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

[Ming Lei <ming.lei@redhat.com>]

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