netdev - Re: [PATCH v2 net-next 0/3] tcp: add rx/tx cache to reduce lock contention

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Message-ID: <CAF=yD-LKAsPOFBG28ARt+EhX7GaNG_Ymp9yRDnRib99hKwk33A@mail.gmail.com>
Date:   Thu, 21 Mar 2019 21:54:36 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Florian Westphal <fw@...len.de>,
        Tom Herbert <tom@...bertland.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH v2 net-next 0/3] tcp: add rx/tx cache to reduce lock contention

On Thu, Mar 21, 2019 at 8:16 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On hosts with many cpus we can observe a very serious contention
> on spinlocks used in mm slab layer.
>
> The following can happen quite often :
>
> 1) TX path
>   sendmsg() allocates one (fclone) skb on CPU A, sends a clone.
>   ACK is received on CPU B, and consumes the skb that was in the retransmit
>   queue.
>
> 2) RX path
>   network driver allocates skb on CPU C
>   recvmsg() happens on CPU D, freeing the skb after it has been delivered
>   to user space.
>
> In both cases, we are hitting the asymetric alloc/free pattern
> for which slab has to drain alien caches. At 8 Mpps per second,
> this represents 16 Mpps alloc/free per second and has a huge penalty.
>
> In an interesting experiment, I tried to use a single kmem_cache for all the skbs
> (in skb_init() : skbuff_fclone_cache = skbuff_head_cache =
>                   kmem_cache_create("skbuff_fclone_cache", sizeof(struct sk_buff_fclones),);
> qnd most of the contention disappeared, since cpus could better use
> their local slab per-cpu cache.
>
> But we can do actually better, in the following patches.
>
> TX : at ACK time, no longer free the skb but put it back in a tcp socket cache,
>      so that next sendmsg() can reuse it immediately.
>
> RX : at recvmsg() time, do not free the skb but put it in a tcp socket cache
>    so that it can be freed by the cpu feeding the incoming packets in BH.
>
> This increased the performance of small RPC benchmark by about 10 % on a host
> with 112 hyperthreads.
>
> v2 : - Solved a race condition : sk_stream_alloc_skb() to make sure the prior
>        clone has been freed.
>      - Really test rps_needed in sk_eat_skb() as claimed.
>      - Fixed rps_needed use in drivers/net/tun.c
>
> Eric Dumazet (3):
>   net: convert rps_needed and rfs_needed to new static branch api
>   tcp: add one skb cache for tx
>   tcp: add one skb cache for rx

Acked-by: Willem de Bruijn <willemb@...gle.com>

Thanks Eric!