linux-kernel - Re: [PATCH v2 17/23] sched/cache: Record the number of active threads per process for cache-aware scheduling

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Message-Id: <20251217094053.GA2073381@bytedance.com>
Date: Wed, 17 Dec 2025 17:40:53 +0800
From: "Aaron Lu" <ziqianlu@...edance.com>
To: "Chen Yu" <yu.c.chen@...el.com>, "Tim Chen" <tim.c.chen@...ux.intel.com>
Cc: "Peter Zijlstra" <peterz@...radead.org>, 
	"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>, "Vern Hao" <haoxing990@...il.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>, 
	"Adam Li" <adamli@...amperecomputing.com>, 
	"Tim Chen" <tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 17/23] sched/cache: Record the number of active threads per process for cache-aware scheduling

On Wed, Dec 03, 2025 at 03:07:36PM -0800, Tim Chen wrote:
> @@ -1501,6 +1507,7 @@ static void __no_profile task_cache_work(struct callback_head *work)
>  		mm->mm_sched_cpu = m_a_cpu;
>  	}
>  
> +	update_avg(&mm->nr_running_avg, nr_running);

update_avg() doesn't appear to deal with small numbers well and can have
an error as large as 7, e.g. when nr_running < 8, nr_running_avg will
always be 0 and when nr_running >= 8 && < 16, nr_running_avg will be
1 - 8, etc.

AMD Genoa has 8 cores per LLC and this will break exceed_llc_nr() there.

>  	free_cpumask_var(cpus);
>  }