* Patch "dm: fix excessive dm-mq context switching" has been added to the 4.5-stable tree
@ 2016-04-09 23:45 gregkh
0 siblings, 0 replies; only message in thread
From: gregkh @ 2016-04-09 23:45 UTC (permalink / raw)
To: snitzer, axboe, gregkh, sagig; +Cc: stable, stable-commits
This is a note to let you know that I've just added the patch titled
dm: fix excessive dm-mq context switching
to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
dm-fix-excessive-dm-mq-context-switching.patch
and it can be found in the queue-4.5 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@vger.kernel.org> know about it.
>From 6acfe68bac7e6f16dc312157b1fa6e2368985013 Mon Sep 17 00:00:00 2001
From: Mike Snitzer <snitzer@redhat.com>
Date: Fri, 5 Feb 2016 08:49:01 -0500
Subject: dm: fix excessive dm-mq context switching
From: Mike Snitzer <snitzer@redhat.com>
commit 6acfe68bac7e6f16dc312157b1fa6e2368985013 upstream.
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")
Reported-by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
block/blk-core.c | 2 +-
drivers/md/dm.c | 13 ++++++-------
2 files changed, 7 insertions(+), 8 deletions(-)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2198,7 +2198,7 @@ int blk_insert_cloned_request(struct req
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;
}
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -1109,12 +1109,8 @@ static void rq_completed(struct mapped_d
* 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 r
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);
}
/*
Patches currently in stable-queue which might be from snitzer@redhat.com are
queue-4.5/dm-thin-metadata-don-t-issue-prefetches-if-a-transaction-abort-has-failed.patch
queue-4.5/dm-snapshot-disallow-the-cow-and-origin-devices-from-being-identical.patch
queue-4.5/dm-fix-excessive-dm-mq-context-switching.patch
queue-4.5/dm-fix-rq_end_stats-null-pointer-in-dm_requeue_original_request.patch
queue-4.5/dm-cache-make-sure-every-metadata-function-checks-fail_io.patch
queue-4.5/sd-fix-discard-granularity-when-lbprz-1.patch
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2016-04-09 23:45 Patch "dm: fix excessive dm-mq context switching" has been added to the 4.5-stable tree gregkh
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