From c6dcd97c8be75f052a1ca52cf79b03e7292962f1 Mon Sep 17 00:00:00 2001 From: "Shubhang Kaushik (Ampere)" Date: Fri, 7 Aug 2026 13:38:52 -0700 Subject: [PATCH] sched/core: Skip rq->avg_idle update without a valid idle_stamp 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") Signed-off-by: Shubhang Kaushik (Ampere) Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: K Prateek Nayak Reviewed-by: Vincent Guittot Acked-by: John Stultz Link: https://patch.msgid.link/20260807-master-v3-1-c328354efed3@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 f78275192036..74724501c623 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3742,11 +3742,17 @@ 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;