Re: [PATCH 05/14] blk-mq-sched: don't dequeue request until all in ->dispatch are flushed

Bart Van Assche
2017-07-31T23:43:09Z

On Tue, 2017-08-01 at 00:51 +0800, Ming Lei wrote:
> During dispatch, we moved all requests from hctx->dispatch to
> one temporary list, then dispatch them one by one from this list.
> Unfortunately duirng this period, run queue from other contexts
> may think the queue is idle and start to dequeue from sw/scheduler
> queue and try to dispatch because ->dispatch is empty.
>
> This way will hurt sequential I/O performance because requests are
> dequeued when queue is busy.
>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
> block/blk-mq-sched.c | 24 ++++++++++++++++++------
> include/linux/blk-mq.h | 1 +
> 2 files changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 3510c01cb17b..eb638063673f 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -112,8 +112,15 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> */
> if (!list_empty_careful(&hctx->dispatch)) {
> spin_lock(&hctx->lock);
> - if (!list_empty(&hctx->dispatch))
> + if (!list_empty(&hctx->dispatch)) {
> list_splice_init(&hctx->dispatch, &rq_list);
> +
> + /*
> + * BUSY won't be cleared until all requests
> + * in hctx->dispatch are dispatched successfully
> + */
> + set_bit(BLK_MQ_S_BUSY, &hctx->state);
> + }
> spin_unlock(&hctx->lock);
> }
>
> @@ -129,15 +136,20 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> if (!list_empty(&rq_list)) {
> blk_mq_sched_mark_restart_hctx(hctx);
> can_go = blk_mq_dispatch_rq_list(q, &rq_list);
> - } else if (!has_sched_dispatch && !q->queue_depth) {
> - blk_mq_flush_busy_ctxs(hctx, &rq_list);
> - blk_mq_dispatch_rq_list(q, &rq_list);
> - can_go = false;
> + if (can_go)
> + clear_bit(BLK_MQ_S_BUSY, &hctx->state);
> }
>
> - if (!can_go)
> + /* can't go until ->dispatch is flushed */
> + if (!can_go || test_bit(BLK_MQ_S_BUSY, &hctx->state))
> return;
>
> + if (!has_sched_dispatch && !q->queue_depth) {
> + blk_mq_flush_busy_ctxs(hctx, &rq_list);
> + blk_mq_dispatch_rq_list(q, &rq_list);
> + return;
> + }

Hello Ming,

Since setting, clearing and testing of BLK_MQ_S_BUSY can happen concurrently
and since clearing and testing happens without any locks held I'm afraid this
patch introduces the following race conditions:
* Clearing of BLK_MQ_S_BUSY immediately after this bit has been set, resulting
in this bit not being set although there are requests on the dispatch list.
* Checking BLK_MQ_S_BUSY after requests have been added to the dispatch list
but before that bit is set, resulting in test_bit(BLK_MQ_S_BUSY, &hctx->state)
reporting that the BLK_MQ_S_BUSY has not been set although there are requests
on the dispatch list.
* Checking BLK_MQ_S_BUSY after requests have been removed from the dispatch list
but before that bit is cleared, resulting in test_bit(BLK_MQ_S_BUSY, &hctx->state)
reporting that the BLK_MQ_S_BUSY
has been set although there are no requests
on the dispatch list.

Bart.