Re: [RFC 0/1] Try to add memory allocation info for cgroup oom kill

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Aug 21, 2025 at 12:53 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> On Thu, Aug 21, 2025 at 12:18:00PM -0700, Yueyang Pan wrote:
> > On Thu, Aug 21, 2025 at 11:35:19AM -0700, Shakeel Butt wrote:
> > > On Thu, Aug 14, 2025 at 10:11:56AM -0700, Yueyang Pan wrote:
> > > > Right now in the oom_kill_process if the oom is because of the cgroup
> > > > limit, we won't get memory allocation infomation. In some cases, we
> > > > can have a large cgroup workload running which dominates the machine.
> > > > The reason using cgroup is to leave some resource for system. When this
> > > > cgroup is killed, we would also like to have some memory allocation
> > > > information for the whole server as well. This is reason behind this
> > > > mini change. Is it an acceptable thing to do? Will it be too much
> > > > information for people? I am happy with any suggestions!
> > >
> > > For a single patch, it is better to have all the context in the patch
> > > and there is no need for cover letter.
> >
> > Thanks for your suggestion Shakeel! I will change this in the next version.
> >
> > >
> > > What exact information you want on the memcg oom that will be helpful
> > > for the users in general? You mentioned memory allocation information,
> > > can you please elaborate a bit more.
> > >
> >
> > As in my reply to Suren, I was thinking the system-wide memory usage info
> > provided by show_free_pages and memory allocation profiling info can help
> > us debug cgoom by comparing them with historical data. What is your take on
> > this?
> >
>
> I am not really sure about show_free_areas(). More specifically how the
> historical data diff will be useful for a memcg oom. If you have a
> concrete example, please give one. For memory allocation profiling, is
> it possible to filter for the given memcg? Do we save memcg information
> in the memory allocation profiling?

Actually I was thinking about making memory profiling memcg-aware but
it would be quite costly both from memory and performance points of
view. Currently we have a per-cpu counter for each allocation in the
kernel codebase. To make it work for each memcg we would have to add
memcg dimension to the counters, so each counter becomes per-cpu plus
per-memcg. I'll be thinking about possible optimizations since many of
these counters will stay at 0 but any such optimization would come at
a performance cost, which we tried to keep at the absolute minimum.

I'm CC'ing Sourav and Pasha since they were also interested in making
memory allocation profiling memcg-aware. Would Meta folks (Usama,
Shakeel, Johannes) be interested in such enhancement as well? Would it
be preferable to have such accounting for a specific memcg which we
pre-select (less memory and performance overhead) or we need that for
all memcgs as a generic feature? We have some options here but I want
to understand what would be sufficient and add as little overhead as
possible.
Thanks,
Suren.