Commit 7386cdbf authored Oct 10, 2012 by Michal Hocko Committed by Thomas Gleixner Oct 10, 2012

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



Git commit 09a1d34f "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@de.ibm.com>
Signed-off-by: Michal Hocko <mhocko@suse.cz>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/r/20121010061820.8999.57245.stgit@srivatsabhat.in.ibm.com


Cc: deepthi@linux.vnet.ibm.com
Cc: stable@vger.kernel.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent f784e8a7

fs/proc/stat.c

+10 −4

Original line number	Diff line number	Diff line
		@@ -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];
		else
		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];
		else
		iowait = usecs_to_cputime64(iowait_time);