From aefa6ec890f0d39a89fc80e95908ec1996337eee Mon Sep 17 00:00:00 2001 From: Heiko Carstens Date: Wed, 18 Feb 2026 15:20:06 +0100 Subject: [PATCH] s390/idle: Add comment for non obvious code Add a comment to update_timer_idle() which describes why wall time (not steal time) is added to steal_timer. This is not obvious and was reported by Frederic Weisbecker. Reported-by: Frederic Weisbecker Closes: https://lore.kernel.org/all/aXEVM-04lj0lntMr@localhost.localdomain/ Reviewed-by: Sven Schnelle Signed-off-by: Heiko Carstens Signed-off-by: Vasily Gorbik --- arch/s390/kernel/idle.c | 9 +++++++++ 1 file changed, 9 insertions(+) diff --git a/arch/s390/kernel/idle.c b/arch/s390/kernel/idle.c index 0f9e53f0a068..4e09f126d4fc 100644 --- a/arch/s390/kernel/idle.c +++ b/arch/s390/kernel/idle.c @@ -34,6 +34,15 @@ void update_timer_idle(void) this_cpu_add(mt_cycles[i], cycles_new[i] - idle->mt_cycles_enter[i]); } + /* + * This is a bit subtle: Forward last_update_clock so it excludes idle + * time. For correct steal time calculation in do_account_vtime() add + * passed wall time before idle_enter to steal_timer: + * During the passed wall time before idle_enter CPU time may have + * been accounted to system, hardirq, softirq, etc. lowcore fields. + * The accounted CPU times will be subtracted again from steal_timer + * when accumulated steal time is calculated in do_account_vtime(). + */ lc->steal_timer += idle->clock_idle_enter - lc->last_update_clock; lc->last_update_clock = lc->int_clock;