => kworker-2013
...
Fixing blk_insert_clone_request()'s blk_mq_insert_request() call to
_not_ use kblockd to submit the cloned requests isn't enough to
eliminate the observed context switches.
In addition to this dm-mq specific blk-core fix, there are 2 DM core
fixes to dm-mq that (when paired with the blk-core fix) completely
eliminate the observed context switching:
1) don't blk_mq_run_hw_queues in blk-mq request completion
Motivated by desire to reduce overhead of dm-mq, punting to kblockd
just increases context switches.
In my testing against a really fast null_blk device there was no benefit
to running blk_mq_run_hw_queues() on completion (and no other blk-mq
driver does this). So hopefully this change doesn't induce the need for
yet another revert like commit 621739b00e16ca2d !
2) use blk_mq_complete_request() in dm_complete_request()
blk_complete_request() doesn't offer the traditional q->mq_ops vs
.request_fn branching pattern that other historic block interfaces
do (e.g. blk_get_request). Using blk_mq_complete_request() for
blk-mq requests is important for performance. It should be noted
that, like blk_complete_request(), blk_mq_complete_request() doesn't
natively handle partial completions -- but the request-based
DM-multipath target does provide the required partial completion
support by dm.c:end_clone_bio() triggering requeueing of the request
via dm-mpath.c:multipath_end_io()'s return of DM_ENDIO_REQUEUE.
dm-mq fix #2 is _much_ more important than #1 for eliminating the
context switches.
Before: cpu : usr=15.10%, sys=59.39%, ctx=7905181, majf=0, minf=475
After: cpu : usr=20.60%, sys=79.35%, ctx=2008, majf=0, minf=472
With these changes multithreaded async read IOPs improved from ~950K
to ~1350K for this dm-mq stacked on null_blk test-case. The raw read
IOPs of the underlying null_blk device for the same workload is ~1950K.
Fixes: 7fb4898e0 ("block: add blk-mq support to blk_insert_cloned_request()")
Fixes: bfebd1cdb ("dm: add full blk-mq support to request-based DM")
Cc: stable@vger.kernel.org # 4.1+
Reported-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
-rw-r--r-- | block/blk-core.c | 2 | ||||
-rw-r--r-- | drivers/md/dm.c | 13 |
2 files changed, 7 insertions, 8 deletions
diff --git a/block/blk-core.c b/block/blk-core.c index b83d29755b5a..45f4d7efbf34 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -2198,7 +2198,7 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq) if (q->mq_ops) { if (blk_queue_io_stat(q)) blk_account_io_start(rq, true); - blk_mq_insert_request(rq, false, true, true); + blk_mq_insert_request(rq, false, true, false); return 0; } diff --git a/drivers/md/dm.c b/drivers/md/dm.c index a712fd73a55f..db7a51a20870 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -1109,12 +1109,8 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue) * back into ->request_fn() could deadlock attempting to grab the * queue lock again. */ - if (run_queue) { - if (md->queue->mq_ops) - blk_mq_run_hw_queues(md->queue, true); - else - blk_run_queue_async(md->queue); - } + if (!md->queue->mq_ops && run_queue) + blk_run_queue_async(md->queue); /* * dm_put() must be at the end of this function. See the comment above @@ -1336,7 +1332,10 @@ static void dm_complete_request(struct request *rq, int error) struct dm_rq_target_io *tio = tio_from_request(rq); tio->error = error; - blk_complete_request(rq); + if (!rq->q->mq_ops) + blk_complete_request(rq); + else + blk_mq_complete_request(rq, error); } /* |