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

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Message-ID: <kyy6mxg4g6aer2mht3xawiq56ytveg7vllg7o6f7dgivkoh52z@ccinqivomtyl>
Date: Wed, 20 Aug 2025 14:34:29 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Matyas Hurtik <matyas.hurtik@...77.com>
Cc: Tejun Heo <tj@...nel.org>, 
	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>, 
	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
Subject: Re: [PATCH v4] memcg: expose socket memory pressure in a cgroup

On Wed, Aug 20, 2025 at 10:37:49PM +0200, Matyas Hurtik wrote:
> Hello,
> 
> On 8/20/25 9:03 PM, Tejun Heo wrote:
> > On Wed, Aug 20, 2025 at 06:51:07PM +0200, Matyas Hurtik wrote:
> > > And the read side:   total_duration = 0;   for (;
> > > !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg))    
> > > total_duration +=
> > > atomic_long_read(&memcg->socket_pressure_duration); Would that work?
> > This doesn't make sense to me. Why would a child report the numbers from
> > its ancestors?
> 
> Result of mem_cgroup_under_socket_pressure() depends on
> whether self or any ancestors have had socket_pressure set.
> 
> So any duration of an ancestor being throttled would also
> mean the child was being throttled.
> 
> By summing our and our ancestors socket_pressure_duration
> we should get our total time being throttled
> (possibly more because of overlaps).

This is not how memcg stats (and their semantics) work and maybe that is
not what you want. In the memcg stats semactics for a given memcg the
socket_pressure_duration metric is not the stall duration faced by
sockets in memcg but instead it will be stall duration caused by the
memcg and its descendants. If that is not what we want, we need to do
something different and orthogonal to memcg stats.