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

[Dongjoo Seo <dongjoo.linux.dev@gmail.com>]

On Thu, Oct 24, 2024 at 10:24:56AM +0200, Michal Hocko wrote:
> On Wed 23-10-24 21:54:37, Dongjoo Seo wrote:
> > 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?
> 
> I still do not understand the actual problem. CPU-less nodes are nothing
> really special. They just never have local allocations for obvious
> reasons. NUMA_HIT which your patch is special casing has a very well
> defined meaning and that is that the memory allocated matches the
> preferred node. That doesn't have any notion of CPU at all. Say somebody
> explicitly requests to allocate from a CPU less node. Why should you
> consider thiat as NUMA_OTHER just because that node has no CPUs? That
> just seems completely wrong.

Thank you for your feedback. After reviewing ur reply and [1], I realize 
my misunderstanding of numa_* stats. I mistakenly assumed node referred to
CPU locality. The current logic is indeed memory-centric and operates 
correctly as it is. 
I appreciate the clarification, and I now understand that no changes are 
needed to special case CPU-less nodes in this context. 

Thanks again for pointing this out.

[1] https://docs.kernel.org/admin-guide/numastat.html

> -- 
> Michal Hocko
> SUSE Labs