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/
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
---
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;
--
2.51.0