linux-kernel - Re: [PATCH v4 0/2] Avoid spurious freezer wakeups

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Message-ID: <ZRwQw7YP5MsiPoTg@gmail.com>
Date:   Tue, 3 Oct 2023 15:01:55 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Elliot Berman <quic_eberman@...cinc.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Carlos Llamas <cmllamas@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Pavel Machek <pavel@....cz>,
        Thomas Gleixner <tglx@...utronix.de>, kernel@...cinc.com,
        linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org,
        Prakash Viswalingam <quic_prakashv@...cinc.com>
Subject: Re: [PATCH v4 0/2] Avoid spurious freezer wakeups


* Elliot Berman <quic_eberman@...cinc.com> wrote:

> Hi Peter,
> 
> On 9/26/2023 1:02 PM, Peter Zijlstra wrote:
> > On Tue, Sep 26, 2023 at 04:17:33PM +0000, Carlos Llamas wrote:
> >> On Fri, Sep 08, 2023 at 03:49:14PM -0700, Elliot Berman wrote:
> >>> After commit f5d39b020809 ("freezer,sched: Rewrite core freezer logic"),
> >>> tasks that transition directly from TASK_FREEZABLE to TASK_FROZEN  are
> >>> always woken up on the thaw path. Prior to that commit, tasks could ask
> >>> freezer to consider them "frozen enough" via freezer_do_not_count(). The
> >>> commit replaced freezer_do_not_count() with a TASK_FREEZABLE state which
> >>> allows freezer to immediately mark the task as TASK_FROZEN without
> >>> waking up the task.  This is efficient for the suspend path, but on the
> >>> thaw path, the task is always woken up even if the task didn't need to
> >>> wake up and goes back to its TASK_(UN)INTERRUPTIBLE state. Although
> >>> these tasks are capable of handling of the wakeup, we can observe a
> >>> power/perf impact from the extra wakeup.
> >>
> >> This issue is hurting the performance of our stable 6.1 releases. Does
> >> it make sense to backport these patches into stable branches once they
> >> land in mainline? I would assume we want to fix the perf regression
> >> there too?
> > 
> > Note that these patches are in tip/sched/core, slated for the next merge
> > window.
> 
> Can the changes be scheduled for the next 6.6-rc? I'd like to get the
> changes backported to stable sooner since we observed the regression on
> real systems.

These are pretty risky and go beyond fixes for regressions introduced
recently: the original commit is more than a year old.

But I agree with having the fixes in stable once it hits upstream in the v6.7 
merge window - the difference would only be a couple of days vs. -final.

Thanks,

	Ingo