Re: [Patch v4 16/22] sched/cache: Disable cache aware scheduling for processes with high thread counts

[Tim Chen]

On Thu, 2026-04-09 at 14:43 +0200, Peter Zijlstra wrote:
> On Wed, Apr 01, 2026 at 02:52:28PM -0700, Tim Chen wrote:
> > @@ -1507,7 +1513,8 @@ void account_mm_sched(struct rq *rq, struct task_struct *p, s64 delta_exec)
> >  	 */
> >  	if (time_after(epoch,
> >  		       READ_ONCE(mm->sc_stat.epoch) + EPOCH_LLC_AFFINITY_TIMEOUT) ||
> > -	    get_nr_threads(p) <= 1) {
> > +	    get_nr_threads(p) <= 1 ||
> > +	    exceed_llc_nr(mm, cpu_of(rq))) {
> >  		if (mm->sc_stat.cpu != -1)
> >  			mm->sc_stat.cpu = -1;
> >  	}
> 
> > @@ -1608,6 +1633,13 @@ static void task_cache_work(struct callback_head *work)
> >  	if (p->flags & PF_EXITING)
> >  		return;
> >  
> > +	if (get_nr_threads(p) <= 1) {
> > +		if (mm->sc_stat.cpu != -1)
> > +			mm->sc_stat.cpu = -1;
> > +
> > +		return;
> > +	}
> > +
> >  	if (!zalloc_cpumask_var(&cpus, GFP_KERNEL))
> >  		return;
> >  
> 
> > @@ -10105,6 +10144,13 @@ static enum llc_mig can_migrate_llc_task(int src_cpu, int dst_cpu,
> >  	if (cpu < 0 || cpus_share_cache(src_cpu, dst_cpu))
> >  		return mig_unrestricted;
> >  
> > +	/* skip cache aware load balance for single/too many threads */
> > +	if (get_nr_threads(p) <= 1 || exceed_llc_nr(mm, dst_cpu)) {
> > +		if (mm->sc_stat.cpu != -1)
> > +			mm->sc_stat.cpu = -1;
> > +		return mig_unrestricted;
> > +	}
> > +
> >  	if (cpus_share_cache(dst_cpu, cpu))
> >  		to_pref = true;
> >  	else if (cpus_share_cache(src_cpu, cpu))
> 
> This is what that made me notice that weird get_nr_threads() <= 1 thing.
> 

With a single thread, we already have code like scanning in the same LLC
in wake up, and migrate_degrades_locality() to keep it where its cache
is hot.  Adding a preferred_llc seems unnecessary.

Tim