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

[Tim Chen]

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>
> 
> [ ... ]
> 
> > 
> > To address the issue of scanning overhead, there is a more targeted 
> > approach: only scanning the CPUs actually accessed by the process, and 
> > evicting these CPUs when they remain unaccessed for a specific period of 
> > time.
> > 
> 
> Thanks for the patch. This approach looks quite sensible to me, and
> 
> [ ... ]
> 
> > 
> >          /*
> >           * The update to mm->sc_stat should not be reordered
> > @@ -1582,6 +1584,7 @@ void account_mm_sched(struct rq *rq, struct 
> > 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

+				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