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

[Peter Zijlstra]

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.