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

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Date:   Thu, 21 Mar 2019 18:25:30 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        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 net-next 0/3] tcp: add rx/tx cache to reduce lock contention

On Thu, Mar 21, 2019 at 6:17 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On hosts with many cpus we can observe a very verious 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 alocates 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.
>
> 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: Soheil Hassas Yeganeh <soheil@...gle.com>

This is a really impressive improvement! Thank you, Eric!

>  include/linux/netdevice.h  |  4 ++--
>  include/net/sock.h         | 13 +++++++++-
>  net/core/dev.c             | 10 ++++----
>  net/core/net-sysfs.c       |  4 ++--
>  net/core/sysctl_net_core.c |  8 +++----
>  net/ipv4/af_inet.c         |  4 ++++
>  net/ipv4/tcp.c             | 49 +++++++++++++++++---------------------
>  net/ipv4/tcp_ipv4.c        | 11 +++++++--
>  net/ipv6/tcp_ipv6.c        | 12 +++++++---
>  9 files changed, 69 insertions(+), 46 deletions(-)
>
> --
> 2.21.0.225.g810b269d1ac-goog
>