linux-kernel - Re: [RFC][PATCH] sched: Cache aware load-balancing

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Message-ID: <20250326104223.GB12071@noisy.programming.kicks-ass.net>
Date: Wed, 26 Mar 2025 11:42:23 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, linux-kernel@...r.kernel.org,
	tim.c.chen@...ux.intel.com, tglx@...utronix.de, len.brown@...el.com,
	gautham.shenoy@....com, mingo@...nel.org, kprateek.nayak@....com,
	yu.chen.surf@...mail.com
Subject: Re: [RFC][PATCH] sched: Cache aware load-balancing

On Wed, Mar 26, 2025 at 11:25:53AM +0100, Peter Zijlstra wrote:
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1286,8 +1286,8 @@ static void task_cache_work(struct callb
>  	struct task_struct *p = current;
>  	struct mm_struct *mm = p->mm;
>  	unsigned long m_a_occ = 0;
> -	int cpu, m_a_cpu = -1;
> -	cpumask_var_t cpus;
> +	int m_a_cpu = -1;
> +	int cpu;
>  
>  	WARN_ON_ONCE(work != &p->cache_work);
>  
> @@ -1296,46 +1296,46 @@ static void task_cache_work(struct callb
>  	if (p->flags & PF_EXITING)
>  		return;
>  
> -	if (!alloc_cpumask_var(&cpus, GFP_KERNEL))
> -		return;
> -
>  	scoped_guard (cpus_read_lock) {
> -		cpumask_copy(cpus, cpu_online_mask);
>  
> -		for_each_cpu(cpu, cpus) {
> -			/* XXX sched_cluster_active */
> -			struct sched_domain *sd = per_cpu(sd_llc, cpu);
> -			unsigned long occ, m_occ = 0, a_occ = 0;
> -			int m_cpu = -1, nr = 0, i;
> +		for_each_online_cpu(cpu) {
> +			struct sched_domain *sd;
> +			struct sched_domain_shared *sds;
> +			unsigned long occ;
> +
> +			for_each_domain(cpu, sd) {
> +				if (!(sd->flags & SD_SHARE_LLC))
> +					break;
>  
> -			for_each_cpu(i, sched_domain_span(sd)) {
> +				sds = sd->shared;
>  				occ = fraction_mm_sched(cpu_rq(i),
>  							per_cpu_ptr(mm->pcpu_sched, i));
> -				a_occ += occ;
> -				if (occ > m_occ) {
> -					m_occ = occ;
> -					m_cpu = i;
> -				}
> -				nr++;
> -				trace_printk("(%d) occ: %ld m_occ: %ld m_cpu: %d nr: %d\n",
> -					     per_cpu(sd_llc_id, i), occ, m_occ, m_cpu, nr);
> -			}
> -
> -			a_occ /= nr;
> -			if (a_occ > m_a_occ) {
> -				m_a_occ = a_occ;
> -				m_a_cpu = m_cpu;
> +				sds->sum_occ += occ + 1;
>  			}
> +		}
>  
> -			trace_printk("(%d) a_occ: %ld m_a_occ: %ld\n",
> -				     per_cpu(sd_llc_id, cpu), a_occ, m_a_occ);
> +		for_each_online_cpu(cpu) {
> +			struct sched_domain *sd;
> +			struct sched_domain_shared *sds;
> +
> +			for_each_domain(cpu, sd) {
> +				if (!(sd->flags & SD_SHARE_LLC))
> +					break;
> +
> +				sds = sd->shared;
> +				if (sds->agg_occ) {
> +					sds->avg_occ = (sds->agg_occ - sd->span_weight) /
> +						       sd->span_weight;
> +					sds->sum_occ = 0;
> +				}

s/agg_occ/sum_occ/g, stupid last minute renames etc.. :-)

>  
> -			for_each_cpu(i, sched_domain_span(sd)) {
> -				/* XXX threshold ? */
> -				per_cpu_ptr(mm->pcpu_sched, i)->occ = a_occ;
> +				if (sd == per_cpu(sd_llc, cpu)) {
> +					if (sds->avg_occ > m_a_occ) {
> +						m_a_occ = sds->avg_occ;
> +						m_a_cpu = cpu;
> +					}
> +				}
>  			}
> -
> -			cpumask_andnot(cpus, cpus, sched_domain_span(sd));
>  		}
>  	}
>  
> @@ -1346,8 +1346,6 @@ static void task_cache_work(struct callb
>  		m_a_cpu = -1;
>  
>  	mm->mm_sched_cpu = m_a_cpu;
> -
> -	free_cpumask_var(cpus);
>  }
>  
>  void init_sched_mm(struct task_struct *p)