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: <20251219033204.GB2598944@bytedance.com>
Date: Fri, 19 Dec 2025 11:32:04 +0800
From: "Aaron Lu" <ziqianlu@...edance.com>
To: "Chen,  Yu C" <yu.c.chen@...el.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>, 
	"Tim Chen" <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v2 17/23] sched/cache: Record the number of active threads per process for cache-aware scheduling

On Wed, Dec 17, 2025 at 08:51:50PM +0800, Chen, Yu C wrote:
> On 12/17/2025 5:40 PM, Aaron Lu wrote:
> > 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.
> > 
> 
> Ah, you are right, thanks for pointing this out, dividing by 8 would make
> convergence slow for small LLC system. Maybe consider the number of Cores

Not just slow but the error is too large for a small LLC.

> in the LLC, the smaller the number is, the more we should honor the diff
> between two invoking of update_avg()?
> 
> static inline void sched_cache_update_avg(u64 *avg, u64 sample)
> {
> 	s64 diff = sample - *avg;
> 	u32 divisor = clamp_t(u32, nr_cores_llc/4, 2, 8);
> 
> 	*avg += diff / divisor;
> }
> 
> For <=8 cores per LLC, the divisor is 2,
> for 16 cores per LLC, the divisor is 4,
> for >=32 cores per LLC, the divisor is 8

Yeah I guess it works. The error can be as large as 'divisor - 1' but
since this avg is an estimate, it may be OK.