linux-kernel - Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

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Message-ID: <7363e7bd-942e-4e6e-9147-86b620a62120@amd.com>
Date: Wed, 29 Oct 2025 09:30:06 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: "Chen, Yu C" <yu.c.chen@...el.com>
CC: 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>, Tim Chen
	<tim.c.chen@...ux.intel.com>, Peter Zijlstra <peterz@...radead.org>, "Ingo
 Molnar" <mingo@...hat.com>, "Gautham R . Shenoy" <gautham.shenoy@....com>
Subject: Re: [PATCH 07/19] sched/fair: Track LLC-preferred tasks per runqueue

Hello Chenyu,

On 10/28/2025 8:45 PM, Chen, Yu C wrote:
>> Instead of relying on reset_llc_stats() hack, I think a better approach
>> would be to have a "p->se.llc_sched_active" flag similar to how uclamp
>> has "uc_se->active" and we set this in account_llc_enqueue() which will
>> still check for sched_cache_enabled() but account_llc_dequeue() would
>> only check for "p->se.llc_sched_active" to decrement the stats and then
>> unset the flag.
>>
>> That way, we cannot have an imbalanced accounting. Thoughts?
>>
> 
> I suppose what you mean is to avoid the race condition between
> enabling sched_cache and EQ/DE_LLC, similar to uclamp:
> 
>         enqueue(taskA)
>         // sched_cache gets enabled
>         enqueue(taskB)
>         dequeue(taskA)
>         // Must not decrement rq->llc_pref for taskA
>         dequeue(taskB)

Yup! We can have

  enqueue(p)
    account_llc_enqueue(p)
      if (sched_cache_enabled())
        p->se.llc_sched_active = true;
        rq->nr_llc_running += (p->preferred_llc != -1);
        rq->nr_pref_llc_running += (p->preferred_llc == task_llc(p));

    ...

  dequeue(p)
    account_llc_dequeue(p)
      if (p->se.llc_sched_active)
        rq->nr_llc_running -= (p->preferred_llc != -1);
        rq->nr_pref_llc_running -= (p->preferred_llc == task_llc(p));


We can also have a single bit for "llc_sched_active" in the
task_struct next to the "sched_task_hot" bit instead of using the
hole in sched_entity after "sched_delayed"

-- 
Thanks and Regards,
Prateek