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

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Date:   Fri, 7 Feb 2020 07:26:49 -0800
From:   Bart Van Assche <bvanassche@....org>
To:     Salman Qazi <sqazi@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Ming Lei <ming.lei@...hat.com>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Jesse Barnes <jsbarnes@...gle.com>,
        Gwendal Grignou <gwendal@...gle.com>,
        Hannes Reinecke <hare@...e.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] block: Limit number of items taken from the I/O scheduler
 in one go

On 2020-02-06 13:12, Salman Qazi wrote:
> + *
> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.

Please elaborate this comment and explain why this is necessary (to
avoid that flush processing is postponed forever).

> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.

Same comment here.

> +again:
> +	run_again = false;
> +
>  	/*
>  	 * If we have previous entries on our dispatch list, grab them first for
>  	 * more fair dispatch.
> @@ -208,19 +234,28 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>  		blk_mq_sched_mark_restart_hctx(hctx);
>  		if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
>  			if (has_sched_dispatch)
> -				blk_mq_do_dispatch_sched(hctx);
> +				run_again = blk_mq_do_dispatch_sched(hctx);
>  			else
> -				blk_mq_do_dispatch_ctx(hctx);
> +				run_again = blk_mq_do_dispatch_ctx(hctx);
>  		}
>  	} else if (has_sched_dispatch) {
> -		blk_mq_do_dispatch_sched(hctx);
> +		run_again = blk_mq_do_dispatch_sched(hctx);
>  	} else if (hctx->dispatch_busy) {
>  		/* dequeue request one by one from sw queue if queue is busy */
> -		blk_mq_do_dispatch_ctx(hctx);
> +		run_again = blk_mq_do_dispatch_ctx(hctx);
>  	} else {
>  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
>  		blk_mq_dispatch_rq_list(q, &rq_list, false);
>  	}
> +
> +	if (run_again) {
> +		if (!restarted) {
> +			restarted = true;
> +			goto again;
> +		}
> +
> +		blk_mq_run_hw_queue(hctx, true);
> +	}

So this patch changes blk_mq_sched_dispatch_requests() such that it
iterates at most two times? How about implementing that loop with an
explicit for-loop? I think that will make
blk_mq_sched_dispatch_requests() easier to read. As you may know forward
goto's are accepted in kernel code but backward goto's are frowned upon.

Thanks,

Bart.