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@xxxxxxxxxxxxxxx> know about it. >From 6acfe68bac7e6f16dc312157b1fa6e2368985013 Mon Sep 17 00:00:00 2001 From: Mike Snitzer <snitzer@xxxxxxxxxx> Date: Fri, 5 Feb 2016 08:49:01 -0500 Subject: dm: fix excessive dm-mq context switching From: Mike Snitzer <snitzer@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxx> Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> Acked-by: Jens Axboe <axboe@xxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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@xxxxxxxxxx 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 -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html