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

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Message-ID: <ai34al6aocctgjxob7pshhm4lyqfnzlytjefhlum6dxwjcx26o@fj2ca5jdqxa7>
Date: Wed, 6 Aug 2025 16:51:33 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
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 06, 2025 at 04:40:45PM -0700, Kuniyuki Iwashima wrote:
> On Wed, Aug 6, 2025 at 4:34 PM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > On Wed, Aug 06, 2025 at 03:01:44PM -0700, Kuniyuki Iwashima wrote:
> > > 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 ?
> >
> > We don't really need lock protection for sk_pressure_duration. The lock
> > is only giving us consistent value of diff. Once we have computed the
> > diff, we can add it to sk_pressure_duration of a memcg and all of its
> > ancestor without lock.
> 
> Maybe I'm wrong but I was assuming two vmpressure() called
> concurrently for cgroup-C and cgroup-D, and one updates
> cgroup-C's duration and another updates C&D duration.
> 
> cgroup-A -> cgroup-B -> cgroup-C -> cgroup-D
> 
> Could that happen ?  Even if it's yes, we could use atomic ops.

I am not getting the hierarchy you are using but yes concurrent updates
to sk_pressure_duration can happen and simple atomic_add is good enough
without any locking.