Re: [RFC PATCH 0/5] mm: Select victim memcg using BPF_OOM_POLICY

[Roman Gushchin]

On Fri, Jul 28, 2023 at 10:06:38AM +0200, Michal Hocko wrote:
> On Thu 27-07-23 21:30:01, Roman Gushchin wrote:
> > On Thu, Jul 27, 2023 at 10:15:16AM +0200, Michal Hocko wrote:
> > > On Thu 27-07-23 15:36:27, Chuyi Zhou wrote:
> > > > This patchset tries to add a new bpf prog type and use it to select
> > > > a victim memcg when global OOM is invoked. The mainly motivation is
> > > > the need to customizable OOM victim selection functionality so that
> > > > we can protect more important app from OOM killer.
> > > 
> > > This is rather modest to give an idea how the whole thing is supposed to
> > > work. I have looked through patches very quickly but there is no overall
> > > design described anywhere either.
> > > 
> > > Please could you give us a high level design description and reasoning
> > > why certain decisions have been made? e.g. why is this limited to the
> > > global oom sitation, why is the BPF program forced to operate on memcgs
> > > as entities etc...
> > > Also it would be very helpful to call out limitations of the BPF
> > > program, if there are any.
> > 
> > One thing I realized recently: we don't have to make a victim selection
> > during the OOM, we [almost always] can do it in advance.
> > 
> > Kernel OOM's must guarantee the forward progress under heavy memory pressure
> > and it creates a lot of limitations on what can and what can't be done in
> > these circumstances.
> > 
> > But in practice most policies except maybe those which aim to catch very fast
> > memory spikes rely on things which are fairly static: a logical importance of
> > several workloads in comparison to some other workloads, "age", memory footprint
> > etc.
> > 
> > So I wonder if the right path is to create a kernel interface which allows
> > to define a OOM victim (maybe several victims, also depending on if it's
> > a global or a memcg oom) and update it periodically from an userspace.
> 
> We already have that interface. Just echo OOM_SCORE_ADJ_MAX to any tasks
> that are to be killed with a priority...
> Not a great interface but still something available.
> 
> > In fact, the second part is already implemented by tools like oomd, systemd-oomd etc.
> > Someone might say that the first part is also implemented by the oom_score
> > interface, but I don't think it's an example of a convenient interface.
> > It's also not a memcg-level interface.
> 
> What do you mean by not memcg-level interface? What kind of interface
> would you propose instead?

Something like memory.oom.priority, which is 0 by default, but if set to 1,
the memory cgroup is considered a good oom victim. Idk if we need priorities
or just fine with a binary thing.

Under oom conditions the kernel can look if we have a pre-selected oom target
and if not fallback to the current behavior.

Thanks!