linux-kernel - Re: [PATCH] sched/fair: Fix pelt lost idle time detection

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] [day] [month] [year] [list]
Message-ID: <CAKfTPtDLUHqChcFwWfPJwLqdkU1jANEYVfHhS6fuD5f0Gy4KRg@mail.gmail.com>
Date: Fri, 5 Dec 2025 16:08:37 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Samuel Wu <wusamuel@...gle.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org, 
	Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH] sched/fair: Fix pelt lost idle time detection

On Tue, 2 Dec 2025 at 01:24, Samuel Wu <wusamuel@...gle.com> wrote:
>
> On Wed, Oct 8, 2025 at 6:12 AM Vincent Guittot
> <vincent.guittot@...aro.org> wrote:
> >
> > The check for some lost idle pelt time should be always done when
> > pick_next_task_fair() fails to pick a task and not only when we call it
> > from the fair fast-path.
> >
> > The case happens when the last running task on rq is a RT or DL task. When
> > the latter goes to sleep and the /Sum of util_sum of the rq is at the max
> > value, we don't account the lost of idle time whereas we should.
> >
> > Fixes: 67692435c411 ("sched: Rework pick_next_task() slow-path")
> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> > ---
> >
> > I Noticed this while reviewing [1]
> >
> > [1] https://lore.kernel.org/all/20251006105453.648473106@infradead.org/
> >
> >  kernel/sched/fair.c | 26 +++++++++++++-------------
> >  1 file changed, 13 insertions(+), 13 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index b3be1e2749ce..dd0ea01af730 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -8920,21 +8920,21 @@ pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf
> >         return p;
> >
> >  idle:
> > -       if (!rf)
> > -               return NULL;
> > -
> > -       new_tasks = sched_balance_newidle(rq, rf);
> > +       if (rf) {
> > +               new_tasks = sched_balance_newidle(rq, rf);
> >
> > -       /*
> > -        * Because sched_balance_newidle() releases (and re-acquires) rq->lock, it is
> > -        * possible for any higher priority task to appear. In that case we
> > -        * must re-start the pick_next_entity() loop.
> > -        */
> > -       if (new_tasks < 0)
> > -               return RETRY_TASK;
> > +               /*
> > +                * Because sched_balance_newidle() releases (and re-acquires)
> > +                * rq->lock, it is possible for any higher priority task to
> > +                * appear. In that case we must re-start the pick_next_entity()
> > +                * loop.
> > +                */
> > +               if (new_tasks < 0)
> > +                       return RETRY_TASK;
> >
> > -       if (new_tasks > 0)
> > -               goto again;
> > +               if (new_tasks > 0)
> > +                       goto again;
> > +       }
> >
> >         /*
> >          * rq is about to be idle, check if we need to update the
> > --
> > 2.43.0
> >
>
> Hi all,
>
> I am seeing a power regression I've observed with this patch. This

The problem is that this patch is about fixing a wrong load tracking
which can be underestimated on systems that become loaded.

> test was performed on Pixel 6 running android-mainline (6.18.0-rc7
> based); all scheduling vendor hooks are disabled, and I'm not seeing
> any obvious sched code differences compared to the vanilla upstream
> kernel. I am still actively working to see if I can find a simpler
> sequence to reproduce this on mainline Linux.
>
> The Wattson tool is reporting an increased average power (~30-40%)
> with the patch vs baseline (patch reverted). This regression

For a use case in particular ?

> correlates with two other metrics:
> 1. Increased residency at higher CPU frequencies
> 2. A significant increase in sugov invocations (at least 10x)
>
> Data in the tables below are collected from a 10s run of a bouncing
> ball animation, with and without the patch.
> +-----------------------------------+--------------+-------------------+
> |                                           | with patch |  without patch |
> +-----------------------------------+-------------+--------------------+
> | sugov invocation rate (Hz) |       133.5 |                   3.7 |
> +-----------------------------------+-------------+--------------------+
>
> +--------------+----------------------+----------------------+
> |                   |         with patch: |    without patch: |
> | Freq (kHz) | time spent (ms) |  time spent (ms) |
> +--------------+----------------------+----------------------+
> |     738000 |                   4869 |                  9869 |
> |   1803000 |                   2936 |                      68 |
> |   1598000 |                   1072 |                        0 |
> |   1704000 |                     674 |                        0 |
> |              ... |                        ... |                       ... |
> +--------------+----------------------+---------------------+
>
> Thanks!
> Sam