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

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Message-ID: <5a45424b-dc25-42b7-899c-26a9f09d0124@intel.com>
Date: Thu, 29 May 2025 13:04:49 +0800
From: "Chen, Yu C" <yu.c.chen@...el.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
CC: <peterz@...radead.org>, <akpm@...ux-foundation.org>, <mkoutny@...e.com>,
	<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 5/28/2025 1:48 AM, Shakeel Butt wrote:
> On Sun, May 25, 2025 at 08:35:24PM +0800, Chen, Yu C wrote:
>> On 5/25/2025 1:32 AM, Shakeel Butt wrote:
> [...]
>>> can you please give an end-to-end> flow/story of all these events
>> happening on a timeline.
>>>
>>
>> Yes, sure, let me have a try.
>>
>> The goal of NUMA balancing is to co-locate a task and its
>> memory pages on the same NUMA node. There are two strategies:
>> migrate the pages to the task's node, or migrate the task to
>> the node where its pages reside.
>>
>> Suppose a task p1 is running on Node 0, but its pages are
>> located on Node 1. NUMA page fault statistics for p1 reveal
>> its "page footprint" across nodes. If NUMA balancing detects
>> that most of p1's pages are on Node 1:
>>
>> 1.Page Migration Attempt:
>> The Numa balance first tries to migrate p1's pages to Node 0.
>> The numa_page_migrate counter increments.
>>
>> 2.Task Migration Strategies:
>> After the page migration finishes, Numa balance checks every
>> 1 second to see if p1 can be migrated to Node 1.
>>
>> Case 2.1: Idle CPU Available
>> If Node 1 has an idle CPU, p1 is directly scheduled there. This event is
>> logged as numa_task_migrated.
>> Case 2.2: No Idle CPU (Task Swap)
>> If all CPUs on Node1 are busy, direct migration could cause CPU contention
>> or load imbalance. Instead:
>> The Numa balance selects a candidate task p2 on Node 1 that prefers
>> Node 0 (e.g., due to its own page footprint).
>> p1 and p2 are swapped. This cross-node swap is recorded as
>> numa_task_swapped.
>>
> 
> Thanks for the explanation, this is really helpful and I would like this
> to be included in the commit message.
> 

OK, just sent out a v6 with the commit message enhanced.

Thanks,
Chenyu