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

[Matyas Hurtik <matyas.hurtik@cdn77.com>]

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;
   old_socket_pressure = atomic_long_xchg(
     &memcg->socket_pressure, new_socket_pressure);

   duration_to_add = jiffies_to_usecs(
     min(new_socket_pressure - old_socket_pressure, HZ));

   do {
     atomic_long_add(duration_to_add, &memcg->socket_pressure_duration);
   } while ((memcg = parent_mem_cgroup(memcg)));
}

memcg->socket_pressure would need to be changed into atomic_long_t,
but we avoid adding the memcg->net_pressure_lock.

> We don't need memcg->net_pressure_lock's protection for
> sk_pressure_duration of the memcg and its ancestors if additions to
> sk_pressure_duration are atomic.

With regards to the hierarchical propagation I noticed during testing that
vmpressure() was sometimes called with memcgs, created for systemd oneshot
services, that were at that time no longer present in the /sys/fs/cgroup 
tree.
This then made their parent counters a lot larger than just sum of the 
subtree
plus value of self. Would this behavior be correct?


Thanks,

Matyas