nohz: 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 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>

1 files changed, 10 insertions, 4 deletions

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 64c3b3172367..e296572c73ed 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];
         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);