Re: [PATCH] io_uring: be smarter about waking multiple CQ ring waiters

Pavel Begunkov
2021-08-10T01:42:41Z

On 8/6/21 9:19 PM, Jens Axboe wrote:
> Currently we only wake the first waiter, even if we have enough entries
> posted to satisfy multiple waiters. Improve that situation so that
> every waiter knows how much the CQ tail has to advance before they can
> be safely woken up.
>
> With this change, if we have N waiters each asking for 1 event and we get
> 4 completions, then we wake up 4 waiters. If we have N waiters asking
> for 2 completions and we get 4 completions, then we wake up the first
> two. Previously, only the first waiter would've been woken up.
>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>
> ---
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index bf548af0426c..04df4fa3c75e 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -1435,11 +1435,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
>
> static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
> {
> - /* see waitqueue_active() comment */
> - smp_mb();
> -
> - if (waitqueue_active(&ctx->cq_wait))
> - wake_up(&ctx->cq_wait);
> + /*
> + * wake_up_all() may seem excessive, but io_wake_function() and
> + * io_should_wake() handle the termination of the loop and only
> + * wake as many waiters as we need to.
> + */
> + if (wq_has_sleeper(&ctx->cq_wait))
> + wake_up_all(&ctx->cq_wait);
> if (ctx->sq_data && waitqueue_active(&ctx->sq_data->wait))
> wake_up(&ctx->sq_data->wait);
> if (io_should_trigger_evfd(ctx))
> @@ -6968,20 +6970,21 @@ static int io_sq_thread(void *data)
> struct io_wait_queue {
> struct wait_queue_entry wq;
> struct io_ring_ctx *ctx;
> - unsigned to_wait;
> + unsigned cq_tail;
> unsigned nr_timeouts;
> };
>
> static inline bool io_should_wake(struct io_wait_queue *iowq)
> {
> struct io_ring_ctx *ctx = iowq->ctx;
> + unsigned tail = ctx->cached_cq_tail + atomic_read(&ctx->cq_timeouts);

Seems, adding cq_timeouts can be dropped from here and iowq.cq_tail

>
> /*
> * Wake up if we have enough events, or if a timeout occurred since we
> * started waiting. For timeouts, we always want to return to userspace,
> * regardless of event count.
> */
> - return io_cqring_events(ctx) >= iowq->to_wait ||

Don't we miss smp_rmb() previously provided my io_cqring_events()?

> + return tail >= iowq->cq_tail ||

tails might overflow

> atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
> }
>
> @@ -7045,7 +7048,6 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
> .entry = LIST_HEAD_INIT(iowq.wq.entry),
> },
> .ctx = ctx,
> - .to_wait = min_events,
> };
> struct io_rings *rings = ctx->rings;
> signed long timeout = MAX_SCHEDULE_TIMEOUT;
> @@ -7081,6 +7083,8 @@ static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
> }
>
> iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
> + iowq.cq_tail = READ_ONCE(ctx->rings->cq.head) + min_events +
> + iowq.nr_timeouts;
> trace_io_uring_cqring_wait(ctx, min_events);
> do {
> /* if we can't even flush overflow, don't wait for more */
>

--
Pavel Begunkov