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

[Suren Baghdasaryan]

On Tue, May 4, 2021 at 7:45 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
>
> On Tue, May 4, 2021 at 6:26 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> >
> > On Tue, May 4, 2021 at 5:37 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote:
> > >
> > > On Wed, Apr 21, 2021 at 7:29 AM Michal Hocko <mhocko@xxxxxxxx> 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,
>
> Your understanding is indeed 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.
>
> Yes I agree.
>
> > 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).
>
> I think "mempool ... always guarantee memory availability" is
> something I should quantify with some experiments.
>
> > 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.
>
> This is indeed the downside i.e. the tradeoff between acceptable stall
> vs frequent wakeups.
>
> > 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.
>
> This is correct but IMHO killing spree is not worse than oomd/lmkd
> getting stuck for some other reason.
>
> > If there is a sure way to identify when a process gets stuck
> > due to memory shortages then this could work better.
>
> Hmm are you saying looking at the stack traces of the userspace
> oom-killer or some metrics related to oom-killer? It will complicate
> the code.

Well, I don't know of a sure and easy way to identify the reasons for
process blockage but maybe there is one I don't know of? My point is
that we would need some additional indications of memory being the
culprit for the process blockage before resorting to kill.

>
> > 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?
>
> Your point of additional complexity is correct. Regarding oom_score I
> think you meant oom_score_adj, I would avoid putting more
> policies/complexity in the kernel but I got your point that the
> simplest watchdog might not be helpful at all.
>
> > Thanks,
> > Suren.
> >
> > >
> > > I would prefer the direction which oomd and lmkd are open to adopt.
> > >
> > > Any suggestions?