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

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Message-ID: <Zxls4HqdkV_yBYxZ@tiehlicka>
Date: Wed, 23 Oct 2024 23:38:40 +0200
From: Michal Hocko <mhocko@...e.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Dongjoo Seo <dongjoo.linux.dev@...il.com>, 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 Wed 23-10-24 13:41:21, Andrew Morton wrote:
> On Wed, 23 Oct 2024 20:03:24 +0200 Michal Hocko <mhocko@...e.com> wrote:
> 
> > On Wed 23-10-24 10:50:37, Dongjoo Seo wrote:
> > > This patch corrects this issue by:
> > 
> > What is this issue? Please describe the problem first,
> 
> Actually, relocating the author's second-last paragraph to
> top-of-changelog produced a decent result ;)
> 
> > ideally describe
> > the NUMA topology, workload and what kind of misaccounting happens
> > (expected values vs. really reported values).
> 
> I think the changelog covered this adequately?
> 
> So with these changelog alterations I've queued this for 6.12-rcX with
> a cc:stable.  As far as I can tell this has been there since 2018.
> 
> : In the case of memoryless node, when a process prefers a node with no
> : memory(e.g., because it is running on a CPU local to that node), the
> : kernel treats a nearby node with memory as the preferred node.  As a
> : result, such allocations do not increment the numa_foreign counter on the
> : memoryless node, leading to skewed NUMA_HIT, NUMA_MISS, and NUMA_FOREIGN
> : stats for the nearest node.

I am sorry but I still do not underastand that. Especially when I do
look at the patch which would like to treat cpuless nodes specially.
Let me be more specific. Why ...

> -     if (zone_to_nid(z) != numa_node_id())
> +     if (zone_to_nid(z) != numa_node_id() || z_is_cpuless)
>               local_stat = NUMA_OTHER;
>
> -     if (zone_to_nid(z) == zone_to_nid(preferred_zone))
> +     if (zone_to_nid(z) == zone_to_nid(preferred_zone) && !z_is_cpuless)
>               __count_numa_events(z, NUMA_HIT, nr_account);
>       else {
>               __count_numa_events(z, NUMA_MISS, nr_account);
> -             __count_numa_events(preferred_zone, NUMA_FOREIGN, nr_account);
> +             if (!pref_is_cpuless)
> +                     __count_numa_events(preferred_zone, NUMA_FOREIGN, nr_account);

... a (well?) established meaning of local needs to be changed? Why
prefrerred policy should have a different meaning for cpuless policies?
Those are memory specific rather than cpu specific right?

Quite some quiestions to have it in linux-next IMHO....
-- 
Michal Hocko
SUSE Labs