linux-kernel - Re: [tip:timers/core] [posix] 1535cb8028: stress-ng.epoll.ops_per

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Message-ID: <877c4azyez.ffs@tglx>
Date: Thu, 27 Mar 2025 09:10:44 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Eric Dumazet <edumazet@...gle.com>, Mateusz Guzik <mjguzik@...il.com>
Cc: kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
 lkp@...el.com, linux-kernel@...r.kernel.org, x86@...nel.org, Benjamin
 Segall <bsegall@...gle.com>, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [tip:timers/core] [posix] 1535cb8028:
 stress-ng.epoll.ops_per_sec 36.2% regression

On Thu, Mar 27 2025 at 07:21, Eric Dumazet wrote:
> On Wed, Mar 26, 2025 at 10:11 PM Mateusz Guzik <mjguzik@...il.com> wrote:
>> On Wed, Mar 26, 2025 at 09:07:51AM +0100, Thomas Gleixner wrote:
>> > Unfortunately I can't reproduce any of it. I checked the epoll test
>> > source and it uses a posix timer, but that commit makes the hash less
>> > contended so there is zero explanation.
>> >
>>
>> The short summary is:
>> 1. your change is fine
>
> Let me rephrase this.
>
> Absolutely wonderful series, thanks a lot Thomas for doing it.

Thank you!

> Next bottlenecks are now these ones, but showing up in synthetic
> benchmarks only.

Right. I saw them too when working on this.

>     33.36%  timer_storm      [kernel.kallsyms]           [k]
> inc_rlimit_get_ucounts
>
>     32.85%  timer_storm      [kernel.kallsyms]           [k]
> dec_rlimit_put_ucounts

These two are not really posix-timer specific. They are also the
standouts for any signal micro benchmark.

I stared at the implementation a bit, but there is not much we can do
about that I fear.

Thanks,

        tglx