linux-kernel - Re: [RFC PATCH v4 06/28] sched: Save the per LLC utilization for better cache aware scheduling

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Message-ID: <c7af901f-e2d5-4544-8acc-600645655b5c@intel.com>
Date: Tue, 30 Sep 2025 12:34:01 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
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>, Libo Chen <libo.chen@...cle.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>, Tim Chen <tim.c.chen@...ux.intel.com>, Aubrey Li
	<aubrey.li@...el.com>, Zhao Liu <zhao1.liu@...el.com>, Chen Yu
	<yu.chen.surf@...il.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v4 06/28] sched: Save the per LLC utilization for
 better cache aware scheduling

On 9/29/2025 10:09 PM, Peter Zijlstra wrote:
> On Sat, Aug 09, 2025 at 01:02:54PM +0800, Chen Yu wrote:
> 
>> +#ifdef CONFIG_SCHED_CACHE
>> +/*
>> + * Save this sched group's statistic for later use:
>> + * The task wakeup and load balance can make better
>> + * decision based on these statistics.
>> + */
>> +static void update_sg_if_llc(struct lb_env *env, struct sg_lb_stats *sgs,
>> +			     struct sched_group *group)
>> +{
>> +	/* Find the sched domain that spans this group. */
>> +	struct sched_domain *sd = env->sd->child;
>> +	struct sched_domain_shared *sd_share;
>> +
>> +	if (!sched_feat(SCHED_CACHE) || env->idle == CPU_NEWLY_IDLE)
>> +		return;
>> +
>> +	/* only care the sched domain that spans 1 LLC */
>> +	if (!sd || !(sd->flags & SD_SHARE_LLC) ||
>> +	    !sd->parent || (sd->parent->flags & SD_SHARE_LLC))
>> +		return;
> 
> Did you want to write:
> 
> 	if (sd != per_cpu(sd_llc))
> 		return;
> 
> Or something?
> 

Ah right, will do it.

>> +	sd_share = rcu_dereference(per_cpu(sd_llc_shared,
>> +				  cpumask_first(sched_group_span(group))));
>> +	if (!sd_share)
>> +		return;
>> +
>> +	if (likely(READ_ONCE(sd_share->util_avg) != sgs->group_util))
>> +		WRITE_ONCE(sd_share->util_avg, sgs->group_util);
> 
> If you expect it to be different, does that whole load and compare still
> matter?
> 

OK, will write to it directly.

thanks,
Chenyu>> +}