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

[Shakeel Butt]

On Tue, Apr 20, 2021 at 7:58 PM Roman Gushchin <guro@xxxxxx> wrote:
>
[...]
> >
> > Michal has suggested ALLOC_OOM which is less risky.
>
> The problem is that even if you'll serve the oom daemon task with pages
> from a reserve/custom pool, it doesn't guarantee anything, because the task
> still can wait for a long time on some mutex, taken by another process,
> throttled somewhere in the reclaim.

I am assuming here by mutex you are referring to locks which
oom-killer might have to take to read metrics or any possible lock
which oom-killer might have to take which some other process can take
too.

Have you observed this situation happening with oomd on production?

> You're basically trying to introduce a
> "higher memory priority" and as always in such cases there will be priority
> inversion problems.
>
> So I doubt that you can simple create a common mechanism which will work
> flawlessly for all kinds of allocations, I anticipate many special cases
> requiring an individual approach.
>
[...]
>
> First, I need to admit that I didn't follow the bpf development too close
> for last couple of years, so my knowledge can be a bit outdated.
>
> But in general bpf is great when there is a fixed amount of data as input
> (e.g. skb) and a fixed output (e.g. drop/pass the packet). There are different
> maps which are handy to store some persistent data between calls.
>
> However traversing complex data structures is way more complicated. It's
> especially tricky if the data structure is not of a fixed size: bpf programs
> have to be deterministic, so there are significant constraints on loops.
>
> Just for example: it's easy to call a bpf program for each task in the system,
> provide some stats/access to some fields of struct task and expect it to return
> an oom score, which then the kernel will look at to select the victim.
> Something like this can be done with cgroups too.
>
> Writing a kthread, which can sleep, poll some data all over the system and
> decide what to do (what oomd/... does),  will be really challenging.
> And going back, it will not provide any guarantees unless we're not taking
> any locks, which is already quite challenging.
>

Thanks for the info and I agree this direction needs much more thought
and time to be materialized.

thanks,
Shakeel