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

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Message-ID: <23a9460132e9221bdfe91c334d4850e15ac6178c.camel@linux.intel.com>
Date: Tue, 28 Oct 2025 10:06:28 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: K Prateek Nayak <kprateek.nayak@....com>, Peter Zijlstra	
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, "Gautham R .
 Shenoy"	 <gautham.shenoy@....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>, Chen Yu	
 <yu.c.chen@...el.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 07/19] sched/fair: Track LLC-preferred tasks per runqueue

On Mon, 2025-10-27 at 11:34 +0530, K Prateek Nayak wrote:
> Hello Tim,
> 
> On 10/11/2025 11:54 PM, Tim Chen wrote:
> > @@ -3999,6 +4038,7 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  		struct rq *rq = rq_of(cfs_rq);
> >  
> >  		account_numa_enqueue(rq, task_of(se));
> > +		account_llc_enqueue(rq, task_of(se));
> >  		list_add(&se->group_node, &rq->cfs_tasks);
> >  	}
> >  	cfs_rq->nr_queued++;
> > @@ -4010,9 +4050,14 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
> >  	update_load_sub(&cfs_rq->load, se->load.weight);
> >  	if (entity_is_task(se)) {
> >  		account_numa_dequeue(rq_of(cfs_rq), task_of(se));
> > +		account_llc_dequeue(rq_of(cfs_rq), task_of(se));
> >  		list_del_init(&se->group_node);
> >  	}
> >  	cfs_rq->nr_queued--;
> > +
> > +	/* safeguard to clear the cache aware data */
> > +	if (!parent_entity(se) && !cfs_rq->nr_queued)
> > +		reset_llc_stats(rq_of(cfs_rq));
> 
> 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?

With our current accounting method, we should not have imbalanced
accounting even if you turn on sched_cache after the
scheduler has started running (see my reply to Chen Yu's follow up).
That reset_llc_stats() hack
should not be needed as Peter pointed out.

We will change that check to warning under debug option.

Tim

> 
> >  }
> >