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

[Abel Wu]

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