linux-kernel - [RESEND PATCH] sched: consider missed ticks when updating global cpu load

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Date:	Fri, 25 Sep 2015 17:52:37 +0900
From:	<byungchul.park@....com>
To:	mingo@...nel.org, peterz@...radead.org
CC:	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	tglx@...utronix.de, Byungchul Park <byungchul.park@....com>
Subject: [RESEND PATCH] sched: consider missed ticks when updating global cpu load

From: Byungchul Park <byungchul.park@....com>

hello,

i have already sent this patch about 1 month ago.
(see https://lkml.org/lkml/2015/8/13/160)

now, i am resending the same patch with adding some additional commit 
message.

thank you,
byungchul

----->8-----
>From 8ece9a0482e74a39cd2e9165bf8eec1d04665fa9 Mon Sep 17 00:00:00 2001
From: Byungchul Park <byungchul.park@....com>
Date: Fri, 25 Sep 2015 17:10:10 +0900
Subject: [RESEND PATCH] sched: consider missed ticks when updating global cpu
 load

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@....com>
---
 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 4d5f97b..829282f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4356,12 +4356,15 @@ 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);
+	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);
 }
 
 /*
-- 
1.7.9.5

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