Re: [PATCH v9 1/2] sched/cache: Reduce the overhead of task_cache_work by only scan the visisted cpus

[Chen, Yu C]

Hi Gengkun,
On 8/11/2026 10:27 AM, Luo Gengkun wrote:

I did a further study on this and have two minor questions:

> > @@ -1711,6 +1722,9 @@ 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;
> > +        pcpu_sched->epoch_last_visit = epoch;
> > +        if (!cpumask_test_cpu(cpu_of(rq), mm->sc_stat.visited_cpus))
> > +            cpumask_set_cpu(cpu_of(rq), mm->sc_stat.visited_cpus);

visited_cpus bits are only cleared inside fraction_mm_sched(), which is
reachable in task_cache_work() - but that loop is skipped when 
invalid_llc_nr()
returns true for any single-threaded process. As a result, single-threaded
processes keep setting bits in account_mm_sched() without using them.
Maybe a gate would be useful:

if (get_nr_threads(p) > 1 &&
    !cpumask_test_cpu(cpu_of(rq), mm->sc_stat.visited_cpus))
	cpumask_set_cpu(cpu_of(rq), mm->sc_stat.visited_cpus);


[ ... ]

> > @@ -1866,7 +1835,18 @@ static void task_cache_work(struct 
> > callback_head *work)
> >       scoped_guard (cpus_read_lock) {
> >           guard(rcu)();
> > -        get_scan_cpumasks(cpus, p);

I'm thinking of if this could bring cross-node bouncing. Is it doable
to honor the result from NUMA preference:
	get_scan_cpumasks(cpus, p);
	cpumask_and(cpus, cpus, mm->sc_stat.visited_cpus);

thanks,
Chenyu