linux-kernel - Re: [PATCH net-next 2/2] sock: Fix improper heuristic on raising memory

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Message-ID: <83352a11-cb54-477d-b19a-5f9e0d00b54d@bytedance.com>
Date:   Wed, 11 Oct 2023 11:04:01 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Kuniyuki Iwashima <kuniyu@...zon.com>,
        Breno Leitao <leitao@...ian.org>,
        Alexander Mikhalitsyn <alexander@...alicyn.com>,
        David Howells <dhowells@...hat.com>,
        Jason Xing <kernelxing@...cent.com>,
        Xin Long <lucien.xin@...il.com>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH net-next 2/2] sock: Fix improper heuristic on raising
 memory

Gentle ping :)

在 10/3/23 8:49 PM, Abel Wu Wrote:
> On 9/24/23 3:28 PM, Shakeel Butt wrote:
>> On Fri, Sep 22, 2023 at 06:10:06PM +0800, Abel Wu wrote:
>> [...]
>>>
>>> After a second thought, it is still vague to me about the position
>>> the memcg pressure should be in socket memory allocation. It lacks
>>> convincing design. I think the above hunk helps, but not much.
>>>
>>> I wonder if we should take option (3) first. Thoughts?
>>>
>>
>> Let's take a step further. Let's decouple the memcg accounting and
>> global skmem accounting. __sk_mem_raise_allocated is already very hard
>> to reason. There are couple of heuristics in it which may or may not
>> apply to both accounting infrastructures.
>>
>> Let's explicitly document what heurisitics allows to forcefully succeed
>> the allocations i.e. irrespective of pressure or over limit for both
>> accounting infras. I think decoupling them would make the flow of the
>> code very clear.
> 
> I can't agree more.
> 
>>
>> There are three heuristics:
> 
> I found all of them were first introduced in linux-2.4.0-test7pre1 for
> TCP only, and then migrated to socket core in linux-2.6.8-rc1 without
> functional change.
> 
>>
>> 1. minimum buffer size even under pressure.
> 
> This is required by RFC 7323 (TCP Extensions for High Performance) to
> make features like Window Scale option work as expected, and should be
> succeeded under global pressure by tcp_{r,w}mem's definition. And IMHO
> for same reason, it should also be succeeded under memcg pressure, or
> else workloads might suffer performance drop due to bottleneck on
> network.
> 
> The allocation must not be succeeded either exceed global or memcg's
> hard limit, or else a DoS attack can be taken place by spawning lots
> of sockets that are under minimum buffer size.
> 
>>
>> 2. allow allocation for a socket whose usage is below average of the
>> system.
> 
> Since 'average' is within the scope of global accounting, this one
> only makes sense under global memory pressure. Actually this exists
> before cgroup was born, hence doesn't take memcg into consideration.
> 
> While OTOH the intention of throttling under memcg pressure is to
> relief the memcg from heavy reclaim pressure, this heuristic does no
> help. And there also seems to be no reason to succeed the allocation
> when global or memcg's hard limit is exceeded.
> 
>>
>> 3. socket is over its sndbuf.
> 
> TBH I don't get its point..
> 
>>
>> Let's discuss which heuristic applies to which accounting infra and
>> under which state (under pressure or over limit).
> 
> I will follow your suggestion to post a patch to explicitly document
> the behaviors once things are cleared.
> 
> Thanks,
>      Abel