linux-kernel - Re: [PATCH v2 09/23] sched/cache: Count tasks prefering destination LLC in a sched group

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Message-ID: <d21dd22e8eab22d13280cd83022ffcf64523a5d3.camel@linux.intel.com>
Date: Wed, 10 Dec 2025 11:00:53 -0800
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Peter Zijlstra <peterz@...radead.org>, "Chen, Yu C" <yu.c.chen@...el.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 09/23] sched/cache: Count tasks prefering destination
 LLC in a sched group

On Wed, 2025-12-10 at 16:16 +0100, Peter Zijlstra wrote:
> On Wed, Dec 10, 2025 at 11:05:33PM +0900, Chen, Yu C wrote:
> > On 12/10/2025 9:52 PM, Peter Zijlstra wrote:
> > > On Wed, Dec 03, 2025 at 03:07:28PM -0800, Tim Chen wrote:
> > > > During LLC load balancing, tabulate the number of tasks on each runqueue
> > > > that prefer the LLC contains the env->dst_cpu in a sched group.
> > > > 
> > > > For example, consider a system with 4 LLC sched groups (LLC0 to LLC3)
> > > > balancing towards LLC3. LLC0 has 3 tasks preferring LLC3, LLC1 has
> > > > 2, and LLC2 has 1. LLC0, having the most tasks preferring LLC3, is
> > > > selected as the busiest source to pick tasks from.
> > > > 
> > > > Within a source LLC, the total number of tasks preferring a destination
> > > > LLC is computed by summing counts across all CPUs in that LLC. For
> > > > instance, if LLC0 has CPU0 with 2 tasks and CPU1 with 1 task preferring
> > > > LLC3, the total for LLC0 is 3.
> > > > 
> > > > These statistics allow the load balancer to choose tasks from source
> > > > sched groups that best match their preferred LLCs.
> > > > 
> > > > Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> > > > ---
> > > > 
> > > > Notes:
> > > >      v1->v2:
> > > >          Convert nr_pref_llc array in sg_lb_stats to a single
> > > >          variable as only the dst LLC stat is needed.
> > > >          (K Prateek Nayak)
> > > > 
> > > >   kernel/sched/fair.c | 12 ++++++++++++
> > > >   1 file changed, 12 insertions(+)
> > > > 
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index b0e87616e377..4d7803f69a74 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -10445,6 +10445,9 @@ struct sg_lb_stats {
> > > >   	unsigned int nr_numa_running;
> > > >   	unsigned int nr_preferred_running;
> > > >   #endif
> > > > +#ifdef CONFIG_SCHED_CACHE
> > > > +	unsigned int nr_pref_llc;
> > > > +#endif
> > > 
> > > At this point I have to note that rq->nr_pref_llc seems like a horrible
> > > misnomer, for it being an array, and not an actual number like the
> > > naming suggests.
> > 
> > In the v2 it seems that rq->nr_pref_llc is not an array anymore, it
> 
> From two patches ago:
> 
> +       unsigned int            *nr_pref_llc;
> 
> Its a pointer of some sort.


Perhaps I should used a different name here when I update this patch
for v2.  

rq->nr_pref_llc[]   is an array as it records the number of tasks preferring each LLC.
However
sgs->nr_pref_llc  is a single number representing the number of tasks
preferring the current domain preferring the destination LLC.

Sorry for using the same name that may have created this confusion.

> 
> 
> > indicates
> > the number of tasks that want to be migrated to the env->dst_cpu (dst_llc),
> > because
> > these tasks' preferred LLC are env->dst_cpu(dst_llc). Maybe renaming it to
> > rq->nr_pref_dst_llc?
> 
> Like I said in:
> 
>   https://lkml.kernel.org/r/20251210125114.GS3707891@noisy.programming.kicks-ass.net
> 
> it might make sense to put it in struct sched_domain instead of struct
> rq, since then you can allocate and swap it right along with the rest of
> the domain tree.

Sent a separate reply to that comment to clarify why I think we need nr_pref_llc[] per
run queue.

Tim