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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 21 Apr 2021 06:26:37 -0700
Message-ID: <CALvZod7x=QR=p73BAxWPbFm+V8KFwYCcxNp-bOBvGaAttxjm7g@mail.gmail.com>
Subject: Re: [RFC] memory reserve for userspace oom-killer
To:     Roman Gushchin <guro@fb.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Cgroups <cgroups@vger.kernel.org>,
        David Rientjes <rientjes@google.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Greg Thelen <gthelen@google.com>,
        Dragos Sbirlea <dragoss@google.com>,
        Priya Duraisamy <padmapriyad@google.com>
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On Tue, Apr 20, 2021 at 7:58 PM Roman Gushchin <guro@fb.com> 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 for last couple of years, so my knowledge can be a bit outdated.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