On 24/09/2019 11:27, 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. > If io_cqring_events() == 8, only the last two would be woken up in both implementations, as to_wait/threshold is specified per waiter. Isn't it? Agree with waiting, I don't see a good real-life case for that, that couldn't be done efficiently in userspace. -- Yours sincerely, Pavel Begunkov
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