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

commit 7386cdbf upstream.

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
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/proc/stat.c

@@ -24,11 +24,14 @@
 
 static cputime64_t get_idle_time(int cpu)
 {
-	u64 idle_time = get_cpu_idle_time_us(cpu, NULL);
+	u64 idle_time = -1ULL;
 	cputime64_t idle;
 
+	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 = kstat_cpu(cpu).cpustat.idle;
 		idle = cputime64_add(idle, arch_idle_time(cpu));
 	} else
@@ -39,11 +42,14 @@ static cputime64_t get_idle_time(int cpu)
 
 static cputime64_t get_iowait_time(int cpu)
 {
-	u64 iowait_time = get_cpu_iowait_time_us(cpu, NULL);
+	u64 iowait_time = -1ULL;
 	cputime64_t iowait;
 
+	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 = kstat_cpu(cpu).cpustat.iowait;
 	else
 		iowait = usecs_to_cputime64(iowait_time);