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From:   Eric Dumazet <edumazet@google.com>
Date:   Fri, 7 Sep 2018 00:21:46 -0700
Message-ID: <CANn89iKgZkfwQ8nAGEfOzubOh69y285TNKB5Q518Wf_phbq2Yg@mail.gmail.com>
Subject: Re: [PATCH] net/sock: move memory_allocated over to percpu_counter variables
To:     Olof Johansson <olof@lixom.net>
Cc:     Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>,
        Neil Horman <nhorman@tuxdriver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
        Vladislav Yasevich <vyasevich@gmail.com>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        linux-crypto@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        linux-sctp@vger.kernel.org, netdev <netdev@vger.kernel.org>,
        linux-decnet-user@lists.sourceforge.net,
        kernel-team <kernel-team@fb.com>,
        Yuchung Cheng <ycheng@google.com>,
        Neal Cardwell <ncardwell@google.com>
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On Fri, Sep 7, 2018 at 12:03 AM Eric Dumazet <edumazet@google.com> wrote:

> Problem is : we have platforms with more than 100 cpus, and
> sk_memory_allocated() cost will be too expensive,
> especially if the host is under memory pressure, since all cpus will
> touch their private counter.
>
> per cpu variables do not really scale, they were ok 10 years ago when
> no more than 16 cpus were the norm.
>
> I would prefer change TCP to not aggressively call
> __sk_mem_reduce_allocated() from tcp_write_timer()
>
> Ideally only tcp_retransmit_timer() should attempt to reduce forward
> allocations, after recurring timeout.
>
> Note that after 20c64d5cd5a2bdcdc8982a06cb05e5e1bd851a3d ("net: avoid
> sk_forward_alloc overflows")
> we have better control over sockets having huge forward allocations.
>
> Something like :

Or something less risky :

diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 7fdf222a0bdfe9775970082f6b5dcdcc82b2ae1a..0aee80b6966cb2898e46350c761f9eb431ff1206
100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -604,7 +604,8 @@ void tcp_write_timer_handler(struct sock *sk)
        }

 out:
-       sk_mem_reclaim(sk);
+       if (tcp_under_memory_pressure(sk))
+               sk_mem_reclaim(sk);
 }

 static void tcp_write_timer(struct timer_list *t)