Re: [RESEND PATCH v2] nohz: fix idle ticks in cpu summary line of/proc/stat

From: Srivatsa S. Bhat
Date: Tue Sep 25 2012 - 14:51:16 EST


On 09/10/2012 04:43 PM, Srivatsa S. Bhat wrote:
> From: Michal Hocko <mhocko@xxxxxxx>
>
> 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.
>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxx>
> [srivatsa.bhat@xxxxxxxxxxxxxxxxxx: Rebased to current mainline]
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> ---
>
> Hi Thomas,
>
> This is a resend of the patch posted by Michal at [1]. Martin had explained
> the importance of this patch for fixing the bug for x86 case in [2]. (The s390
> fix is already upstream, commit id cb85a6ed67e9). Could you kindly consider
> taking this fix?
>

Hi Thomas,
Any thoughts on this?

Regards,
Srivatsa S. Bhat

> [1]. http://thread.gmane.org/gmane.linux.kernel/1265374/focus=1266457
> [2]. http://thread.gmane.org/gmane.linux.kernel/1265374/focus=1276336
>
> fs/proc/stat.c | 14 ++++++++++----
> 1 files changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/fs/proc/stat.c b/fs/proc/stat.c
> index 64c3b31..e296572 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);
>

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