Re: [RFC] memory reserve for userspace oom-killer

[Suren Baghdasaryan <surenb@google.com>]

On Tue, May 4, 2021 at 5:37 PM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Wed, Apr 21, 2021 at 7:29 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> [...]
> > > > What if the pool is depleted?
> > >
> > > This would mean that either the estimate of mempool size is bad or
> > > oom-killer is buggy and leaking memory.
> > >
> > > I am open to any design directions for mempool or some other way where
> > > we can provide a notion of memory guarantee to oom-killer.
> >
> > OK, thanks for clarification. There will certainly be hard problems to
> > sort out[1] but the overall idea makes sense to me and it sounds like a
> > much better approach than a OOM specific solution.
> >
> >
> > [1] - how the pool is going to be replenished without hitting all
> > potential reclaim problems (thus dependencies on other all tasks
> > directly/indirectly) yet to not rely on any background workers to do
> > that on the task behalf without a proper accounting etc...
> > --
>
> I am currently contemplating between two paths here:
>
> First, the mempool, exposed through either prctl or a new syscall.
> Users would need to trace their userspace oom-killer (or whatever
> their use case is) to find an appropriate mempool size they would need
> and periodically refill the mempools if allowed by the state of the
> machine. The challenge here is to find a good value for the mempool
> size and coordinating the refilling of mempools.
>
> Second is a mix of Roman and Peter's suggestions but much more
> simplified. A very simple watchdog with a kill-list of processes and
> if userspace didn't pet the watchdog within a specified time, it will
> kill all the processes in the kill-list. The challenge here is to
> maintain/update the kill-list.

IIUC this solution is designed to identify cases when oomd/lmkd got
stuck while allocating memory due to memory shortages and therefore
can't feed the watchdog. In such a case the kernel goes ahead and
kills some processes to free up memory and unblock the blocked
process. Effectively this would limit the time such a process gets
stuck by the duration of the watchdog timeout. If my understanding of
this proposal is correct, then I see the following downsides:
1. oomd/lmkd are still not prevented from being stuck, it just limits
the duration of this blocked state. Delaying kills when memory
pressure is high even for short duration is very undesirable. I think
having mempool reserves could address this issue better if it can
always guarantee memory availability (not sure if it's possible in
practice).
2. What would be performance overhead of this watchdog? To limit the
duration of a process being blocked to a small enough value we would
have to have quite a small timeout, which means oomd/lmkd would have
to wake up quite often to feed the watchdog. Frequent wakeups on a
battery-powered system is not a good idea.
3. What if oomd/lmkd gets stuck for some memory-unrelated reason and
can't feed the watchdog? In such a scenario the kernel would assume
that it is stuck due to memory shortages and would go on a killing
spree. If there is a sure way to identify when a process gets stuck
due to memory shortages then this could work better.
4. Additional complexity of keeping the list of potential victims in
the kernel. Maybe we can simply reuse oom_score to choose the best
victims?
Thanks,
Suren.

>
> I would prefer the direction which oomd and lmkd are open to adopt.
>
> Any suggestions?