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

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Message-ID: <4g63mbix4aut7ye7b7s4m5q7aewfxq542i2vygniow7l5a3zmd@bvis5wmifscy>
Date: Tue, 22 Jul 2025 12:05:41 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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 Tue, Jul 22, 2025 at 11:27:39AM -0700, 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 ?

1 second is the current implementation and it can be more if the memcg
remains in memory pressure. Regarding usefullness I think the periodic
stat collectors (like cadvisor or Google's internal borglet+rumbo) would
be interested in scraping this interface. If this is still not useful,
what will be better? Some kind of trace which tracks the state of socket
pressure state of a memcg (i.e. going into and out of pressure)?