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. > > 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 <sqazi@xxxxxxxxxx> > --- > 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