On Mon, Jan 11, 2021 at 05:47:17PM +0100, Jan Kara wrote:
> Currently when non-mq aware IO scheduler (BFQ, mq-deadline) is used for
> a queue with multiple HW queues, the performance it rather bad. The
> problem is that these IO schedulers use queue-wide locking and their
> dispatch function does not respect the hctx it is passed in and returns
> any request it finds appropriate. Thus locality of request access is
> broken and dispatch from multiple CPUs just contends on IO scheduler
> locks. For these IO schedulers there's little point in dispatching from
> multiple CPUs. Instead dispatch always only from a single CPU to limit
> contention.
>
> Below is a comparison of dbench runs on XFS filesystem where the storage
> is a raid card with 64 HW queues and to it attached a single rotating
> disk. BFQ is used as IO scheduler:
>
> clients MQ SQ MQ-Patched
> Amean 1 39.12 (0.00%) 43.29 * -10.67%* 36.09 * 7.74%*
> Amean 2 128.58 (0.00%) 101.30 * 21.22%* 96.14 * 25.23%*
> Amean 4 577.42 (0.00%) 494.47 * 14.37%* 508.49 * 11.94%*
> Amean 8 610.95 (0.00%) 363.86 * 40.44%* 362.12 * 40.73%*
> Amean 16 391.78 (0.00%) 261.49 * 33.25%* 282.94 * 27.78%*
> Amean 32 324.64 (0.00%) 267.71 * 17.54%* 233.00 * 28.23%*
> Amean 64 295.04 (0.00%) 253.02 * 14.24%* 242.37 * 17.85%*
> Amean 512 10281.61 (0.00%) 10211.16 * 0.69%* 10447.53 * -1.61%*
>
> Numbers are times so lower is better. MQ is stock 5.10-rc6 kernel. SQ is
> the same kernel with megaraid_sas.host_tagset_enable=0 so that the card
> advertises just a single HW queue. MQ-Patched is a kernel with this
> patch applied.
>
> You can see multiple hardware queues heavily hurt performance in
> combination with BFQ. The patch restores the performance.
>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
> block/blk-mq.c | 66 ++++++++++++++++++++++++++++++++++++----
> block/kyber-iosched.c | 1 +
> include/linux/elevator.h | 2 ++
> 3 files changed, 63 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 57f3482b2c26..580601787f7a 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1646,6 +1646,42 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
> }
> EXPORT_SYMBOL(blk_mq_run_hw_queue);
>
> +/*
> + * Is the request queue handled by an IO scheduler that does not respect
> + * hardware queues when dispatching?
> + */
> +static bool blk_mq_has_sqsched(struct request_queue *q)
> +{
> + struct elevator_queue *e = q->elevator;
> +
> + if (e && e->type->ops.dispatch_request &&
> + !(e->type->elevator_features & ELEVATOR_F_MQ_AWARE))
> + return true;
> + return false;
> +}
> +
> +/*
> + * Return prefered queue to dispatch from (if any) for non-mq aware IO
> + * scheduler.
> + */
> +static struct blk_mq_hw_ctx *blk_mq_get_sq_hctx(struct request_queue *q)
> +{
> + struct blk_mq_hw_ctx *hctx;
> +
> + /*
> + * If the IO scheduler does not respect hardware queues when
> + * dispatching, we just don't bother with multiple HW queues and
> + * dispatch from hctx for the current CPU since running multiple queues
> + * just causes lock contention inside the scheduler and pointless cache
> + * bouncing.
> + */
> + hctx = blk_mq_map_queue_type(q, HCTX_TYPE_DEFAULT,
> + raw_smp_processor_id());
> + if (!blk_mq_hctx_stopped(hctx))
> + return hctx;
> + return NULL;
> +}
> +
> /**
> * blk_mq_run_hw_queues - Run all hardware queues in a request queue.
> * @q: Pointer to the request queue to run.
> @@ -1653,14 +1689,23 @@ EXPORT_SYMBOL(blk_mq_run_hw_queue);
> */
> void blk_mq_run_hw_queues(struct request_queue *q, bool async)
> {
> - struct blk_mq_hw_ctx *hctx;
> + struct blk_mq_hw_ctx *hctx, *sq_hctx;
> int i;
>
> + sq_hctx = NULL;
> + if (blk_mq_has_sqsched(q))
> + sq_hctx = blk_mq_get_sq_hctx(q);
> queue_for_each_hw_ctx(q, hctx, i) {
> if (blk_mq_hctx_stopped(hctx))
> continue;
> -
> - blk_mq_run_hw_queue(hctx, async);
> + /*
> + * Dispatch from this hctx either if there's no hctx preferred
> + * by IO scheduler or if it has requests that bypass the
> + * scheduler.
> + */
> + if (!sq_hctx || sq_hctx == hctx ||
> + !list_empty_careful(&hctx->dispatch))
> + blk_mq_run_hw_queue(hctx, async);
> }
> }
> EXPORT_SYMBOL(blk_mq_run_hw_queues);
> @@ -1672,14 +1717,23 @@ EXPORT_SYMBOL(blk_mq_run_hw_queues);
> */
> void blk_mq_delay_run_hw_queues(struct request_queue *q, unsigned long msecs)
> {
> - struct blk_mq_hw_ctx *hctx;
> + struct blk_mq_hw_ctx *hctx, *sq_hctx;
> int i;
>
> + sq_hctx = NULL;
> + if (blk_mq_has_sqsched(q))
> + sq_hctx = blk_mq_get_sq_hctx(q);
> queue_for_each_hw_ctx(q, hctx, i) {
> if (blk_mq_hctx_stopped(hctx))
> continue;
> -
> - blk_mq_delay_run_hw_queue(hctx, msecs);
> + /*
> + * Dispatch from this hctx either if there's no hctx preferred
> + * by IO scheduler or if it has requests that bypass the
> + * scheduler.
> + */
> + if (!sq_hctx || sq_hctx == hctx ||
> + !list_empty_careful(&hctx->dispatch))
> + blk_mq_delay_run_hw_queue(hctx, msecs);
> }
> }
> EXPORT_SYMBOL(blk_mq_delay_run_hw_queues);
> diff --git a/block/kyber-iosched.c b/block/kyber-iosched.c
> index dc89199bc8c6..c25c41d0d061 100644
> --- a/block/kyber-iosched.c
> +++ b/block/kyber-iosched.c
> @@ -1029,6 +1029,7 @@ static struct elevator_type kyber_sched = {
> #endif
> .elevator_attrs = kyber_sched_attrs,
> .elevator_name = "kyber",
> + .elevator_features = ELEVATOR_F_MQ_AWARE,
> .elevator_owner = THIS_MODULE,
> };
>
> diff --git a/include/linux/elevator.h b/include/linux/elevator.h
> index bacc40a0bdf3..1fe8e105b83b 100644
> --- a/include/linux/elevator.h
> +++ b/include/linux/elevator.h
> @@ -172,6 +172,8 @@ extern struct request *elv_rb_find(struct rb_root *, sector_t);
>
> /* Supports zoned block devices sequential write constraint */
> #define ELEVATOR_F_ZBD_SEQ_WRITE (1U << 0)
> +/* Supports scheduling on multiple hardware queues */
> +#define ELEVATOR_F_MQ_AWARE (1U << 1)
>
> #endif /* CONFIG_BLOCK */
> #endif
> --
> 2.26.2
>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
--
Ming
