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

[Daniel Sedlak <daniel.sedlak@cdn77.com>]

On 7/22/25 9:17 AM, Eric Dumazet wrote:
> 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

Oops, missed that. I will add it to the v4.

Thanks!
Daniel