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 <[email protected]>
Signed-off-by: Michal Hocko <[email protected]>
Reviewed-by: Srivatsa S. Bhat <[email protected]>
Signed-off-by: Srivatsa S. Bhat <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Thomas Gleixner <[email protected]>

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 64c3b3172367abbd1c1464b9372f33ee4efa7acb..e296572c73ed651dd7d81cc33eb3dd1b64de5d0d 100644 (file)
--- 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);