netdev - Re: [PATCH v4] memcg: expose socket memory pressure in a cgroup

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Message-ID: <5e32aded-bcff-40d2-aef1-20a1cf6f4f8e@cdn77.com>
Date: Thu, 7 Aug 2025 12:42:23 +0200
From: Daniel Sedlak <daniel.sedlak@...77.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>,
 Jonathan Corbet <corbet@....net>, Neal Cardwell <ncardwell@...gle.com>,
 Kuniyuki Iwashima <kuniyu@...gle.com>, David Ahern <dsahern@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Yosry Ahmed <yosry.ahmed@...ux.dev>, linux-mm@...ck.org,
 netdev@...r.kernel.org, Johannes Weiner <hannes@...xchg.org>,
 Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, cgroups@...r.kernel.org,
 Tejun Heo <tj@...nel.org>, Michal Koutný
 <mkoutny@...e.com>, Matyas Hurtik <matyas.hurtik@...77.com>
Subject: Re: [PATCH v4] memcg: expose socket memory pressure in a cgroup

On 8/6/25 1:02 AM, Shakeel Butt wrote:
> On Tue, Aug 05, 2025 at 08:44:29AM +0200, Daniel Sedlak wrote:
>> This patch is a result of our long-standing debug sessions, where it all
>> started as "networking is slow", and TCP network throughput suddenly
>> dropped from tens of Gbps to few Mbps, and we could not see anything in
>> the kernel log or netstat counters.
>>
>> Currently, we have two memory pressure counters for TCP sockets [1],
>> which we manipulate only when the memory pressure is signalled through
>> the proto struct [2]. However, the memory pressure can also be signaled
>> through the cgroup memory subsystem, which we do not reflect in the
>> netstat counters. In the end, when the cgroup memory subsystem signals
>> that it is under pressure, we silently reduce the advertised TCP window
>> with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
>> throughput reduction.
>>
>> Keep in mind that when the cgroup memory subsystem signals the socket
>> memory pressure, it affects all sockets used in that cgroup.
>>
>> This patch exposes a new file for each cgroup in sysfs which signals
>> the cgroup socket memory pressure. The file is accessible in
>> the following path.
>>
>>    /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure
> 
> let's keep the name concise. Maybe memory.net.pressure?
> 

Sure, I can rename it for v5.

Thanks!
Daniel