linux-kernel - [PATCH] fix idle ticks in cpu summary line of /proc/stat

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Date:	Sun, 11 Mar 2012 18:26:21 -0400
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Michal Hocko <mhocko@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: [PATCH] fix idle ticks in cpu summary line of /proc/stat

Git commit 09a1d34f8535ecf9 "nohz: Make idle/iowait counter update conditional"
introduced a bug in regard to cpu hotplug. The effect is that the number
of idle ticks in the cpu summary line in /proc/stat is still counting
ticks for offline cpus. 

Reproduction is easy, just start a workload that keeps all cpus busy,
switch off one or more cpus and then watch the idle field in top.
On a dual-core with one cpu 100% busy and one offline cpu you will get
something like this:

%Cpu(s): 48.7 us,  1.3 sy,  0.0 ni, 50.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st

The problem is that an offline cpu still has ts->idle_active == 1.
To fix this get_cpu_idle_time_us and get_cpu_iowait_time_us
should check for offline cpus.

Cc: Michal Hocko <mhocko@...e.cz>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel <linux-kernel@...r.kernel.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>
---
 kernel/time/tick-sched.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -213,7 +213,7 @@ u64 get_cpu_idle_time_us(int cpu, u64 *l
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 	ktime_t now, idle;

-	if (!tick_nohz_enabled)
+	if (!tick_nohz_enabled || !cpu_online(cpu))
 		return -1;

 	now = ktime_get();
@@ -254,7 +254,7 @@ u64 get_cpu_iowait_time_us(int cpu, u64
 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
 	ktime_t now, iowait;

-	if (!tick_nohz_enabled)
+	if (!tick_nohz_enabled || !cpu_online(cpu))
 		return -1;

 	now = ktime_get();

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