netdev - Re: [PATCH v10 bpf-next/net 3/6] net-memcg: Introduce net.core.memcg

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Message-ID: <vu55dawwasbnogwqsdjomwci3nstz6pcl7a3l7pp5gouo5ijv3@cw2msu4ks4aa>
Date: Thu, 25 Sep 2025 15:52:25 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>, 
	Andrii Nakryiko <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, 
	Martin KaFai Lau <martin.lau@...ux.dev>, John Fastabend <john.fastabend@...il.com>, 
	Stanislav Fomichev <sdf@...ichev.me>, Johannes Weiner <hannes@...xchg.org>, 
	Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Neal Cardwell <ncardwell@...gle.com>, Willem de Bruijn <willemb@...gle.com>, 
	Mina Almasry <almasrymina@...gle.com>, Kuniyuki Iwashima <kuni1840@...il.com>, bpf@...r.kernel.org, 
	netdev@...r.kernel.org
Subject: Re: [PATCH v10 bpf-next/net 3/6] net-memcg: Introduce
 net.core.memcg_exclusive sysctl.

On Sat, Sep 20, 2025 at 12:07:17AM +0000, Kuniyuki Iwashima wrote:
> If net.core.memcg_exclusive is 1 when sk->sk_memcg is allocated,
> the socket is flagged with SK_MEMCG_EXCLUSIVE internally and skips
> the global per-protocol memory accounting.
> 
> OTOH, for accept()ed child sockets, this flag is inherited from
> the listening socket in sk_clone_lock() and set in __inet_accept().
> This is to preserve the decision by BPF which will be supported later.
> 
> Given sk->sk_memcg can be accessed in the fast path, it would
> be preferable to place the flag field in the same cache line as
> sk->sk_memcg.
> 
> However, struct sock does not have such a 1-byte hole.
> 
> Let's store the flag in the lowest bit of sk->sk_memcg and check
> it in mem_cgroup_sk_exclusive().
> 
> Tested with a script that creates local socket pairs and send()s a
> bunch of data without recv()ing.
> 
> Setup:
> 
>   # mkdir /sys/fs/cgroup/test
>   # echo $$ >> /sys/fs/cgroup/test/cgroup.procs
>   # sysctl -q net.ipv4.tcp_mem="1000 1000 1000"
> 
> Without net.core.memcg_exclusive, charged to memcg & tcp_mem:
> 
>   # prlimit -n=524288:524288 bash -c "python3 pressure.py" &
>   # cat /sys/fs/cgroup/test/memory.stat | grep sock
>   sock 22642688 <-------------------------------------- charged to memcg
>   # cat /proc/net/sockstat| grep TCP
>   TCP: inuse 2006 orphan 0 tw 0 alloc 2008 mem 5376 <-- charged to tcp_mem
>   # ss -tn | head -n 5
>   State Recv-Q Send-Q Local Address:Port  Peer Address:Port
>   ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53188
>   ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:49972
>   ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53868
>   ESTAB 2000   0          127.0.0.1:34479    127.0.0.1:53554
>   # nstat | grep Pressure || echo no pressure
>   TcpExtTCPMemoryPressures        1                  0.0
> 
> With net.core.memcg_exclusive=1, only charged to memcg:
> 
>   # sysctl -q net.core.memcg_exclusive=1
>   # prlimit -n=524288:524288 bash -c "python3 pressure.py" &
>   # cat /sys/fs/cgroup/test/memory.stat | grep sock
>   sock 2757468160 <------------------------------------ charged to memcg
>   # cat /proc/net/sockstat | grep TCP
>   TCP: inuse 2006 orphan 0 tw 0 alloc 2008 mem 0 <- NOT charged to tcp_mem
>   # ss -tn | head -n 5
>   State Recv-Q Send-Q  Local Address:Port  Peer Address:Port
>   ESTAB 111000 0           127.0.0.1:36019    127.0.0.1:49026
>   ESTAB 110000 0           127.0.0.1:36019    127.0.0.1:45630
>   ESTAB 110000 0           127.0.0.1:36019    127.0.0.1:44870
>   ESTAB 111000 0           127.0.0.1:36019    127.0.0.1:45274
>   # nstat | grep Pressure || echo no pressure
>   no pressure
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...gle.com>

Reviewed-by: Shakeel Butt <shakeel.butt@...ux.dev>