linux-kernel - Re: [PATCH 08/19] sched/fair: Introduce per runqueue task LLC preference counter

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Message-ID: <20251015122125.GU3289052@noisy.programming.kicks-ass.net>
Date: Wed, 15 Oct 2025 14:21:25 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tim Chen <tim.c.chen@...ux.intel.com>
Cc: Ingo Molnar <mingo@...hat.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	"Gautham R . Shenoy" <gautham.shenoy@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Madadi Vineeth Reddy <vineethr@...ux.ibm.com>,
	Hillf Danton <hdanton@...a.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Jianyong Wu <jianyong.wu@...look.com>,
	Yangyu Chen <cyy@...self.name>,
	Tingyin Duan <tingyin.duan@...il.com>,
	Vern Hao <vernhao@...cent.com>, Len Brown <len.brown@...el.com>,
	Aubrey Li <aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>,
	Chen Yu <yu.chen.surf@...il.com>, Chen Yu <yu.c.chen@...el.com>,
	Libo Chen <libo.chen@...cle.com>,
	Adam Li <adamli@...amperecomputing.com>,
	Tim Chen <tim.c.chen@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/19] sched/fair: Introduce per runqueue task LLC
 preference counter

On Sat, Oct 11, 2025 at 11:24:45AM -0700, Tim Chen wrote:
> Each runqueue is assigned a static array where each element tracks
> the number of tasks preferring a given LLC, indexed from 0 to
> NR_LLCS.
> 
> For example, rq->nr_pref_llc[3] = 2 signifies that there are 2 tasks on
> this runqueue which prefer to run within LLC3 (indexed from 0 to NR_LLCS
> 
> The load balancer can use this information to identify busy runqueues
> and migrate tasks to their preferred LLC domains.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  kernel/sched/fair.c  | 35 +++++++++++++++++++++++++++++++++++
>  kernel/sched/sched.h |  1 +
>  2 files changed, 36 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fd315937c0cf..b7a68fe7601b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1235,22 +1235,51 @@ static inline int llc_idx(int cpu)
>  	return per_cpu(sd_llc_idx, cpu);
>  }
>  
> +static inline int pref_llc_idx(struct task_struct *p)
> +{
> +	return llc_idx(p->preferred_llc);
> +}
> +
>  static void account_llc_enqueue(struct rq *rq, struct task_struct *p)
>  {
> +	int pref_llc;
> +
>  	if (!sched_cache_enabled())
>  		return;
>  
>  	rq->nr_llc_running += (p->preferred_llc != -1);
>  	rq->nr_pref_llc_running += (p->preferred_llc == task_llc(p));
> +
> +	if (p->preferred_llc < 0)
> +		return;
> +
> +	pref_llc = pref_llc_idx(p);
> +	if (pref_llc < 0)
> +		return;
> +
> +	++rq->nr_pref_llc[pref_llc];
>  }
>  
>  static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
>  {
> +	int pref_llc;
> +
>  	if (!sched_cache_enabled())
>  		return;
>  
>  	rq->nr_llc_running -= (p->preferred_llc != -1);
>  	rq->nr_pref_llc_running -= (p->preferred_llc == task_llc(p));
> +
> +	if (p->preferred_llc < 0)
> +		return;
> +
> +	pref_llc = pref_llc_idx(p);
> +	if (pref_llc < 0)
> +		return;
> +
> +	/* avoid negative counter */
> +	if (rq->nr_pref_llc[pref_llc] > 0)
> +		--rq->nr_pref_llc[pref_llc];

How!? Also, please use post increment/decrement operators.

>  }
>  
>  void mm_init_sched(struct mm_struct *mm, struct mm_sched __percpu *_pcpu_sched)
> @@ -1524,10 +1553,16 @@ void init_sched_mm(struct task_struct *p)
>  
>  void reset_llc_stats(struct rq *rq)
>  {
> +	int i = 0;
> +
>  	if (!sched_cache_enabled())
>  		return;
>  
>  	rq->nr_llc_running = 0;
> +
> +	for (i = 0; i < max_llcs; ++i)
> +		rq->nr_pref_llc[i] = 0;
> +
>  	rq->nr_pref_llc_running = 0;
>  }

Still don't understand why this thing exists..

>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3ab64067acc6..b801d32d5fba 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1101,6 +1101,7 @@ struct rq {
>  #ifdef CONFIG_SCHED_CACHE
>  	unsigned int		nr_pref_llc_running;
>  	unsigned int		nr_llc_running;
> +	unsigned int		nr_pref_llc[NR_LLCS];

Gah, yeah, lets not do this. Just (re)alloc the thing on topology
changes or something.