Re: [PATCH v2] sched/core: Skip rq->avg_idle update without a valid idle_stamp

On Thu, Aug 6, 2026 at 5:26 PM Shubhang Kaushik (Ampere) <[email protected]> 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 the scheduler switches to the idle task through a
> path that did not set rq->idle_stamp via newidle_balance(), for example
> during 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 <[email protected]>
> Signed-off-by: Shubhang Kaushik (Ampere) <[email protected]>
> ---
> 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/[email protected]
>
> 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 forced/proxy idle accounting
> concerns.

Thanks for sending this out!

Acked-by: John Stultz <[email protected]>