[ 3.5.yuz extended stable ] Patch "nohz: Fix idle ticks in cpu summary line of /proc/stat" has been added to staging queue

Herton Ronaldo Krzesinski herton.krzesinski at canonical.com
Thu Nov 22 04:45:23 UTC 2012

This is a note to let you know that I have just added a patch titled

    nohz: Fix idle ticks in cpu summary line of /proc/stat

to the linux-3.5.y-queue branch of the 3.5.yuz extended stable tree 
which can be found at:


If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.5.yuz tree, see



>From 378cbb4a911105fbfcfa5560ee54ae4f14d72141 Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko at suse.cz>
Date: Wed, 10 Oct 2012 11:51:09 +0530
Subject: [PATCH] nohz: Fix idle ticks in cpu summary line of /proc/stat

commit 7386cdbf2f57ea8cff3c9fde93f206e58b9fe13f upstream.

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 we should make sure that the cpu is online when calling
get_cpu_idle_time_us and get_cpu_iowait_time_us.

[Srivatsa: Rebased to current mainline]

Reported-by: Martin Schwidefsky <schwidefsky at de.ibm.com>
Signed-off-by: Michal Hocko <mhocko at suse.cz>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat at linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20121010061820.8999.57245.stgit@srivatsabhat.in.ibm.com
Cc: deepthi at linux.vnet.ibm.com
Signed-off-by: Thomas Gleixner <tglx at linutronix.de>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski at canonical.com>
 fs/proc/stat.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 64c3b31..e296572 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -45,10 +45,13 @@ static cputime64_t get_iowait_time(int cpu)

 static u64 get_idle_time(int cpu)
-	u64 idle, idle_time = get_cpu_idle_time_us(cpu, NULL);
+	u64 idle, idle_time = -1ULL;
+	if (cpu_online(cpu))
+		idle_time = get_cpu_idle_time_us(cpu, NULL);

 	if (idle_time == -1ULL)
-		/* !NO_HZ so we can rely on cpustat.idle */
+		/* !NO_HZ or cpu offline so we can rely on cpustat.idle */
 		idle = kcpustat_cpu(cpu).cpustat[CPUTIME_IDLE];
 		idle = usecs_to_cputime64(idle_time);
@@ -58,10 +61,13 @@ static u64 get_idle_time(int cpu)

 static u64 get_iowait_time(int cpu)
-	u64 iowait, iowait_time = get_cpu_iowait_time_us(cpu, NULL);
+	u64 iowait, iowait_time = -1ULL;
+	if (cpu_online(cpu))
+		iowait_time = get_cpu_iowait_time_us(cpu, NULL);

 	if (iowait_time == -1ULL)
-		/* !NO_HZ so we can rely on cpustat.iowait */
+		/* !NO_HZ or cpu offline so we can rely on cpustat.iowait */
 		iowait = kcpustat_cpu(cpu).cpustat[CPUTIME_IOWAIT];
 		iowait = usecs_to_cputime64(iowait_time);

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