Re: [RFC] simple_lmk: Introduce Simple Low Memory Killer for Android

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Mar 12, 2019 at 1:05 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 11-03-19 15:15:35, Suren Baghdasaryan wrote:
> > On Mon, Mar 11, 2019 at 1:46 PM Sultan Alsawaf <sultan@kerneltoast.com> wrote:
> > >
> > > On Mon, Mar 11, 2019 at 01:10:36PM -0700, Suren Baghdasaryan wrote:
> > > > The idea seems interesting although I need to think about this a bit
> > > > more. Killing processes based on failed page allocation might backfire
> > > > during transient spikes in memory usage.
> > >
> > > This issue could be alleviated if tasks could be killed and have their pages
> > > reaped faster. Currently, Linux takes a _very_ long time to free a task's memory
> > > after an initial privileged SIGKILL is sent to a task, even with the task's
> > > priority being set to the highest possible (so unwanted scheduler preemption
> > > starving dying tasks of CPU time is not the issue at play here). I've
> > > frequently measured the difference in time between when a SIGKILL is sent for a
> > > task and when free_task() is called for that task to be hundreds of
> > > milliseconds, which is incredibly long. AFAIK, this is a problem that LMKD
> > > suffers from as well, and perhaps any OOM killer implementation in Linux, since
> > > you cannot evaluate effect you've had on memory pressure by killing a process
> > > for at least several tens of milliseconds.
> >
> > Yeah, killing speed is a well-known problem which we are considering
> > in LMKD. For example the recent LMKD change to assign process being
> > killed to a cpuset cgroup containing big cores cuts the kill time
> > considerably. This is not ideal and we are thinking about better ways
> > to expedite the cleanup process.
>
> If you design is relies on the speed of killing then it is fundamentally
> flawed AFAICT. You cannot assume anything about how quickly a task dies.
> It might be blocked in an uninterruptible sleep or performin an
> operation which takes some time. Sure, oom_reaper might help here but
> still.

That's what I was considering. This is not a silver bullet but
increased speed would not hurt.

> The only way to control the OOM behavior pro-actively is to throttle
> allocation speed. We have memcg high limit for that purpose. Along with
> PSI, I can imagine a reasonably working user space early oom
> notifications and reasonable acting upon that.

That makes sense and we are working in this direction.

> --
> Michal Hocko
> SUSE Labs

Thanks,
Suren.