linux-kernel - Re: [PATCH 08/19] sched/fair: Introduce per runqueue task LLC preference counter

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Message-ID: <68a7c8f5-25d8-4cce-92f4-99dd6a50add3@intel.com>
Date: Thu, 23 Oct 2025 14:07:57 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>, Tim Chen
	<tim.c.chen@...ux.intel.com>
CC: Peter Zijlstra <peterz@...radead.org>, 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>, "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>, <haoxing990@...il.com>
Subject: Re: [PATCH 08/19] sched/fair: Introduce per runqueue task LLC
 preference counter

On 10/21/2025 4:28 PM, Madadi Vineeth Reddy wrote:
> On 16/10/25 02:11, Tim Chen wrote:
>> On Wed, 2025-10-15 at 14:21 +0200, Peter Zijlstra wrote:
>>> On Sat, Oct 11, 2025 at 11:24:45AM -0700, Tim Chen wrote:
>>>> Each runqueue is assigned a static array where each element tracks
>>>> the number of tasks preferring a given LLC, indexed from 0 to
>>>> NR_LLCS.
>>>>
> 
> [snip]
> 
>>>
>>>>   
>>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>>> index 3ab64067acc6..b801d32d5fba 100644
>>>> --- a/kernel/sched/sched.h
>>>> +++ b/kernel/sched/sched.h
>>>> @@ -1101,6 +1101,7 @@ struct rq {
>>>>   #ifdef CONFIG_SCHED_CACHE
>>>>   	unsigned int		nr_pref_llc_running;
>>>>   	unsigned int		nr_llc_running;
>>>> +	unsigned int		nr_pref_llc[NR_LLCS];
>>>
>>> Gah, yeah, lets not do this. Just (re)alloc the thing on topology
>>> changes or something.
>>
>> Will have to think about how to keep the tasks' preference
>> consistent with nr_pref_llc with the new array.  Perhaps
>> make it size of NR_CPUS so we will allocate
>> once and don't have to resize and reallocate it, and
>> fill it back up with the right data.
>>
>> Tim
> 
> IIUC, what Peter meant is to dynamically allocate the array size based on
> the actual number of LLCs computed in build_sched_domains() or somesuch
> rather than statically allocating NR_LLCS which is 64 by default.
> 

OK, this might involve dynamic adjustment/data synchronization of the
nr_pref_llc buffer during CPU hotplug, and we'll make some modifications 
to it.

thanks,
Chenyu

> Making it NR_CPUS would be even larger and waste more memory on systems
> with few LLCs.
> 
> Thanks,
> Madadi Vineeth Reddy