Re: [PATCH] block: Limit number of items taken from the I/O scheduler in one go

[Salman Qazi <sqazi@google.com>]

On Tue, Feb 4, 2020 at 1:20 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> 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@google.com>
> > ---
> >  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.

I will update the commit message, drop max_sched_batch and just turn
it into a simple check and add the same
thing to the dispatch from the sw queue.

>
>
> Thanks,
> Ming
>