[PATCH v3 2/2] sched: consider missed ticks when updating global cpu load

[byungchul.park]

From: Byungchul Park <byungchul.park@xxxxxxx>

in hrtimer_interrupt(), the first tick_program_event() can be failed
because the next timer could be already expired due to,
(see the comment in hrtimer_interrupt())

- tracing
- long lasting callbacks
- being scheduled away when running in a VM

in the case that the first tick_program_event() is failed, the second
tick_program_event() set the expired time to more than one tick later.
then next tick can happen after more than one tick, even though tick is
not stopped by e.g. NOHZ.

when the next tick occurs, update_process_times() -> scheduler_tick()
-> update_cpu_load_active() is performed, assuming the distance between
last tick and current tick is 1 tick! it's wrong in this case. thus,
this abnormal case should be considered in update_cpu_load_active().

Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
---
 kernel/sched/fair.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9e76871..14b9757 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4388,12 +4388,15 @@ void update_cpu_load_nohz(void)
  */
 void update_cpu_load_active(struct rq *this_rq)
 {
+	unsigned long curr_jiffies = READ_ONCE(jiffies);
+	unsigned long pending_updates;
 	unsigned long load = weighted_cpuload(cpu_of(this_rq));
 	/*
 	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
 	 */
-	this_rq->last_load_update_tick = jiffies;
-	__update_cpu_load(this_rq, load, 1, 1);
+	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
+	this_rq->last_load_update_tick = curr_jiffies;
+	__update_cpu_load(this_rq, load, pending_updates, 1);
 }
 
 /*
-- 
1.7.9.5

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