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

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Message-ID: <094de5bd4f1b5cd4552ae024f9254df26c9e47be.camel@linux.intel.com>
Date: Tue, 28 Oct 2025 08:46:06 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: "Chen, Yu C" <yu.c.chen@...el.com>, K Prateek Nayak
 <kprateek.nayak@....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, 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

On Tue, 2025-10-28 at 23:15 +0800, Chen, Yu C wrote:
> On 10/27/2025 2:04 PM, 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?
> > 
> 
> 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

For this case, task A is already on rq when sched cache get
enabled. But task A's preferred_llc is still -1. 

If we dequeue it while its preferred_llc is still -1, it won't
affect rq->llc_pref.

If we change its preferred_llc to llc_i before we dequeue it,
then rq->llc_pref[llc_i] will be incremented first.

Then when we dequeue task A, we will decrement it. We are
still accounting rq->llc_pref[llc_i] correctly with current
code.

The trickier case is if we need to dynamically resize
rq->llc_pref[]. We need to make sure that we lock the rq
to prevent enqueue/dequeue, switch it to a larger size
rq->llc_pref[], copy the old data over, then switch over
to the larger sized rq->llc_pref[] and unlock rq to keep
the accounting straight.

Tim 

>          dequeue(taskB)
> 
> We'll think more about this.




> 
> thanks,
> Chenyu