Re: Re: [RFC PATCH 0/5] mm: Select victim memcg using BPF_OOM_POLICY

[Abel Wu <wuyun.abel@bytedance.com>]

On 7/28/23 12:30 PM, 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.

I agree. We take precautions against memory shortage on over-committed
machines through oomd-like userspace tools, to mitigate possible SLO
violations on important services. The kernel OOM-killer in our scenario
works as a last resort, since userspace tools are not that reliable.
IMHO it would be useful for kernel to provide such flexibility.

> 
> 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.

Something like [1] proposed by Chuyi? IIUC there is still lack of some
triggers to invoke the procedure so we can actually do this in advance.

[1] 
https://lore.kernel.org/lkml/f8f44103-afba-10ee-b14b-a8e60a7f33d8@bytedance.com/

Thanks & Best,
	Abel