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

Ming Lei
2020-02-04T09:20:23Z

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.

Thanks,
Ming