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authorMike Snitzer <snitzer@redhat.com>2016-02-05 16:49:01 +0300
committerMike Snitzer <snitzer@redhat.com>2016-02-22 19:04:40 +0300
commit6acfe68bac7e6f16dc312157b1fa6e2368985013 (patch)
tree76021143405230f21cf73dcc29b06a99c4b122f0 /drivers
parent956a4025808df4abfe2fe25a11feb4c8f33fc336 (diff)
downloadlinux-6acfe68bac7e6f16dc312157b1fa6e2368985013.tar.xz
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>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/md/dm.c13
1 files changed, 6 insertions, 7 deletions
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);
}
/*