Messages in this thread | | | Date | Wed, 11 Oct 2023 11:04:01 +0800 | Subject | Re: [PATCH net-next 2/2] sock: Fix improper heuristic on raising memory | From | Abel Wu <> |
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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
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