[PATCH 01/14] sched/idle: Handle offlining first in idle loop

Frederic Weisbecker
2026-03-31T13:16:40Z

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>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Tested-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 3681b6ad9276..521a076d8845 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();

--
2.53.0