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

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Message-ID: <nju55eqv56g6gkmxuavc2z2pcr26qhpmgrt76jt5dte5g4trxs@tjxld2iwdc5c>
Date: Wed, 6 Aug 2025 14:54:37 -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 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);

	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);
		// }
	}

	spin_unlock(memcg->net_pressure_lock);

Regarding the hierarchical, we can avoid rstat infra as this code path
is not really performance critical.