linux-kernel - Re: [PATCH v5 2/2] sched/numa: add statistics of numa balance task

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Message-ID: <6e52340a-cabf-48db-b9f1-8300c1c13997@intel.com>
Date: Thu, 19 Jun 2025 21:03:55 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Michal Koutný <mkoutny@...e.com>
CC: Shakeel Butt <shakeel.butt@...ux.dev>, <peterz@...radead.org>,
	<akpm@...ux-foundation.org>, <mingo@...hat.com>, <tj@...nel.org>,
	<hannes@...xchg.org>, <corbet@....net>, <mgorman@...e.de>,
	<mhocko@...nel.org>, <muchun.song@...ux.dev>, <roman.gushchin@...ux.dev>,
	<tim.c.chen@...el.com>, <aubrey.li@...el.com>, <libo.chen@...cle.com>,
	<kprateek.nayak@....com>, <vineethr@...ux.ibm.com>, <venkat88@...ux.ibm.com>,
	<ayushjai@....com>, <cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<yu.chen.surf@...mail.com>
Subject: Re: [PATCH v5 2/2] sched/numa: add statistics of numa balance task

On 6/17/2025 5:30 PM, Michal Koutný wrote:
> On Tue, Jun 03, 2025 at 10:46:06PM +0800, "Chen, Yu C" <yu.c.chen@...el.com> wrote:
>> My understanding is that the "misplaced" container is not strictly tied
>> to set_mempolicy or cpuset configuration, but is mainly caused by the
>> scheduler's generic load balancer.
> 
> You are convincing me with this that, cpu.stat fits the concept better.
> Doesn't that sound like that to you?
> 
>> Regarding the threaded subtrees mode, I was previously unfamiliar with
>> it and have been trying to understand it better.
> 
> No problem.
> 
>> If I understand correctly, if threads within a single process are
>> placed in different cgroups via cpuset, we might need to scan
>> /proc/<PID>/sched to collect NUMA task migration/swap statistics.
> 
> The premise of your series was that you didn't want to do that :-)
> 
>> I agree with your prior point that NUMA balancing task activity is not
>> directly
>> associated with either the Memory controller or the CPU controller. Although
>> showing this data in cpu.stat might seem more appropriate, we expose it in
>> memory.stat due to the following trade-offs(or as an exception for
>> NUMA balancing):
>>
>> 1.It aligns with existing NUMA-related metrics already present in
>> memory.stat.
> 
> That one I'd buy into. OTOH, I'd hope this could be overcome with
> documentation.
> 
>> 2.It simplifies code implementation.
> 
> I'd say that only applies when accepting memory.stat as the better
> place. I think the appropriately matching API should be picked first and
> implementation is only secondary to that.

Thanks for this guidance.

>  From your reasoning above, I think that the concept is closer to be in
> cpu.stat ¯\_(ツ)_/¯
> 
OK. Since this change has already been addressed in upstream kernel,
I can update the numa_task_migrated/numa_task_swapped fields in
Documentation/admin-guide/cgroup-v2.rst to mention that, these
activities are not memory related but put here because they are
closer to numa balance's page statistics.
Or do you want me to submit a patch to move the items from
memory.stat to cpu.stat?

thanks,
Chenyu



> Michal