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

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Message-ID: <20251210094246.GQ3707891@noisy.programming.kicks-ass.net>
Date: Wed, 10 Dec 2025 10:42:46 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: 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>,
	Chen Yu <yu.c.chen@...el.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 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.


> > > +	pref_llc = p->preferred_llc;
> > > +	if (pref_llc < 0)
> > > +		return;
> > > +
> > > +	rq->nr_llc_running--;
> > > +	rq->nr_pref_llc_running -= (pref_llc == task_llc(p));
> > > +	p->sched_llc_active = false;
> > > +}