Re: [PATCH rfc 10/12] mm: introduce bpf_out_of_memory() bpf kfunc

[Roman Gushchin <roman.gushchin@linux.dev>]

On Wed, Apr 30, 2025 at 09:27:39AM +0200, Michal Hocko wrote:
> On Tue 29-04-25 21:31:35, Roman Gushchin wrote:
> > On Tue, Apr 29, 2025 at 01:46:07PM +0200, Michal Hocko wrote:
> > > On Mon 28-04-25 03:36:15, Roman Gushchin wrote:
> > > > Introduce bpf_out_of_memory() bpf kfunc, which allows to declare
> > > > an out of memory events and trigger the corresponding kernel OOM
> > > > handling mechanism.
> > > > 
> > > > It takes a trusted memcg pointer (or NULL for system-wide OOMs)
> > > > as an argument, as well as the page order.
> > > > 
> > > > Only one OOM can be declared and handled in the system at once,
> > > > so if the function is called in parallel to another OOM handling,
> > > > it bails out with -EBUSY.
> > > 
> > > This makes sense for the global OOM handler because concurrent handlers
> > > are cooperative. But is this really correct for memcg ooms which could
> > > happen for different hierarchies? Currently we do block on oom_lock in
> > > that case to make sure one oom doesn't starve others. Do we want the
> > > same behavior for custom OOM handlers?
> > 
> > It's a good point and I had similar thoughts when I was working on it.
> > But I think it's orthogonal to the customization of the oom handling.
> > Even for the existing oom killer it makes no sense to serialize memcg ooms
> > in independent memcg subtrees. But I'm worried about the dmesg reporting,
> > it can become really messy for 2+ concurrent OOMs.
> > 
> > Also, some memory can be shared, so one OOM can eliminate a need for another
> > OOM, even if they look independent.
> > 
> > So my conclusion here is to leave things as they are until we'll get signs
> > of real world problems with the (lack of) concurrency between ooms.
> 
> How do we learn about that happening though? I do not think we have any
> counters to watch to suspect that some oom handlers cannot run.

The bpf program which declares an OOM can handle this: e.g. retry, wait
and retry, etc. We can also try to mimick the existing behavior and wait
on oom_lock (potentially splitting it into multiple locks to support
concurrent ooms in various memcgs). Do you think it's preferable?