Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go
From: Ming Lei <hidden>
Date: 2020-02-04 09:20:21
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On Mon, Feb 03, 2020 at 12:59:50PM -0800, Salman Qazi wrote:
We observed that it is possible for a flush to bypass the I/O scheduler and get added to hctx->dispatch in blk_mq_sched_bypass_insert.
We always bypass io scheduler for flush request.
This can happen while a kworker is running blk_mq_do_dispatch_sched call in blk_mq_sched_dispatch_requests. However, the blk_mq_do_dispatch_sched call doesn't end in bounded time. As a result, the flush can sit there indefinitely, as the I/O scheduler feeds an arbitrary number of requests to the hardware.
blk-mq supposes to handle requests in hctx->dispatch with higher priority, see blk_mq_sched_dispatch_requests(). However, there is single hctx->run_work, so async run queue for dispatching flush request can only be run until another async run queue from scheduler is done.
quoted hunk ↗ jump to hunk
The solution is to periodically poll hctx->dispatch in blk_mq_do_dispatch_sched, to put a bound on the latency of the commands sitting there. Signed-off-by: Salman Qazi <redacted> --- block/blk-mq-sched.c | 6 ++++++ block/blk-mq.c | 4 ++++ block/blk-sysfs.c | 33 +++++++++++++++++++++++++++++++++ include/linux/blkdev.h | 2 ++ 4 files changed, 45 insertions(+)diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c index ca22afd47b3d..75cdec64b9c7 100644 --- a/block/blk-mq-sched.c +++ b/block/blk-mq-sched.c@@ -90,6 +90,7 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) struct request_queue *q = hctx->queue; struct elevator_queue *e = q->elevator; LIST_HEAD(rq_list); + int count = 0; do { struct request *rq;@@ -97,6 +98,10 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) if (e->type->ops.has_work && !e->type->ops.has_work(hctx)) break; + if (count > 0 && count % q->max_sched_batch == 0 && + !list_empty_careful(&hctx->dispatch)) + break;
q->max_sched_batch may not be needed, and it is reasonable to always disaptch requests in hctx->dispatch first. Also such trick is missed in dispatch from sw queue. Thanks, Ming