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

[Michal Hocko]

On Sun 11-03-12 18:26:21, Martin Schwidefsky wrote:
> 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 get_cpu_idle_time_us and get_cpu_iowait_time_us
> should check for offline cpus.

Goot catch. But I think that the following fix should be better because
it doesn't change the semantic of the function. What do you think?
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