linux-kernel - Re: [RFT][PATCH v2 0/3] cpuidle: teo: Do not check timers unconditionally every time

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Wed, 9 Aug 2023 17:43:51 -0700
From:   Doug Smythies <dsmythies@...us.net>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Doug Smythies <dsmythies@...us.net>
Subject: Re: [RFT][PATCH v2 0/3] cpuidle: teo: Do not check timers
 unconditionally every time

On Wed, Aug 9, 2023 at 9:24 AM Anna-Maria Behnsen
<anna-maria@...utronix.de> wrote:
> On Tue, 8 Aug 2023, Doug Smythies wrote:
> > On Tue, Aug 8, 2023 at 9:43 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
> > > On Tue, Aug 8, 2023 at 5:22 PM Doug Smythies <dsmythies@...us.net> wrote:
>
> [...]
>
> > > > Conclusions: Overall, I am not seeing a compelling reason to
> > > > proceed with this patch set.
> > >
> > > On the other hand, if there is a separate compelling reason to do
> > > that, it doesn't appear to lead to a major regression.
> >
> > Agreed.
> >
>
> Regarding the compelling reason:
>
> On a fully loaded machine with 256 CPUs tick_nohz_next_event() is executed
> ~48000 times per second. With this patchset it is reduced to ~120 times per
> second. The factor for the difference is 400. This is already an
> improvement.
>
> tick_nohz_next_event() marks timer bases idle, whenever possible - even if
> the tick is not stopped afterwards. When a timer is enqueued remote into an
> idle timer base an IPI is sent. Calling tick_nohz_next_event() only when
> the system is not that busy, prevents those unnecessary IPIs.
>
> Beside of those facts, I'm working on the timer pull model [0]. With this,
> non pinned timers can also be expired by other CPUs and do not prevent CPUs
> from going idle. Those timers will be enqueued on the local CPU without any
> heuristics. This helps to improve behavior when a system is idle (regarding
> power). But the call of tick_nohz_next_event() will be more expensive which
> led to a regression during testing. This regression is gone with the new
> teo implementation - it seems that there is also an improvement under
> load. I do not have finalized numbers, as it is still WIP (I came across
> some other possible optimizations during analyzing the regression, which
> I'm evaluating at the moment).
>
> Thanks,
>
>         Anna-Maria
>
>
> [0] https://lore.kernel.org/lkml/20230524070629.6377-1-anna-maria@linutronix.de/

Thank you for the context and the link.

... Doug