Re: : [PATCH v8 1/2] sched/cache: Reduce the overhead of task_cache_work by only scan the visisted cpus

[Chen Yu <chen.yu@linux.dev>]

On Tue, Jul 28, 2026 at 04:53:59PM +0800, Luo Gengkun wrote:
> 
> On 2026/7/27 9:09, Chen, Yu C wrote:
> > On 7/23/2026 12:04 PM, Luo Gengkun wrote:
> > 
> > [ ... ]
> > 
> > >       guard(raw_spinlock_irqsave)(&rq->cpu_epoch_lock);
> > >       __update_mm_sched(rq, pcpu_sched);
> > > +    /* Skip the rq that has not been hit for a long time */
> > > +    if ((rq->cpu_epoch - pcpu_sched->epoch_last_visit) > llc_epoch_affinity_timeout) {
> > 
> > In v2 there is a check if the cpu has been set before writing:
> > cpumask_test_cpu(cpu_of(rq), &mm->sc_stat.visited_cpus)
> > https://lore.kernel.org/all/20260414150745.225416-1-luogengkun2@huawei.com/
> > do we need to bring that back?
> > 
> I don't think we need it back. Here is why:
> 
> In v2, for_each_cpu was used instead of for_each_cpu_and in the inner loop,
> meaning some CPUs being checked might not have been set. Therefore,
> cpumask_test_cpu was necessary to filter out those cases.
> 
> Now, with for_each_cpu_and(i, sched_domain_span(sd), &mm->sc_stat.visited_cpus),
> we can ensure each scanned CPU is set, so the issue no longer exists.
> Furthermore, the only place where the visited_cpus bits are cleared is
> task_cache_work(), which is only called once per scan period, there is no
> risk of the bit being cleared concurrently mid-loop.
>

Make sense.
 
> However, is there a possibility that the current task_cache_work() execution
> hasn't finished yet when the next scan window arrives? For instance, if the
> current task work is heavily delayed or preempted by unexpected interrupt,
> jiffies could advance past next_scan before the loop completes.
> If we move the `work->next = work;` to the very end of task_cache_work(),
> would that resolve this issue? By doing so, the existing `work->next == work`
> check in task_tick_cache() should fail and no new task work will be submitted.
> 
> Please let me know if I'm missing something.
>

There are two layers of protection: first a cheap timeout gate (time_before)
that skips scanning until the next period, and then a try_cmpxchg that atomically
picks a single winner among the threads that pass the timeout — this actually
guarantees only one scanner per mm at a time, no?

thanks,
Chenyu