[PATCH v2] sched/cputime: Don't warn when the tick period starts after idle exit

[Stian Halseth <stian@itx.no>]

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 <pmenzel@molgen.mpg.de>
Closes: https://lore.kernel.org/all/bfe9b14a-6e65-41c5-b638-26f6e033788b@molgen.mpg.de/
Reported-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/all/auTCQauaU733PY5z@gmail.com/
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/CAHk-=winC2u2D1FAX5grfq9F=bxmARxhuiQ4MvHAiJFu3iF6gw@mail.gmail.com/
Signed-off-by: Stian Halseth <stian@itx.no>
---
v2: rewrite the commit message, which named the wrong variable and
had the wrong clocksource. The code is unchanged from v1. Add the
reports from Breno and Linus.
v1: https://lore.kernel.org/all/20261025100202.2584654-1-stian@itx.no/

 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 9e0fb8633d823..17f3cbd5d403a 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;
-- 
2.43.0