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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 22 Mar 2019 07:28:33 -0400

On Thu, Mar 21, 2019 at 05:14:41PM -0700, Eric Dumazet 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

Just a thought: would it make sense to flush the cache
in enter_memory_pressure?

> 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
> 
>  drivers/net/tun.c          |  2 +-
>  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             | 54 +++++++++++++++++++-------------------
>  net/ipv4/tcp_ipv4.c        | 11 ++++++--
>  net/ipv6/tcp_ipv6.c        | 12 ++++++---
>  10 files changed, 75 insertions(+), 47 deletions(-)
> 
> -- 
> 2.21.0.225.g810b269d1ac-goog