On 24/09/2019 11:36, Jens Axboe wrote: > On 9/24/19 2:27 AM, Jens Axboe wrote: >> On 9/24/19 2:02 AM, Jens Axboe wrote: >>> On 9/24/19 1:06 AM, Pavel Begunkov wrote: >>>> On 24/09/2019 02:00, Jens Axboe wrote: >>>>>> I think we can do the same thing, just wrapping the waitqueue in a >>>>>> structure with a count in it, on the stack. Got some flight time >>>>>> coming up later today, let me try and cook up a patch. >>>>> >>>>> Totally untested, and sent out 5 min before departure... But something >>>>> like this. >>>> Hmm, reminds me my first version. Basically that's the same thing but >>>> with macroses inlined. I wanted to make it reusable and self-contained, >>>> though. >>>> >>>> If you don't think it could be useful in other places, sure, we could do >>>> something like that. Is that so? >>> >>> I totally agree it could be useful in other places. Maybe formalized and >>> used with wake_up_nr() instead of adding a new primitive? Haven't looked >>> into that, I may be talking nonsense. >>> >>> In any case, I did get a chance to test it and it works for me. Here's >>> the "finished" version, slightly cleaned up and with a comment added >>> for good measure. >> >> Notes: >> >> This version gets the ordering right, you need exclusive waits to get >> fifo ordering on the waitqueue. >> >> Both versions (yours and mine) suffer from the problem of potentially >> waking too many. I don't think this is a real issue, as generally we >> don't do threaded access to the io_urings. But if you had the following >> tasks wait on the cqring: >> >> [min_events = 32], [min_events = 8], [min_events = 8] >> >> and we reach the io_cqring_events() == threshold, we'll wake all three. >> I don't see a good solution to this, so I suspect we just live with >> until proven an issue. Both versions are much better than what we have >> now. > > Forgot an issue around signal handling, version below adds the > right check for that too.
It seems to be a good reason to not keep reimplementing
"prepare_to_wait*() + wait loop" every time, but keep it in sched :)
>
> Curious what your test case was for this?
You mean a performance test case? It's briefly described in a comment
for the second patch. That's just rewritten io_uring-bench, with
1. a thread generating 1 request per call in a loop
2. and the second thread waiting for ~128 events.
Both are pinned to the same core.
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index ca7570aca430..3fbab5692f14 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2768,6 +2768,42 @@ static int io_ring_submit(struct io_ring_ctx *ctx,
> unsigned int to_submit,
> return submit;
> }
>
> +struct io_wait_queue {
> + struct wait_queue_entry wq;
> + struct io_ring_ctx *ctx;
> + struct task_struct *task;
> + unsigned to_wait;
> + unsigned nr_timeouts;
> +};
> +
> +static inline bool io_should_wake(struct io_wait_queue *iowq)
> +{
> + struct io_ring_ctx *ctx = iowq->ctx;
> +
> + /*
> + * Wake up if we have enough events, or if a timeout occured since we
> + * started waiting. For timeouts, we always want to return to userspace,
> + * regardless of event count.
> + */
> + return io_cqring_events(ctx->rings) >= iowq->to_wait ||
> + atomic_read(&ctx->cq_timeouts) != iowq->nr_timeouts;
> +}
> +
> +static int io_wake_function(struct wait_queue_entry *curr, unsigned int mode,
> + int wake_flags, void *key)
> +{
> + struct io_wait_queue *iowq = container_of(curr, struct io_wait_queue,
> + wq);
> +
> + if (io_should_wake(iowq)) {
> + list_del_init(&curr->entry);
> + wake_up_process(iowq->task);
> + return 1;
> + }
> +
> + return -1;
> +}
> +
> /*
> * Wait until events become available, if we don't already have some. The
> * application must reap them itself, as they reside on the shared cq ring.
> @@ -2775,8 +2811,16 @@ static int io_ring_submit(struct io_ring_ctx *ctx,
> unsigned int to_submit,
> static int io_cqring_wait(struct io_ring_ctx *ctx, int min_events,
> const sigset_t __user *sig, size_t sigsz)
> {
> + struct io_wait_queue iowq = {
> + .wq = {
> + .func = io_wake_function,
> + .entry = LIST_HEAD_INIT(iowq.wq.entry),
> + },
> + .task = current,
> + .ctx = ctx,
> + .to_wait = min_events,
> + };
> struct io_rings *rings = ctx->rings;
> - unsigned nr_timeouts;
> int ret;
>
> if (io_cqring_events(rings) >= min_events)
> @@ -2795,15 +2839,18 @@ static int io_cqring_wait(struct io_ring_ctx *ctx,
> int min_events,
> return ret;
> }
>
> - nr_timeouts = atomic_read(&ctx->cq_timeouts);
> - /*
> - * Return if we have enough events, or if a timeout occured since
> - * we started waiting. For timeouts, we always want to return to
> - * userspace.
> - */
> - ret = wait_event_interruptible(ctx->wait,
> - io_cqring_events(rings) >= min_events ||
> - atomic_read(&ctx->cq_timeouts) != nr_timeouts);
> + iowq.nr_timeouts = atomic_read(&ctx->cq_timeouts);
> + prepare_to_wait_exclusive(&ctx->wait, &iowq.wq, TASK_INTERRUPTIBLE);
> + do {
> + if (io_should_wake(&iowq))
> + break;
> + schedule();
> + if (signal_pending(current))
> + break;
> + set_current_state(TASK_INTERRUPTIBLE);
> + } while (1);
> + finish_wait(&ctx->wait, &iowq.wq);
> +
> restore_saved_sigmask_unless(ret == -ERESTARTSYS);
> if (ret == -ERESTARTSYS)
> ret = -EINTR;
>
--
Yours sincerely,
Pavel Begunkov
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