linux-kernel - Re: [PATCH] mm/page_alloc: fix NUMA stats update for cpu-less nodes

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Message-ID: <ZxnTDYiQWU45eX-Y@eqbm-smc020.dtc.local>
Date: Wed, 23 Oct 2024 21:54:37 -0700
From: Dongjoo Seo <dongjoo.linux.dev@...il.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, dave@...olabs.net,
	dan.j.williams@...el.com, nifan@...look.com,
	a.manzanares@...sung.com
Subject: Re: [PATCH] mm/page_alloc: fix NUMA stats update for cpu-less nodes

On Thu, Oct 24, 2024 at 12:23:56AM +0200, Michal Hocko wrote:
> On Wed 23-10-24 15:15:20, Dongjoo Seo wrote:
> > Hi Andrew, Michal,
> > 
> > Thanks for the feedback.
> > 
> > The issue is that CPU-less nodes can lead to incorrect NUMA stats.
> > For example, NUMA_HIT may incorrectly increase for CPU-less nodes
> > because the current logic doesn't account for whether a node has CPUs.
> 
> Define incorrect
> 
> Current semantic doesn't really care about cpu less NUMA nodes because
> current means whatever is required AFIU. This is certainly a long term

I agree that, in the long term, special logging for preferred_zone 
and a separate counter might be necessary for CPU-less nodes.

> semantic. Why does this need to change and why it makes sense to 
> pre-existing users?

This patch doesn't change existing logic; the additional logic only 
applies when a CPU-less node is present, so there shouldn't be 
concerns for pre-existing users. Currently, the NUMA stats for 
configurations with CPU-less nodes are incorrect, as allocations
are not properly accounted for.

I believe this approach improves logging accuracy with minimal impact
on the memory allocation path, but I'm open to alternative solutions.
This isn't the only way to address the issue—any suggestions?

> 
> -- 
> Michal Hocko
> SUSE Labs