On Fri, Nov 10, 2017 at 09:12:18AM -0700, Jens Axboe wrote:
> Currently we are inconsistent in when we decide to run the queue. Using
> blk_mq_run_hw_queues() we check if the hctx has pending IO before
> running it, but we don't do that from the individual queue run function,
> blk_mq_run_hw_queue(). This results in a lot of extra and pointless
> queue runs, potentially, on flush requests and (much worse) on tag
> starvation situations. This is observable just looking at the top
> output, with lots of kworkers active. For the !async runs, it just adds
> to the CPU overhead of blk-mq.
>
> Move the has-pending check into the run function instead of having
> callers do it.
>
> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> ---
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 6f4bdb8209f7..c117bd8fd1f6 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -81,12 +81,7 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
> } else
> clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
>
> - if (blk_mq_hctx_has_pending(hctx)) {
> - blk_mq_run_hw_queue(hctx, true);
> - return true;
> - }
> -
> - return false;
> + return blk_mq_run_hw_queue(hctx, true);
> }
>
> /*
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index bfe24a5b62a3..a2a4271f5ab8 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -61,10 +61,10 @@ static int blk_mq_poll_stats_bkt(const struct request *rq)
> /*
> * Check if any of the ctx's have pending work in this hardware queue
> */
> -bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx)
> {
> - return sbitmap_any_bit_set(&hctx->ctx_map) ||
> - !list_empty_careful(&hctx->dispatch) ||
> + return !list_empty_careful(&hctx->dispatch) ||
> + sbitmap_any_bit_set(&hctx->ctx_map) ||
> blk_mq_sched_has_work(hctx);
> }
>
> @@ -1253,9 +1253,14 @@ void blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, unsigned long msecs)
> }
> EXPORT_SYMBOL(blk_mq_delay_run_hw_queue);
>
> -void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> +bool blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> {
> - __blk_mq_delay_run_hw_queue(hctx, async, 0);
> + if (blk_mq_hctx_has_pending(hctx)) {
> + __blk_mq_delay_run_hw_queue(hctx, async, 0);
> + return true;
> + }
> +
> + return false;
> }
> EXPORT_SYMBOL(blk_mq_run_hw_queue);
>
> @@ -1265,8 +1270,7 @@ void blk_mq_run_hw_queues(struct request_queue *q, bool async)
> int i;
>
> queue_for_each_hw_ctx(q, hctx, i) {
> - if (!blk_mq_hctx_has_pending(hctx) ||
> - blk_mq_hctx_stopped(hctx))
> + if (blk_mq_hctx_stopped(hctx))
> continue;
>
> blk_mq_run_hw_queue(hctx, async);
Minor cosmetic point, I find this double-negative thing confusing,
how about:
queue_for_each_hw_ctx(q, hctx, i) {
if (!blk_mq_hctx_stopped(hctx))
blk_mq_run_hw_queue(hctx, async);
Other than that,
Reviewed-by: Omar Sandoval <osandov@xxxxxx>
