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s390/idle: Add comment for non obvious code

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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 <frederic@kernel.org>
Closes: https://lore.kernel.org/all/aXEVM-04lj0lntMr@localhost.localdomain/
Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
This commit is contained in:
Heiko Carstens 2026-02-18 15:20:06 +01:00 • committed by Vasily Gorbik
parent dbc0fb3567
commit aefa6ec890
1 changed files with 9 additions and 0 deletions
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9
arch/s390/kernel/idle.c
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@ -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;