Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

[Chen, Yu C]

On 10/27/2025 2:04 PM, K Prateek Nayak wrote:
> Hello Tim,
>
> On 10/11/2025 11:54 PM, Tim Chen wrote:
> > @@ -3999,6 +4038,7 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >   		struct rq *rq = rq_of(cfs_rq);
> >   
> >   		account_numa_enqueue(rq, task_of(se));
> > +		account_llc_enqueue(rq, task_of(se));
> >   		list_add(&se->group_node, &rq->cfs_tasks);
> >   	}
> >   	cfs_rq->nr_queued++;
> > @@ -4010,9 +4050,14 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >   	update_load_sub(&cfs_rq->load, se->load.weight);
> >   	if (entity_is_task(se)) {
> >   		account_numa_dequeue(rq_of(cfs_rq), task_of(se));
> > +		account_llc_dequeue(rq_of(cfs_rq), task_of(se));
> >   		list_del_init(&se->group_node);
> >   	}
> >   	cfs_rq->nr_queued--;
> > +
> > +	/* safeguard to clear the cache aware data */
> > +	if (!parent_entity(se) && !cfs_rq->nr_queued)
> > +		reset_llc_stats(rq_of(cfs_rq));
>
> Instead of relying on reset_llc_stats() hack, I think a better approach
> would be to have a "p->se.llc_sched_active" flag similar to how uclamp
> has "uc_se->active" and we set this in account_llc_enqueue() which will
> still check for sched_cache_enabled() but account_llc_dequeue() would
> only check for "p->se.llc_sched_active" to decrement the stats and then
> unset the flag.
>
> That way, we cannot have an imbalanced accounting. Thoughts?

I suppose what you mean is to avoid the race condition between
enabling sched_cache and EQ/DE_LLC, similar to uclamp:

        enqueue(taskA)
        // sched_cache gets enabled
        enqueue(taskB)
        dequeue(taskA)
        // Must not decrement rq->llc_pref for taskA
        dequeue(taskB)

We'll think more about this.

thanks,
Chenyu