[PATCH] 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 current tick
period has started. It fired once during boot on a Dell XPS 13, in
CPU 0's first idle period, right after the switch from refined-jiffies
to tsc-early.

That can happen. The clocksource switch continues ktime from the last
jiffies update, so ktime falls behind the tick by up to a tick, and the
sched timer that NOHZ arms next on tick_next_period can expire more
than a tick after now. In a KVM guest it was 1.5 ticks ahead after a
switch from jiffies to hpet. An idle period that stops the tick and
exits before it fires then sees ts->last_tick - TICK_NSEC after now.

The tick restart keeps ts->last_tick in that case, so the first tick
covers a period after the idle exit and nothing overlaps, which the
code already handles. 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/
Signed-off-by: Stian Halseth <stian@itx.no>
---
I could not hit the window in a KVM guest, but instrumented, the guest
showed the sched timer armed 5.9 ms after now at HZ=250 right after a
jiffies -> hpet switch, and tick stops saving a ts->last_tick more than
a tick ahead. The fix boots without warnings there.

 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