Re: [PATCH v3] memcg: expose socket memory pressure in a cgroup

[Eric Dumazet <edumazet@google.com>]

On Tue, Jul 22, 2025 at 12:12 AM Daniel Sedlak <daniel.sedlak@cdn77.com> wrote:
>
> This patch is a result of our long-standing debug sessions, where it all
> started as "networking is slow", and TCP network throughput suddenly
> dropped from tens of Gbps to few Mbps, and we could not see anything in
> the kernel log or netstat counters.
>
> Currently, we have two memory pressure counters for TCP sockets [1],
> which we manipulate only when the memory pressure is signalled through
> the proto struct [2]. However, the memory pressure can also be signaled
> through the cgroup memory subsystem, which we do not reflect in the
> netstat counters. In the end, when the cgroup memory subsystem signals
> that it is under pressure, we silently reduce the advertised TCP window
> with tcp_adjust_rcv_ssthresh() to 4*advmss, which causes a significant
> throughput reduction.
>
> Keep in mind that when the cgroup memory subsystem signals the socket
> memory pressure, it affects all sockets used in that cgroup.
>
> This patch exposes a new file for each cgroup in sysfs which signals
> the cgroup socket memory pressure. The file is accessible in
> the following path.
>
>   /sys/fs/cgroup/**/<cgroup name>/memory.net.socket_pressure
>
> The output value is an integer matching the internal semantics of the
> struct mem_cgroup for socket_pressure. It is a periodic re-arm clock,
> representing the end of the said socket memory pressure, and once the
> clock is re-armed it is set to jiffies + HZ.
>
> Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/uapi/linux/snmp.h#L231-L232 [1]
> Link: https://elixir.bootlin.com/linux/v6.15.4/source/include/net/sock.h#L1300-L1301 [2]
> Co-developed-by: Matyas Hurtik <matyas.hurtik@cdn77.com>
> Signed-off-by: Matyas Hurtik <matyas.hurtik@cdn77.com>
> Signed-off-by: Daniel Sedlak <daniel.sedlak@cdn77.com>
> ---
> Changes:
> v2 -> v3:
> - Expose the socket memory pressure on the cgroups instead of netstat
> - Split patch
> - Link: https://lore.kernel.org/netdev/20250714143613.42184-1-daniel.sedlak@cdn77.com/
>
> v1 -> v2:
> - Add tracepoint
> - Link: https://lore.kernel.org/netdev/20250707105205.222558-1-daniel.sedlak@cdn77.com/
>
>
>  mm/memcontrol.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 902da8a9c643..8e8808fb2d7a 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -4647,6 +4647,15 @@ static ssize_t memory_reclaim(struct kernfs_open_file *of, char *buf,
>         return nbytes;
>  }
>
> +static int memory_socket_pressure_show(struct seq_file *m, void *v)
> +{
> +       struct mem_cgroup *memcg = mem_cgroup_from_seq(m);
> +
> +       seq_printf(m, "%lu\n", READ_ONCE(memcg->socket_pressure));
> +
> +       return 0;
> +}
> +
>  static struct cftype memory_files[] = {
>         {
>                 .name = "current",
> @@ -4718,6 +4727,11 @@ static struct cftype memory_files[] = {
>                 .flags = CFTYPE_NS_DELEGATABLE,
>                 .write = memory_reclaim,
>         },
> +       {
> +               .name = "net.socket_pressure",
> +               .flags = CFTYPE_NOT_ON_ROOT,
> +               .seq_show = memory_socket_pressure_show,
> +       },
>         { }     /* terminate */
>  };
>

It seems you forgot to update Documentation/admin-guide/cgroup-v2.rst