netdev - Re: [PATCH v3] memcg: expose socket memory pressure in a cgroup

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Message-ID: <3db01bc9-f6ea-41f7-8cbf-fb33e522694a@redhat.com>
Date: Tue, 22 Jul 2025 14:41:38 -0400
From: Waiman Long <llong@...hat.com>
To: Kuniyuki Iwashima <kuniyu@...gle.com>,
 Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Michal Koutný <mkoutny@...e.com>,
 Daniel Sedlak <daniel.sedlak@...77.com>,
 "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>, 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,
 Matyas Hurtik <matyas.hurtik@...77.com>
Subject: Re: [PATCH v3] memcg: expose socket memory pressure in a cgroup

On 7/22/25 2:27 PM, Kuniyuki Iwashima wrote:
> On Tue, Jul 22, 2025 at 10:50 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>> On Tue, Jul 22, 2025 at 10:57:31AM +0200, Michal Koutný wrote:
>>> Hello Daniel.
>>>
>>> On Tue, Jul 22, 2025 at 09:11:46AM +0200, Daniel Sedlak <daniel.sedlak@...77.com> wrote:
>>>>    /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure
>>>>
>>>> The output value is an integer matching the internal semantics of the
>>>> struct mem_cgroup for socket_pressure. It is a periodic re-arm clock,
>>>> representing the end of the said socket memory pressure, and once the
>>>> clock is re-armed it is set to jiffies + HZ.
>>> I don't find it ideal to expose this value in its raw form that is
>>> rather an implementation detail.
>>>
>>> IIUC, the information is possibly valid only during one jiffy interval.
>>> How would be the userspace consuming this?
>>>
>>> I'd consider exposing this as a cummulative counter in memory.stat for
>>> simplicity (or possibly cummulative time spent in the pressure
>>> condition).
>>>
>>> Shakeel, how useful is this vmpressure per-cgroup tracking nowadays? I
>>> thought it's kind of legacy.
>>
>> Yes vmpressure is legacy and we should not expose raw underlying number
>> to the userspace. How about just 0 or 1 and use
>> mem_cgroup_under_socket_pressure() underlying? In future if we change
>> the underlying implementation, the output of this interface should be
>> consistent.
> But this is available only for 1 second, and it will not be useful
> except for live debugging ?

If the new interface is used mainly for debugging purpose, I will 
suggest adding the CFTYPE_DEBUG flag so that it will only show up when 
"cgroup_debug" is specified in the kernel command line.

Cheers,
Longman