Re: [PATCH 04/19] sched/fair: Introduce a static key to enable cache aware only for multi LLCs

[Chen, Yu C]

On 10/28/2025 7:36 AM, Tim Chen wrote:
> On Mon, 2025-10-27 at 20:56 +0800, Chen, Yu C wrote:
> > Hi Prateek,
> >
> > On 10/27/2025 1:42 PM, K Prateek Nayak wrote:
> > > Hello Tim,
> > >
> > > On 10/11/2025 11:54 PM, Tim Chen wrote:
> > > > @@ -2530,10 +2531,12 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
> > > >    				 * between LLCs and memory channels.
> > > >    				 */
> > > >    				nr_llcs = sd->span_weight / child->span_weight;
> > > > -				if (nr_llcs == 1)
> > > > +				if (nr_llcs == 1) {
> > > >    					imb = sd->span_weight >> 3;
> > > > -				else
> > > > +				} else {
> > > >    					imb = nr_llcs;
> > > > +					has_multi_llcs = true;
> > >
> > > One caution: this will not hold if all the CPUs aren't online during boot.
> > > One case I can think of is when the kernel is booted with "maxcpus" cmdline
> > > and CPUs are hotplugged later.
> > >
> > > Unfortunately, I don't think we even have the raw topology data from the
> > > arch/ side under such scenario to accurately make a call if the system
> > > contains single or multiple LLC :(
> > >
> > > I'm not sure if it is feasible but assuming the task_work() cannot run if
> > > &sched_cache_allowed is false, can the fist instance of the task work for
> > > sched_cache do the necessary setup?
> >
> > build_sched_domains() might get invoked to rebuild the corresponding sched
> > domains during CPU hotplug via cpuset subsystem. So if the CPU gets online
> > after bootup, we still have the chance to detect multiple LLCs I suppose?
>
> The case Pratek brought up of adding CPUs and enabling SCHED_CACHE
> should be covered.
>
> The trickier case is if we disable SCHED_CACHE when CPUs are
> offlined and multi_cpus becomes false.  We'll need to clear out rq->nr_pref_llcs
> data and tasks' preferred LLC would need to be cleared.  Or else the accounting
> could be skewed we bring CPU online later and again re-enable SCHED_CACHE.

To safely keep the data consistent, do we need to add hook when 
SCHED_CACHE is
disabled at runtime, to clear all the stale data? In that way, another
race condition between SCHED_CACHE enabling vs EQ/DQ might also be covered.

thanks,
Chenyu
> So far we haven't done that when we disable SCHED_CACHE from an enabled state.