linux-kernel - Re: [RFC PATCH 0/5] mm: Select victim memcg using BPF_OOM

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Message-ID: <ZMKmuLqyjePWESux@dhcp22.suse.cz>
Date:   Thu, 27 Jul 2023 19:17:44 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alan Maguire <alan.maguire@...cle.com>,
        Chuyi Zhou <zhouchuyi@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, wuyun.abel@...edance.com,
        robin.lu@...edance.com
Subject: Re: [RFC PATCH 0/5] mm: Select victim memcg using BPF_OOM_POLICY

On Thu 27-07-23 08:57:03, Alexei Starovoitov wrote:
> On Thu, Jul 27, 2023 at 4:45 AM Alan Maguire <alan.maguire@...cle.com> wrote:
> >
> > On 27/07/2023 08:36, Chuyi Zhou wrote:
> > > This patchset tries to add a new bpf prog type and use it to select
> > > a victim memcg when global OOM is invoked. The mainly motivation is
> > > the need to customizable OOM victim selection functionality so that
> > > we can protect more important app from OOM killer.
> > >
> >
> > It's a nice use case, but at a high level, the approach pursued here
> > is, as I understand it, discouraged for new BPF program development.
> > Specifically, adding a new BPF program type with semantics like this
> > is not preferred. Instead, can you look at using something like
> >
> > - using "fmod_ret" instead of a new program type
> > - use BPF kfuncs instead of helpers.
> > - add selftests in tools/testing/selftests/bpf not samples.
> 
> +1 to what Alan said above and below.
> 
> Also as Michal said there needs to be a design doc with pros and cons.
> We see far too often that people attempt to use BPF in places where it
> shouldn't be.
> If programmability is not strictly necessary then BPF is not a good fit.

To be completely honest I am not sure whether BPF is the right fit
myself. It is definitely a path to be explored but maybe we will learn
not the proper one in the end. The primary reason for considering it
though is that there is endless (+1) different policies how to select a
victim to kill on OOM. So having an interface to hook into and make that
decision sounds very promissing. This might be something very static
like EXPORT_SYMBOL that a kernel module can hook into or a more broader
BPF callback that could be more generic and therefore allow for easier
to define policy AFAIU.
-- 
Michal Hocko
SUSE Labs