linux-kernel - Re: [PATCH 06/19] sched/fair: Assign preferred LLC ID to processes

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Message-ID: <2e7efcfc-9a70-4e26-9559-e5a367ba8ef7@intel.com>
Date: Thu, 16 Oct 2025 11:13:22 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, "K
 Prateek Nayak" <kprateek.nayak@....com>, "Gautham R . Shenoy"
	<gautham.shenoy@....com>, Vern Hao <vernhao@...cent.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>, 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>, 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 06/19] sched/fair: Assign preferred LLC ID to processes

On 10/15/2025 7:15 PM, Peter Zijlstra wrote:
> On Tue, Oct 14, 2025 at 01:16:16PM +0800, Chen, Yu C wrote:
> 
>> The question becomes: how can we figure out the threads that share
>> data? Can the kernel detect this, or get the hint from user space?
> 
> This needs the PMU, then you can steer using cache-miss ratios. But then
> people will hate us for using counters.
> 
>> Yes, the numa_group in NUMA load balancing indicates
>> that several tasks manipulate the same page, which could be an
>> indicator. Besides, if task A frequently wakes up task B, does it
>> mean A and B have the potential to share data? Furthermore, if
>> task A wakes up B via a pipe, it might also indicate that A has
>> something to share with B. I just wonder if we can introduce a
>> structure to gather this information together.
> 
> The wakeup or pipe relation might be small relative to the working set.
> Consider a sharded in memory database, where the query comes in through
> the pipe/socket/wakeup. This query is small, but then it needs to go
> trawl through its memory to find the answer.
> 
> Something we *could* look at -- later -- is an interface to create
> thread groups, such that userspace that is clever enough can communicate
> this. But then there is the ago old question, will there be sufficient
> users to justify the maintenance of said interface.

OK, that could be an enhancement in the future if we encounter more
use cases.

thanks,
Chenyu