linux-kernel - Re: [PATCH v2 06/23] sched/cache: Track LLC-preferred tasks per runqueue

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Message-ID: <b757128b-abb3-4257-b75f-31a9b1c2263e@intel.com>
Date: Tue, 16 Dec 2025 08:20:33 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>, Tim Chen
	<tim.c.chen@...ux.intel.com>
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>, 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>, Aaron Lu <ziqianlu@...edance.com>, Tim Chen
	<tim.c.chen@...el.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 06/23] sched/cache: Track LLC-preferred tasks per
 runqueue

On 12/10/2025 5:42 PM, Peter Zijlstra wrote:
> On Tue, Dec 09, 2025 at 02:55:21PM -0800, Tim Chen wrote:
> 
>>>> +static void account_llc_dequeue(struct rq *rq, struct task_struct *p)
>>>> +{
>>>> +	int pref_llc;
>>>> +
>>>> +	/*
>>>> +	 * Borrow the uc_se->active from uclamp_rq_inc_id(),
>>>> +	 * uclamp_rq_dec_id() to avoid the unbalanced calculation
>>>> +	 * of rq statistics.
>>>> +	 */
>>>> +	if (unlikely(!p->sched_llc_active))
>>>> +		return;
>>>
>>> Another very confusing comment; what? Also, can you please explain (in
>>> the new comment) how we get here without having llc_active set?
>>
>> The comment meant to say that we are using a similar mechanism as
>> accounting done in uc_se->active from uclamp_rq_inc_id(). I agree that
>> it confuses more than making things clearer.
>>
>> How about the following comment to make things clearer:
>>
>> 	/*
>> 	 * Cache aware scheduling was active when the task was enqueued.
>> 	 * Admin has disabled cache aware scheduling before task was dequeued
>> 	 * but the accounting has to be kept straight in case cache aware scheduling
>> 	 * is re-enabled.
>> 	 */
> 
> Is having that sched_cache_enabled() test worth it?
> account_numa_{en,de}queue() don't seem to have any of this.
> 
> 

OK, I think we can remove the sched_cache_enabled() check and
make the account_llc_{en,de}queue() depending on CONFIG_SCHED_CACHE,
so the sched_llc_active can be removed.

thanks,
Chenyu