Re: Re: [PATCH net-next v5 2/3] sock: Always take memcg pressure into consideration

[Abel Wu <wuyun.abel@bytedance.com>]

On 6/4/23 6:36 PM, Willem de Bruijn wrote:
> On Fri, Jun 2, 2023 at 10:42 PM Shakeel Butt <shakeelb@google.com> wrote:
>>
>> On Fri, Jun 02, 2023 at 04:11:34PM +0800, Abel Wu wrote:
>>> The sk_under_memory_pressure() is called to check whether there is
>>> memory pressure related to this socket. But now it ignores the net-
>>> memcg's pressure if the proto of the socket doesn't care about the
>>> global pressure, which may put burden on its memcg compaction or
>>> reclaim path (also remember that socket memory is un-reclaimable).
>>>
>>> So always check the memcg's vm status to alleviate memstalls when
>>> it's in pressure.
>>>
>>
>> This is interesting. UDP is the only protocol which supports memory
>> accounting (i.e. udp_memory_allocated) but it does not define
>> memory_pressure. In addition, it does have sysctl_udp_mem. So
>> effectively UDP supports a hard limit and ignores memcg pressure at the
>> moment. This patch will change its behavior to consider memcg pressure
>> as well. I don't have any objection but let's get opinion of UDP
>> maintainer.
> 
> Others have more experience with memory pressure on UDP, for the
> record. Paolo worked on UDP memory pressure in
> https://lore.kernel.org/netdev/cover.1579281705.git.pabeni@redhat.com/
> 
> It does seem odd to me to modify sk_under_memory_pressure only. See
> for instance its use in __sk_mem_raise_allocated:
> 
>          if (sk_has_memory_pressure(sk)) {
>                  u64 alloc;
> 
>                  if (!sk_under_memory_pressure(sk))
>                          return 1;
> 
> This is not even reached as sk_has_memory_pressure is false for UDP.

I intended to make __sk_mem_raise_allocated() be aware of net-memcg
pressure instead of just this bit [1][2].

[1] 
https://lore.kernel.org/lkml/20230523094652.49411-5-wuyun.abel@bytedance.com/
[2] 
https://lore.kernel.org/lkml/20230523094652.49411-6-wuyun.abel@bytedance.com/

And TBH I am wondering why considering memcg's pressure here, as the
main part in this if statement is to allow the sockets that are below
average memory usage to raise from a *global* memory view, which seems
nothing to do with memcg.

> So this commit only affects the only other protocol-independent
> caller, __sk_mem_reduce_allocated, to possibly call
> sk_leave_memory_pressure if now under the global limit.
> 
> What is the expected behavioral change in practice of this commit?

Be more conservative on sockmem alloc if under memcg pressure, to
avoid worse memstall/latency.

> 
> 
>>> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
>>> ---
>>>   include/net/sock.h | 6 ++----
>>>   1 file changed, 2 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/include/net/sock.h b/include/net/sock.h
>>> index 3f63253ee092..ad1895ffbc4a 100644
>>> --- a/include/net/sock.h
>>> +++ b/include/net/sock.h
>>> @@ -1411,13 +1411,11 @@ static inline bool sk_has_memory_pressure(const struct sock *sk)
>>>
>>>   static inline bool sk_under_memory_pressure(const struct sock *sk)
>>>   {
>>> -     if (!sk->sk_prot->memory_pressure)
>>> -             return false;
>>> -
>>>        if (mem_cgroup_under_socket_pressure(sk->sk_memcg))
>>>                return true;
>>>
>>> -     return !!*sk->sk_prot->memory_pressure;
>>> +     return sk->sk_prot->memory_pressure &&
>>> +             *sk->sk_prot->memory_pressure;
>>>   }
>>>
>>>   static inline long
>>> --
>>> 2.37.3
>>>