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

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Message-ID: <253f00cc-2bea-43b6-aca3-96105014c4be@intel.com>
Date: Wed, 29 Oct 2025 21:19:54 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Aaron Lu <ziqianlu@...edance.com>
CC: Tim Chen <tim.c.chen@...ux.intel.com>, 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>, 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

On 10/29/2025 5:51 PM, Aaron Lu wrote:
> On Mon, Oct 27, 2025 at 10:00:52AM +0800, Chen, Yu C wrote:
>> 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.
> 
> Looks like can_migrate_llc() doesn't care an exact cpu but any cpu in the
> same LLC should do, so either the candidate group's first cpu or any
> other cpus in that group should make no difference.
> 

Yes, actually the cache aware is based on LLC rather than CPU. It was
a historic reason that the original proposal was based on CPU in the
wakeup path.

> It might be more intuitive to prototype can_migrate_llc() with sd_shared
> as param than using cpu, just a thought.

I understand your concern,  will think about this.

thanks,
Chenyu