Re: [PATCH v3] sched/core: Skip rq->avg_idle update without a valid idle_stamp

[Vincent Guittot]

On Fri, 7 Aug 2026 at 22:47, Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx> wrote:
>
> Commit 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved
> to an idle CPU") moved rq->avg_idle accounting out of the wakeup path and
> into put_prev_task_idle(), so that the idle interval is consumed whenever
> the idle task is switched out.
>
> The wakeup-side accounting that it replaced only updated rq->avg_idle
> when rq->idle_stamp was non-zero. The new helper lost that validity
> check and unconditionally computes:
>
>         rq_clock(rq) - rq->idle_stamp
>
> If rq->idle_stamp is zero, this uses rq_clock(rq) as the sample. That is
> not a valid idle duration and can immediately drive rq->avg_idle to its
> clamp.
>
> This can happen when sched_balance_newidle() returns before setting
> rq->idle_stamp, for example when this_rq->ttwu_pending is set. In that
> case the rq can switch to the idle task with idle_stamp still zero and
> leave idle again when the pending wakeup is processed.
>
> Other paths can also switch to the idle task without setting
> rq->idle_stamp via newidle_balance(), for example find_proxy_task() or
> force-idling.
>
> Restore the idle_stamp validity check in update_rq_avg_idle() and skip
> the rq->avg_idle update when there is no measured idle interval.
>
> Fixes: 4b603f1551a73 ("sched: Update rq->avg_idle when a task is moved to an idle CPU")
> Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
> Acked-by: John Stultz <jstultz@xxxxxxxxxx>
> Signed-off-by: Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> ---
> Temporary tracing under hackbench load confirmed that
> update_rq_avg_idle() can be reached with rq->idle_stamp == 0.
> Hackbench showed no material regression versus v7.2-rc5 mainline.
>
> Related discussion:
>   https://lore.kernel.org/r/20260423023322.1293923-1-firelzrd@xxxxxxxxx
>
> This is a narrower variant of the earlier proposal. It keeps the
> rq->idle_stamp guard in update_rq_avg_idle(), but intentionally does not
> stamp idle entry from set_next_task_idle(), preserving the existing
> newidle accounting model and avoiding force-idle/proxy-exec accounting
> concerns.
> ---
> Changes in v3:
>   - Describe the sched_balance_newidle()/ttwu_pending path as an
>     example of entering idle without a valid rq->idle_stamp.
>   - Drop unlikely() from the idle_stamp check.
>   - Add Acked-by from John Stultz.
>
> Link to v2: https://lore.kernel.org/r/20260806-master-v2-1-e1f3a1a0c903@xxxxxxxxxx
>
> Changes in v2:
>   - Add Reviewed-by from Prateek.
>   - Mention find_proxy_task() and force-idling as examples of paths that
>     can switch to the idle task without a valid rq->idle_stamp.
>   - Cc John Stultz.
>
> Link to v1: https://lore.kernel.org/r/20260728-master-v1-1-f95d9b0147d2@xxxxxxxxxx
> ---
>  kernel/sched/core.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 96226707c2f6135341aa779b8262f113e103d8ad..68fa724cd668fb6c4cead329d05fa95e2f1ea5db 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3732,11 +3732,17 @@ static inline void ttwu_do_wakeup(struct task_struct *p)
>
>  void update_rq_avg_idle(struct rq *rq)
>  {
> -       u64 delta = rq_clock(rq) - rq->idle_stamp;
> -       u64 max = 2*rq->max_idle_balance_cost;
> +       u64 idle_stamp = rq->idle_stamp;
> +       u64 delta, max;
> +
> +       if (!idle_stamp)
> +               return;
> +
> +       delta = rq_clock(rq) - idle_stamp;
>
>         update_avg(&rq->avg_idle, delta);
>
> +       max = 2 * rq->max_idle_balance_cost;
>         if (rq->avg_idle > max)
>                 rq->avg_idle = max;
>         rq->idle_stamp = 0;
>
> ---
> base-commit: 3f008280327ba5ad132965abab0c7846283cef0c
> change-id: 20260728-master-55cd7cc13290
>
> Best regards,
> --
> Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>
>
>