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

["Shubhang Kaushik (Ampere)" <sh@gentwo.org>]

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 <kprateek.nayak@amd.com>
Signed-off-by: Shubhang Kaushik (Ampere) <sh@gentwo.org>
---
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@gmail.com

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.
---
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@gentwo.org
---
 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..d8c9a80ffa83ac680bb479f67357ac1ec4a6d55e 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 (unlikely(!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: c0a27675eaf08255017b3cabc28c99c0cd71f468
change-id: 20260728-master-55cd7cc13290

Best regards,
-- 
Shubhang Kaushik (Ampere) <sh@gentwo.org>