Re: [tip:timers/core] proc: stats: Use arch_idle_time for idle andiowait times if available

From: Andrew Morton
Date: Fri Mar 30 2012 - 18:55:25 EST

On Fri, 30 Mar 2012 06:58:25 -0700
tip-bot for Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:

> Commit-ID: cb85a6ed67e979c59a29b7b4e8217e755b951cf4
> Gitweb:
> Author: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> AuthorDate: Fri, 30 Mar 2012 12:23:08 +0200
> Committer: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> CommitDate: Fri, 30 Mar 2012 15:43:33 +0200
> proc: stats: Use arch_idle_time for idle and iowait times if available
> Git commit a25cac5198d4ff28 "proc: Consider NO_HZ when printing idle and
> iowait times" changes the code for /proc/stat to use get_cpu_idle_time_us
> and get_cpu_iowait_time_us if the system is running with nohz enabled.
> For architectures which define arch_idle_time (currently s390 only)
> this is a change for the worse. The result of arch_idle_time is supposed
> to be the exact sleep time of the target cpu and should be used instead
> of the value kept by the scheduler.

So it appears that this patch is a superset of "nohz: fix idle ticks in
cpu summary line of /proc/stat" (below), yes?

> Signed-off-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Reviewed-by: Michal Hocko <mhocko@xxxxxxx>
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Link:
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

No cc:stable? Both 09a1d34f8535ecf9 and a25cac5198d date from
September '11 and 09a1d34f8535ecf9 (at least) was a regression.

From: Michal Hocko <mhocko@xxxxxxx>
Subject: nohz: fix idle ticks in cpu summary line of /proc/stat

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.

Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
Reported-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [3.2.x]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>

fs/proc/stat.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)

diff -puN fs/proc/stat.c~nohz-fix-idle-ticks-in-cpu-summary-line-of-proc-stat fs/proc/stat.c
--- a/fs/proc/stat.c~nohz-fix-idle-ticks-in-cpu-summary-line-of-proc-stat
+++ a/fs/proc/stat.c
@@ -24,10 +24,13 @@

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 += arch_idle_time(cpu);
} else
@@ -38,10 +41,13 @@ 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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