Re: [PATCH v2 net-next 1/2] tcp: account for memory pressure signaled by cgroup

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Jul 16, 2025 at 11:07:44AM -0700, Kuniyuki Iwashima wrote:
> On Wed, Jul 16, 2025 at 9:50 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
> >
> > On Mon, Jul 14, 2025 at 04:36:12PM +0200, Daniel Sedlak 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.
> > >
> > > So this patch adds a new counter to account for memory pressure
> > > signaled by the memory cgroup, so it is much easier to spot.
> > >
> > > 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>
> > > ---
> > >  Documentation/networking/net_cachelines/snmp.rst |  1 +
> > >  include/net/tcp.h                                | 14 ++++++++------
> > >  include/uapi/linux/snmp.h                        |  1 +
> > >  net/ipv4/proc.c                                  |  1 +
> > >  4 files changed, 11 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/Documentation/networking/net_cachelines/snmp.rst b/Documentation/networking/net_cachelines/snmp.rst
> > > index bd44b3eebbef..ed17ff84e39c 100644
> > > --- a/Documentation/networking/net_cachelines/snmp.rst
> > > +++ b/Documentation/networking/net_cachelines/snmp.rst
> > > @@ -76,6 +76,7 @@ unsigned_long  LINUX_MIB_TCPABORTONLINGER
> > >  unsigned_long  LINUX_MIB_TCPABORTFAILED
> > >  unsigned_long  LINUX_MIB_TCPMEMORYPRESSURES
> > >  unsigned_long  LINUX_MIB_TCPMEMORYPRESSURESCHRONO
> > > +unsigned_long  LINUX_MIB_TCPCGROUPSOCKETPRESSURE
> > >  unsigned_long  LINUX_MIB_TCPSACKDISCARD
> > >  unsigned_long  LINUX_MIB_TCPDSACKIGNOREDOLD
> > >  unsigned_long  LINUX_MIB_TCPDSACKIGNOREDNOUNDO
> > > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > > index 761c4a0ad386..aae3efe24282 100644
> > > --- a/include/net/tcp.h
> > > +++ b/include/net/tcp.h
> > > @@ -267,6 +267,11 @@ extern long sysctl_tcp_mem[3];
> > >  #define TCP_RACK_STATIC_REO_WND  0x2 /* Use static RACK reo wnd */
> > >  #define TCP_RACK_NO_DUPTHRESH    0x4 /* Do not use DUPACK threshold in RACK */
> > >
> > > +#define TCP_INC_STATS(net, field)    SNMP_INC_STATS((net)->mib.tcp_statistics, field)
> > > +#define __TCP_INC_STATS(net, field)  __SNMP_INC_STATS((net)->mib.tcp_statistics, field)
> > > +#define TCP_DEC_STATS(net, field)    SNMP_DEC_STATS((net)->mib.tcp_statistics, field)
> > > +#define TCP_ADD_STATS(net, field, val)       SNMP_ADD_STATS((net)->mib.tcp_statistics, field, val)
> > > +
> > >  extern atomic_long_t tcp_memory_allocated;
> > >  DECLARE_PER_CPU(int, tcp_memory_per_cpu_fw_alloc);
> > >
> > > @@ -277,8 +282,10 @@ extern unsigned long tcp_memory_pressure;
> > >  static inline bool tcp_under_memory_pressure(const struct sock *sk)
> > >  {
> > >       if (mem_cgroup_sockets_enabled && sk->sk_memcg &&
> > > -         mem_cgroup_under_socket_pressure(sk->sk_memcg))
> > > +         mem_cgroup_under_socket_pressure(sk->sk_memcg)) {
> > > +             TCP_INC_STATS(sock_net(sk), LINUX_MIB_TCPCGROUPSOCKETPRESSURE);
> > >               return true;
> >
> > Incrementing it here will give a very different semantic to this stat
> > compared to LINUX_MIB_TCPMEMORYPRESSURES. Here the increments mean the
> > number of times the kernel check if a given socket is under memcg
> > pressure for a net namespace. Is that what we want?
> 
> I'm trying to decouple sk_memcg from the global tcp_memory_allocated
> as you and Wei planned before, and the two accounting already have the
> different semantics from day1 and will keep that, so a new stat having a
> different semantics would be fine.
> 
> But I think per-memcg stat like memory.stat.XXX would be a good fit
> rather than pre-netns because one netns could be shared by multiple
> cgroups and multiple sockets in the same cgroup could be spread across
> multiple netns.

Yeah it makes much more sense to have memcg stat for memcg based socket
pressure.