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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