linux-kernel - Re: [PATCH 10/19] sched/fair: Prioritize tasks preferring destination LLC during balancing

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Message-ID: <cc8284e8-c8f2-4f2c-bcab-4920b80a5a87@intel.com>
Date: Mon, 27 Oct 2025 10:00:52 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Aaron Lu <ziqianlu@...edance.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>, 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>, 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 10/19] sched/fair: Prioritize tasks preferring destination
 LLC during balancing

Hi Aaron,

On 10/24/2025 5:32 PM, Aaron Lu wrote:
> Hi Tim,
> 
> On Sat, Oct 11, 2025 at 11:24:47AM -0700, Tim Chen wrote:
>> @@ -10849,11 +10849,45 @@ static void record_sg_llc_stats(struct lb_env *env,
>>   	if (unlikely(READ_ONCE(sd_share->capacity) != sgs->group_capacity))
>>   		WRITE_ONCE(sd_share->capacity, sgs->group_capacity);
>>   }
>> +
>> +/*
>> + * Do LLC balance on sched group that contains LLC, and have tasks preferring
>> + * to run on LLC in idle dst_cpu.
>> + */
>> +static inline bool llc_balance(struct lb_env *env, struct sg_lb_stats *sgs,
>> +			       struct sched_group *group)
>> +{
>> +	struct sched_domain *child = env->sd->child;
>> +	int llc;
>> +
>> +	if (!sched_cache_enabled())
>> +		return false;
>> +
>> +	if (env->sd->flags & SD_SHARE_LLC)
>> +		return false;
>> +
>> +	/* only care about task migration among LLCs */
>> +	if (child && !(child->flags & SD_SHARE_LLC))
>> +		return false;
>> +
>> +	llc = llc_idx(env->dst_cpu);
>> +	if (sgs->nr_pref_llc[llc] > 0 &&
>> +	    can_migrate_llc(env->src_cpu, env->dst_cpu, 0, true) == mig_llc)
> 
> llc_balance() is called from update_sg_lb_stats() and at that time,
> env->src_cpu is not determined yet so should not be used here?
> 

You are right, I think we should check the candidate group's first
CPU rather than the env->src_cpu. Will fix it in the next version.
Thanks a lot!

chenyu
>> +		return true;
>> +
>> +	return false;
>> +}