From b327e63683a2b894b500027375342c25222b58ef Mon Sep 17 00:00:00 2001 From: Stian Halseth Date: Sat, 31 Oct 2026 09:14:07 +0100 Subject: [PATCH] sched/cputime: Don't warn when the tick period starts after idle exit kcpustat_dyntick_stop() warns if idle ends before the tick period that the next tick will account has started. That happens on x86 in a CPU's first idle period with the tick stopped, right after the switch from the jiffies clocksource to tsc-early. On the jiffies clocksource ktime only moves at the tick. When the clocksource switches some time d after a tick, that d is lost: ktime continues from the value of the last tick. The next tick comes TICK_NSEC after the last one, but ktime has only moved TICK_NSEC - d, while tick_periodic() still adds a full TICK_NSEC to tick_next_period, which is now d ahead of ktime. When NOHZ starts, tick_init_jiffy_update() rounds it up to a multiple of TICK_NSEC and the sched timer is armed there, up to nearly two ticks after now. A tick stop saves that as ts->last_tick, and an idle exit before now reaches ts->last_tick - TICK_NSEC sees the tick period in the future. Nothing is accounted twice then. The first tick covers a period that starts after the idle exit, and the code already handles it. Drop the warning. Fixes: 96a494c9cfd8 ("sched/cputime: Don't account idle time twice after dyntick-idle") Reported-by: Paul Menzel Reported-by: Breno Leitao Reported-by: Linus Torvalds Reviewed-by: Kayra Cizmeci Reviewed-by: Frederic Weisbecker Signed-off-by: Stian Halseth Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/all/auTCQauaU733PY5z@gmail.com/ Link: https://lore.kernel.org/all/CAHk-=winC2u2D1FAX5grfq9F=bxmARxhuiQ4MvHAiJFu3iF6gw@mail.gmail.com/ Link: https://patch.msgid.link/20261031081408.3378884-1-stian@itx.no Closes: https://lore.kernel.org/all/bfe9b14a-6e65-41c5-b638-26f6e033788b@molgen.mpg.de/ -- kernel/sched/cputime.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) --- kernel/sched/cputime.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index 9e0fb8633d82..17f3cbd5d403 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -478,7 +478,12 @@ void kcpustat_dyntick_stop(u64 now, u64 tick_start) kc->idle_tick_overlap = 0; } tick_start = max(tick_start, kc->idle_dyntick_entry); - if (!WARN_ON_ONCE(now < tick_start)) + /* + * After a switch from the jiffies clocksource the tick can be + * armed more than a tick ahead, so the period may not have + * started yet. + */ + if (now > tick_start) kc->idle_tick_overlap += now - tick_start; kcpustat_idle_stop(kc, now); kc->idle_dyntick = false;