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

Ming Lei
2020-02-07T02:07:44Z

On Thu, Feb 06, 2020 at 01:12:22PM -0800, Salman Qazi wrote:
> Flushes bypass the I/O scheduler and get added to hctx->dispatch
> in blk_mq_sched_bypass_insert. This can happen while a kworker is running
> hctx->run_work work item and is past the point in
> blk_mq_sched_dispatch_requests where hctx->dispatch is checked.
>
> The blk_mq_do_dispatch_sched call is not guaranteed to end in bounded time,
> because the I/O scheduler can feed an arbitrary number of commands.
>
> Since we have only one hctx->run_work, the commands waiting in
> hctx->dispatch will wait an arbitrary length of time for run_work to be
> rerun.
>
> A similar phenomenon exists with dispatches from the software queue.
>
> The solution is to poll hctx->dispatch in blk_mq_do_dispatch_sched and
> blk_mq_do_dispatch_ctx and return from the run_work handler and let it
> rerun.
>
> Signed-off-by: Salman Qazi <sqazi@google.com>
> ---
> block/blk-mq-sched.c | 47 ++++++++++++++++++++++++++++++++++++++------
> 1 file changed, 41 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index ca22afd47b3d..84dde147f646 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -84,12 +84,16 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
> * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
> * its queue by itself in its completion handler, so we don't need to
> * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> + *
> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.
> */
> -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> +static bool 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);
> + bool ret = false;
>
> do {
> struct request *rq;
> @@ -97,6 +101,11 @@ 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 (!list_empty_careful(&hctx->dispatch)) {
> + ret = true;
> + break;
> + }
> +
> if (!blk_mq_get_dispatch_budget(hctx))
> break;
>
> @@ -113,6 +122,8 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> */
> list_add(&rq->queuelist, &rq_list);
> } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
> +
> + return ret;
> }
>
> static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> @@ -130,16 +141,25 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
> * its queue by itself in its completion handler, so we don't need to
> * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> + *
> + * Returns true if hctx->dispatch was found non-empty and
> + * run_work has to be run again.
> */
> -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> {
> struct request_queue *q = hctx->queue;
> LIST_HEAD(rq_list);
> struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
> + bool ret = false;
>
> do {
> struct request *rq;
>
> + if (!list_empty_careful(&hctx->dispatch)) {
> + ret = true;
> + break;
> + }
> +
> if (!sbitmap_any_bit_set(&hctx->ctx_map))
> break;
>
> @@ -165,6 +185,7 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
>
> WRITE_ONCE(hctx->dispatch_from, ctx);
> + return ret;
> }
>
> void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> @@ -172,6 +193,8 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> struct request_queue *q = hctx->queue;
> struct elevator_queue *e = q->elevator;
> const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
> + bool run_again;
> + bool restarted = false;
> LIST_HEAD(rq_list);
>
> /* RCU or SRCU read lock is needed before checking quiesced flag */
> @@ -180,6 +203,9 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
>
> hctx->run++;
>
> +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);
> + }
> }
>
> bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
> --
> 2.25.0.341.g760bfbb309-goog
>

Reviewed-by: Ming Lei <ming.lei@redhat.com>

--
Ming