Re: [PATCH 01/15] sched/idle: Handle offlining first in idle loop

Shrikanth Hegde
2026-02-18T18:23:06Z

On 2/6/26 7:52 PM, Frederic Weisbecker wrote:
> Offline handling happens from within the inner idle loop,
> after the beginning of dyntick cputime accounting, nohz idle
> load balancing and TIF_NEED_RESCHED polling.
>
> This is not necessary and even buggy because:
>
> * There is no dyntick handling to do. And calling tick_nohz_idle_enter()
> messes up with the struct tick_sched reset that was performed on
> tick_sched_timer_dying().
>
> * There is no nohz idle balancing to do.
>
> * Polling on TIF_RESCHED is irrelevant at this stage, there are no more
> tasks allowed to run.
>
> * No need to check if need_resched() before offline handling since
> stop_machine is done and all per-cpu kthread should be done with
> their job.
>
> Therefore move the offline handling at the beginning of the idle loop.
> This will also ease the idle cputime unification later by not elapsing
> idle time while offline through the call to:
>
> tick_nohz_idle_enter() -> tick_nohz_start_idle()
>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

Makes sense to call it outside the loop.
Once you report idle is dead, there is nothing to do that CPU.

Reviewed-by: Shrikanth Hegde<sshegde@linux.ibm.com>

> ---
> kernel/sched/idle.c | 13 ++++++++-----
> 1 file changed, 8 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index c174afe1dd17..51764cbec6f3 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -260,6 +260,14 @@ static void do_idle(void)
> {
> int cpu = smp_processor_id();
>
> + if (cpu_is_offline(cpu)) {
> + local_irq_disable();
> + /* All per-CPU kernel threads should be done by now. */
> + WARN_ON_ONCE(need_resched());
> + cpuhp_report_idle_dead();
> + arch_cpu_idle_dead();
> + }
> +
> /*
> * Check if we need to update blocked load
> */
> @@ -311,11 +319,6 @@ static void do_idle(void)
> */
> local_irq_disable();
>
> - if (cpu_is_offline(cpu)) {
> - cpuhp_report_idle_dead();
> - arch_cpu_idle_dead();
> - }
> -
> arch_cpu_idle_enter();
> rcu_nocb_flush_deferred_wakeup();
>