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

[Abel Wu]

On 6/4/23 6:36 PM, Willem de Bruijn wrote:
> On Fri, Jun 2, 2023 at 10:42 PM Shakeel Butt <shakeelb@xxxxxxxxxx> 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@xxxxxxxxxx/
>
> 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@xxxxxxxxxxxxx/
[2] 
https://lore.kernel.org/lkml/20230523094652.49411-6-wuyun.abel@xxxxxxxxxxxxx/

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@xxxxxxxxxxxxx>
> > > ---
> > >   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