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Message-ID: <CAAVpQUBNoRgciFXVtqS2rxjCeD44JHOuDNcuN0J__guY33pfjw@mail.gmail.com>
Date: Wed, 16 Jul 2025 11:07:44 -0700
From: Kuniyuki Iwashima <kuniyu@...gle.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Daniel Sedlak <daniel.sedlak@...77.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Simon Horman <horms@...nel.org>, Jonathan Corbet <corbet@....net>, Neal Cardwell <ncardwell@...gle.com>, 
	David Ahern <dsahern@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>, linux-mm@...ck.org, netdev@...r.kernel.org, 
	Matyas Hurtik <matyas.hurtik@...77.com>
Subject: Re: [PATCH v2 net-next 1/2] tcp: account for memory pressure signaled
 by cgroup

On Wed, Jul 16, 2025 at 9:50 AM Shakeel Butt <shakeel.butt@...ux.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@...77.com>
> > Signed-off-by: Matyas Hurtik <matyas.hurtik@...77.com>
> > Signed-off-by: Daniel Sedlak <daniel.sedlak@...77.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.

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