Re: [PATCH v2 2/2] blk-mq: Rerun dispatching in the case of budget contention

[Doug Anderson <dianders@chromium.org>]

Hi,

On Sun, Apr 5, 2020 at 2:15 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> OK, looks it isn't specific on BFQ any more.
>
> Follows another candidate approach for this issue, given it is so hard
> to trigger, we can make it more reliable by rerun queue when has_work()
> returns true after ops->dispath_request() returns NULL.
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 74cedea56034..4408e5d4fcd8 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -80,6 +80,7 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
>         blk_mq_run_hw_queue(hctx, true);
>  }
>
> +#define BLK_MQ_BUDGET_DELAY    3               /* ms units */
>  /*
>   * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
>   * its queue by itself in its completion handler, so we don't need to
> @@ -103,6 +104,9 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>                 rq = e->type->ops.dispatch_request(hctx);
>                 if (!rq) {
>                         blk_mq_put_dispatch_budget(hctx);
> +
> +                       if (e->type->ops.has_work && e->type->ops.has_work(hctx))
> +                               blk_mq_delay_run_hw_queue(hctx, BLK_MQ_BUDGET_DELAY);

I agree that your patch should solve the race.  With the current BFQ's
has_work() it's a bit of a disaster though. It will essentially put
blk-mq into a busy-wait loop (with a 3 ms delay between each poll)
while BFQ's has_work() says "true" but BFQ doesn't dispatch anything.

...so I guess the question that still needs to be answered: does
has_work() need to be exact?  If so then we need the patch you propose
plus one to BFQ.  If not, we should continue along the lines of my
patch.

-Doug