linux-kernel - Re: [PATCH] psi: Only collect online cpu time in collect_percpu

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Date:   Thu, 5 Dec 2019 11:33:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     liu.song11@....com.cn
Cc:     fishland@...yun.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, jiang.xuexin@....com.cn
Subject: Re: [PATCH] psi: Only collect online cpu time in collect_percpu_times

On Thu, Dec 05, 2019 at 10:05:59AM +0800, liu.song11@....com.cn wrote:
> >On Mon, Dec 02, 2019 at 09:09:28PM +0800, Liu Song wrote:
> >> From: Liu Song <liu.song11@....com.cn> 
> >>  
> >> Tasks can only run on the online cpu, so only need to
> >> collect the time of the online cpu.
> >>  
> >> Signed-off-by: Liu Song <liu.song11@....com.cn> 
> >> ---
> >>  kernel/sched/psi.c | 2 +-
> >>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>  
> >> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> >> index 7acc632c3b82..605f02facb7b 100644
> >> --- a/kernel/sched/psi.c
> >> +++ b/kernel/sched/psi.c
> >> @@ -316,7 +316,7 @@ static void collect_percpu_times(struct psi_group *group,
> >>       * the sampling period. This eliminates artifacts from uneven
> >>       * loading, or even entirely idle CPUs.
> >>       */
> >> -    for_each_possible_cpu(cpu) {
> >> +    for_each_online_cpu(cpu) {
> >>          u32 times[NR_PSI_STATES];
> >>          u32 nonidle;
> >>          u32 cpu_changed_states;
> >
> >And who collects the deltas that remain after offline?
> 
> Hi,
> After the cpu goes offline, there is no activity on the cpu and no time will be updated.

But there might have been activitiy before it went offline.

> So the value of deltas is 0 and it will not contribute to the total time.

No, the value will not change, but it need not be 0.