Re: [PATCH v7 linux 1/2] sched/cache: Reduce the overhead of task_cache_work by only scan the visisted cpus

[Tim Chen]

On Tue, 2026-07-21 at 10:53 +0800, Luo Gengkun wrote:
> 
> 
snip

> > > -static unsigned long fraction_mm_sched(struct rq *rq,
> > > -				       struct sched_cache_time *pcpu_sched)
> > > +static unsigned long fraction_mm_sched(int cpu,
> > > +				       struct mm_struct *mm)
> > >   {
> > > +	struct sched_cache_time *pcpu_sched =
> > > +		per_cpu_ptr(mm->sc_stat.pcpu_sched, cpu);
> > > +	struct rq *rq = cpu_rq(cpu);
> > > +
> > >   	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) {
> > > +		cpumask_clear_cpu(cpu, mm->sc_stat.visited_cpus);
> > > +		return 0;
> > > +	}
> > > +
> > >   	/*
> > >   	 * Runtime is a geometric series (r=0.5) and as such will sum to twice
> > >   	 * the accumulation period, this means the multiplcation here should
> > > @@ -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;
> > 
> > We need to make sure that the epoch_last_visit update is seen by the cpu
> > running task_cache_work(), before it attempts to do the epoch comparison
> > and clear the cpu, and causing inconsistency in the visited_cpus.
> > 
> > For example
> > 
> > CPU A (e.g. doing LLC/affinity selection, reading remote pcpu_sched)      CPU B (= `cpu`, running account_mm_sched() locally)
> > --------------------------------------------------------------------      -----------------------------------------------------
> > read pcpu_sched->epoch_last_visit  (stale, old value)
> >    -> looks like it timed out
> >                                                                              pcpu_sched->epoch_last_visit = epoch   (fresh visit!)
> >                                                                              cpumask_set_cpu(cpu, visited_cpus)     (correctly marks it visited)
> > cpumask_clear_cpu(cpu, visited_cpus)   <-- wipes out the fresh set!
> > 
> I don't think such a race condition will occur, because both the read and clear
> operations in fraction_mm_sched() are protected by rq->cpu_epoch_lock, just like
> account_mm_sched(). Please let me know if I'm missing something.
> 

Yes, the epoch lock should prevent the above race from happening.
Missed that on my end.

> 
Thanks.

Tim