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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Sat, May 24, 2025 at 2:07 AM Chen, Yu C <yu.c.chen@intel.com> wrote:
>
> Hi Shakeel,
>
> On 5/24/2025 7:42 AM, Shakeel Butt wrote:
> > On Fri, May 23, 2025 at 08:51:15PM +0800, Chen Yu wrote:
> >> On systems with NUMA balancing enabled, it has been found
> >> that tracking task activities resulting from NUMA balancing
> >> is beneficial. NUMA balancing employs two mechanisms for task
> >> migration: one is to migrate a task to an idle CPU within its
> >> preferred node, and the other is to swap tasks located on
> >> different nodes when they are on each other's preferred nodes.
> >>
> >> The kernel already provides NUMA page migration statistics in
> >> /sys/fs/cgroup/mytest/memory.stat and /proc/{PID}/sched. However,
> >> it lacks statistics regarding task migration and swapping.
> >> Therefore, relevant counts for task migration and swapping should
> >> be added.
> >>
> >> The following two new fields:
> >>
> >> numa_task_migrated
> >> numa_task_swapped
> >>
> >> will be shown in /sys/fs/cgroup/{GROUP}/memory.stat, /proc/{PID}/sched
> >> and /proc/vmstat
> >
> > Hmm these are scheduler events, how are these relevant to memory cgroup
> > or vmstat?
> > Any reason to not expose these in cpu.stat?
> >
>
> I understand that in theory they are scheduling activities.
> The reason for including these statistics here was mainly that
> I assumed there is a close relationship between page migration
> and task migration in Numa Balance. Specifically, task migration
> is triggered when page migration fails.
> Placing these statistics closer to the existing Numa Balance page
> statistics in /sys/fs/cgroup/{GROUP}/memory.stat and /proc/vmstat
> may help users query relevant data from a single file, avoiding
> the need to search through scattered files.
> Notably, these events are associated with a task’s working set
> (footprint) rather than pure CPU cycles IMO. I took a look at
> the cpu_cfs_stat_show() for cpu.stat, it seems that a lot of
> code is needed if we want to expose them in cpu.stat, while
> reusing existing interface of count_memcg_event_mm() is simpler.

Let me address two of your points first:

(1) cpu.stat currently contains cpu cycles stats. I don't see an issue
adding these new events in it as you can see memory.stat exposes stats
and events as well.

(2) You can still use count_memcg_event_mm() and related infra while
exposing the stats/events in cpu.stat.

Now your point on having related stats within a single interface is
more convincing. Let me ask you couple of simple questions:

I am not well versed with numa migration, can you expand a bit more on
these two events (numa_task_migrated & numa_task_swapped)? How are
these related to numa memory migration? You mentioned these events
happen on page migration failure, can you please give an end-to-end
flow/story of all these events happening on a timeline.

Beside that, do you think there might be some other scheduling events
(maybe unrelated to numa balancing) which might be suitable for
memory.stat? Basically I am trying to find if having sched events in
memory.stat be an exception for numa balancing or more general.

thanks,
Shakeel