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

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Message-ID: <CAAVpQUCCg-7kvzMeSSsKp3+Fu8pvvE5U-H5wkt=xMryNmnF5CA@mail.gmail.com>
Date: Wed, 6 Aug 2025 15:01:44 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: 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, 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 Wed, Aug 6, 2025 at 2:54 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
>
> On Wed, Aug 06, 2025 at 12:20:25PM -0700, Kuniyuki Iwashima wrote:
> > > > -                     WRITE_ONCE(memcg->socket_pressure, jiffies + HZ);
> > > > +                     socket_pressure = jiffies + HZ;
> > > > +
> > > > +                     jiffies_diff = min(socket_pressure - READ_ONCE(memcg->socket_pressure), HZ);
> > > > +                     memcg->socket_pressure_duration += jiffies_to_usecs(jiffies_diff);
> > >
> > > KCSAN will complain about this. I think we can use atomic_long_add() and
> > > don't need the one with strict ordering.
> >
> > Assuming from atomic_ that vmpressure() could be called concurrently
> > for the same memcg, should we protect socket_pressure and duration
> > within the same lock instead of mixing WRITE/READ_ONCE() and
> > atomic?  Otherwise jiffies_diff could be incorrect (the error is smaller
> > than HZ though).
> >
>
> Yeah good point. Also this field needs to be hierarchical. So, with lock
> something like following is needed:
>
>         if (!spin_trylock(memcg->net_pressure_lock))
>                 return;
>
>         socket_pressure = jiffies + HZ;
>         diff = min(socket_pressure - READ_ONCE(memcg->socket_pressure), HZ);

READ_ONCE() should be unnecessary here.

>
>         if (diff) {
>                 WRITE_ONCE(memcg->socket_pressure, socket_pressure);
>                 // mod_memcg_state(memcg, MEMCG_NET_PRESSURE, diff);
>                 // OR
>                 // while (memcg) {
>                 //      memcg->sk_pressure_duration += diff;
>                 //      memcg = parent_mem_cgroup(memcg);

The parents' sk_pressure_duration is not protected by the lock
taken by trylock.  Maybe we need another global mutex if we want
the hierarchy ?


>                 // }
>         }
>
>         spin_unlock(memcg->net_pressure_lock);
>
> Regarding the hierarchical, we can avoid rstat infra as this code path
> is not really performance critical.