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

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Message-ID: <CAG2KctpO6VKS6GN4QWDji0t92_gNBJ7HjjXrE+6H+RwRXt=iLg@mail.gmail.com>
Date: Mon, 1 Dec 2025 16:23:51 -0800
From: Samuel Wu <wusamuel@...gle.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
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 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
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
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