netdev - Re: [PATCH v5] memcg: expose socket memory pressure in a cgroup

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Message-ID: <87o6qgnl9w.fsf@linux.dev>
Date: Thu, 09 Oct 2025 08:32:27 -0700
From: Roman Gushchin <roman.gushchin@...ux.dev>
To: Daniel Sedlak <daniel.sedlak@...77.com>
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>,  Shakeel Butt
 <shakeel.butt@...ux.dev>,  Yosry Ahmed <yosry.ahmed@...ux.dev>,
  linux-mm@...ck.org,  netdev@...r.kernel.org,  Johannes Weiner
 <hannes@...xchg.org>,  Michal Hocko <mhocko@...nel.org>,  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 v5] memcg: expose socket memory pressure in a cgroup

Daniel Sedlak <daniel.sedlak@...77.com> writes:

> Hi Roman,
>
> On 10/8/25 8:58 PM, Roman Gushchin wrote:
>>> This patch exposes a new file for each cgroup in sysfs which is a
>>> read-only single value file showing how many microseconds this cgroup
>>> contributed to throttling the throughput of network sockets. The file is
>>> accessible in the following path.
>>>
>>>    /sys/fs/cgroup/**/<cgroup name>/memory.net.throttled_usec
>> Hi Daniel!
>> How this value is going to be used? In other words, do you need an
>> exact number or something like memory.events::net_throttled would be
>> enough for your case?
>
> Just incrementing a counter each time the vmpressure() happens IMO
> provides bad semantics of what is actually happening, because it can
> hide important details, mainly the _time_ for how long the network
> traffic was slowed down.
>
> For example, when memory.events::net_throttled=1000, it can mean that
> the network was slowed down for 1 second or 1000 seconds or something
> between, and the memory.net.throttled_usec proposed by this patch
> disambiguates it.
>
> In addition, v1/v2 of this series started that way, then from v3 we
> rewrote it to calculate the duration instead, which proved to be
> better information for debugging, as it is easier to understand
> implications.

But how are you planning to use this information? Is this just
"networking is under pressure for non-trivial amount of time ->
raise the memcg limit" or something more complicated?

I am bit concerned about making this metric the part of cgroup API
simple because it's too implementation-defined and in my opinion
lack the fundamental meaning.

Vmpressure is calculated based on scanned/reclaimed ratio (which is
also not always the best proxy for the memory pressure level), then
if it reaches some level we basically throttle networking for 1s.
So it's all very arbitrary.

I totally get it from the debugging perspective, but not sure about
usefulness of it as a permanent metric. This is why I'm asking if there
are lighter alternatives, e.g. memory.events or maybe even tracepoints.

Thanks!