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Date:   Mon, 31 Jul 2023 15:12:20 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Cc:     Chuyi Zhou <zhouchuyi@bytedance.com>, hannes@cmpxchg.org,
        ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org,
        bpf@vger.kernel.org, linux-kernel@vger.kernel.org,
        wuyun.abel@bytedance.com, robin.lu@bytedance.com
Subject: Re: [RFC PATCH 0/5] mm: Select victim memcg using BPF_OOM_POLICY
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On Fri 28-07-23 11:42:27, Roman Gushchin wrote:
> On Fri, Jul 28, 2023 at 10:06:38AM +0200, Michal Hocko wrote:
> > On Thu 27-07-23 21:30:01, Roman Gushchin wrote:
> > > On Thu, Jul 27, 2023 at 10:15:16AM +0200, Michal Hocko wrote:
> > > > On Thu 27-07-23 15:36:27, 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.
> > > > 
> > > > This is rather modest to give an idea how the whole thing is supposed to
> > > > work. I have looked through patches very quickly but there is no overall
> > > > design described anywhere either.
> > > > 
> > > > Please could you give us a high level design description and reasoning
> > > > why certain decisions have been made? e.g. why is this limited to the
> > > > global oom sitation, why is the BPF program forced to operate on memcgs
> > > > as entities etc...
> > > > Also it would be very helpful to call out limitations of the BPF
> > > > program, if there are any.
> > > 
> > > One thing I realized recently: we don't have to make a victim selection
> > > during the OOM, we [almost always] can do it in advance.
> > > 
> > > Kernel OOM's must guarantee the forward progress under heavy memory pressure
> > > and it creates a lot of limitations on what can and what can't be done in
> > > these circumstances.
> > > 
> > > But in practice most policies except maybe those which aim to catch very fast
> > > memory spikes rely on things which are fairly static: a logical importance of
> > > several workloads in comparison to some other workloads, "age", memory footprint
> > > etc.
> > > 
> > > So I wonder if the right path is to create a kernel interface which allows
> > > to define a OOM victim (maybe several victims, also depending on if it's
> > > a global or a memcg oom) and update it periodically from an userspace.
> > 
> > We already have that interface. Just echo OOM_SCORE_ADJ_MAX to any tasks
> > that are to be killed with a priority...
> > Not a great interface but still something available.
> > 
> > > In fact, the second part is already implemented by tools like oomd, systemd-oomd etc.
> > > Someone might say that the first part is also implemented by the oom_score
> > > interface, but I don't think it's an example of a convenient interface.
> > > It's also not a memcg-level interface.
> > 
> > What do you mean by not memcg-level interface? What kind of interface
> > would you propose instead?
> 
> Something like memory.oom.priority, which is 0 by default, but if set to 1,
> the memory cgroup is considered a good oom victim. Idk if we need priorities
> or just fine with a binary thing.

Priorities as a general API have been discussed at several occasions
(e.g http://lkml.kernel.org/r/ZFkEqhAs7FELUO3a@dhcp22.suse.cz). Their usage
is rather limited, hiearchical semantic not trivial etc.
-- 
Michal Hocko
SUSE Labs