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

[Luo Gengkun]

On 2026/8/11 15:46, Chen, Yu C wrote:
> 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:
From a technical perspective, adding a gate here is unnecessary.

The overhead is virtually nonexistent, especially since it resides on a path
already burdened by heavier operations like __update_mm_sched(). If we were
to care about performance and optimization, focusing on __update_mm_sched()
would be far more meaningful than adding checks here.

What do you think?
> 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);

I looked closely at get_scan_cpumasks(). The CPU mask it returns is the
union of node(p->numa_preferred_nid), node(mm->sc_stat.cpu), and node(task_cpu(p)).
Its purpose was only to mitigate sc_stat.cpu bouncing — it did not fully
eliminate it. Relying on visited_cpus alone follows the actual footprint
of where the threads really ran, making it even less prone to bouncing.

Additionally, even if the numa node derived from visited_cpus disagrees with a
given thread's numa_preferred_nid, get_pref_llc() still prevents that
thread from being migrated to that node, so it remains safe either way.

Please let me know if I'm missing something.

thanks,
Gengkun
> thanks,
> Chenyu