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

[Shakeel Butt <shakeel.butt@linux.dev>]

On Thu, Aug 14, 2025 at 10:31:39AM -0700, Shakeel Butt wrote:
> On Thu, Aug 14, 2025 at 06:27:22PM +0200, Matyas Hurtik wrote:
> > On 8/7/25 10:52 PM, Shakeel Butt wrote:
> > 
> > > We definitely don't need a global lock. For memcg->net_pressure_lock, we
> > > need to be very clear why we need this lock. Basically we are doing RMW
> > > on memcg->socket_pressure and we want known 'consistently' how much
> > > further we are pushing memcg->socket_pressure. In other words the
> > > consistent value of diff. The lock is one way to get that consistent
> > > diff. We can also play some atomic ops trick to get the consistent value
> > > without lock but I don't think that complexity is worth it.
> > 
> > Hello,
> > 
> > 
> > I tried implementing the second option, making the diff consistent using
> > atomics.
> > Would something like this work?
> > 
> > if (level > VMPRESSURE_LOW) {
> >   unsigned long new_socket_pressure;
> >   unsigned long old_socket_pressure;
> >   unsigned long duration_to_add;
> >   /*
> >     * Let the socket buffer allocator know that
> >     * we are having trouble reclaiming LRU pages.
> >     *
> >     * For hysteresis keep the pressure state
> >     * asserted for a second in which subsequent
> >     * pressure events can occur.
> >     */
> >   new_socket_pressure = jiffies + HZ;
> 
> Add an if condition here if old_socket_pressure is already equal to
> the new_socket_pressure and skip all of the following.
> 
> >   old_socket_pressure = atomic_long_xchg(
> >     &memcg->socket_pressure, new_socket_pressure);
> > 
> >   duration_to_add = jiffies_to_usecs(

One more point, this jiffies_to_usecs() can be done at the read side
i.e. keep socket_pressure_duration in jiffies.

Also TJ suggested to expose this new stat in memory.stat. That will be
easy by just adding seq_buf_printf() at the end of memcg_stat_format().