Re: [Patch v4 02/22] sched/cache: Limit the scan number of CPUs when calculating task occupancy

[Chen, Yu C]

On 4/11/2026 1:12 AM, Tim Chen wrote:
> On Fri, 2026-04-10 at 15:29 +0800, Chen, Yu C wrote:
> > Hi Gengkun,
> >
> > On 4/9/2026 9:17 PM, Luo Gengkun wrote:
> > > On 2026/4/2 5:52, Tim Chen wrote:
> > > > From: Chen Yu <yu.c.chen@xxxxxxxxx>
> > > task_struct *p, s64 delta_exec)
> > >                   pcpu_sched->runtime += delta_exec;
> > >                   rq->cpu_runtime += delta_exec;
> > >                   epoch = rq->cpu_epoch;
> > > +               cpumask_set_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus);
> >
> > I would refer a check before writing to avoid c2c overhead:
> > 	if (!cpumask_test_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus))
> > 		cpumask_set_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus);
>
>
> I think a similar check is also needed for clearing of visited CPU.  It is possible that
> the visited_cpus mask has already been cleared long time back.  Change

Yes, I agree, although we reach here when scanning the visited_cpus
snapshot, there could be a race condition when we are in this loop
but the CPU in visited_cpu has already been cleared.

thanks,
Chenyu

> +				if (llc_epoch_visited_timeout && (rq->cpu_epoch - pcpu_sched->epoch) >
> +				    llc_epoch_visited_timeout) {
> +					cpumask_clear_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus);
> +					continue;
> +				}
>
> to
>
> +				if (llc_epoch_visited_timeout &&
> +				    cpumask_test_cpu(cpu_of(rq), &mm->sc_stat.visitied_cpus) &&
> +				    (rq->cpu_epoch - pcpu_sched->epoch) > llc_epoch_visited_timeout) {
> +					cpumask_clear_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus);
> +					continue;
> +				}
>
> Thanks.
>
> Tim