linux-kernel - Re: [PATCH 0/2] task

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Message-ID: <5b20b94d-13f7-66ee-610a-6f37ec8caa8d@gmail.com>
Date:   Mon, 20 Jul 2020 19:06:28 +0300
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] task_put batching

On 20/07/2020 18:49, Jens Axboe wrote:
> On 7/20/20 9:22 AM, Pavel Begunkov wrote:
>> On 18/07/2020 17:37, Jens Axboe wrote:
>>> On 7/18/20 2:32 AM, Pavel Begunkov wrote:
>>>> For my a bit exaggerated test case perf continues to show high CPU
>>>> cosumption by io_dismantle(), and so calling it io_iopoll_complete().
>>>> Even though the patch doesn't yield throughput increase for my setup,
>>>> probably because the effect is hidden behind polling, but it definitely
>>>> improves relative percentage. And the difference should only grow with
>>>> increasing number of CPUs. Another reason to have this is that atomics
>>>> may affect other parallel tasks (e.g. which doesn't use io_uring)
>>>>
>>>> before:
>>>> io_iopoll_complete: 5.29%
>>>> io_dismantle_req:   2.16%
>>>>
>>>> after:
>>>> io_iopoll_complete: 3.39%
>>>> io_dismantle_req:   0.465%
>>>
>>> Still not seeing a win here, but it's clean and it _should_ work. For
>>> some reason I end up getting the offset in task ref put growing the
>>> fput_many(). Which doesn't (on the surface) make a lot of sense, but
>>> may just mean that we have some weird side effects.
>>
>> It grows because the patch is garbage, the second condition is always false.
>> See the diff. Could you please drop both patches?
> 
> Hah, indeed. With this on top, it looks like it should in terms of
> performance and profiles.

It just shows, that it doesn't really matters for a single-threaded app,
as expected. Worth to throw some contention though. I'll think about
finding some time to get/borrow a multi-threaded one.

> 
> I can just fold this into the existing one, if you'd like.

Would be nice. I'm going to double-check the counter and re-measure anyway.
BTW, how did you find it? A tool or a proc file would be awesome.

-- 
Pavel Begunkov