Subject: Re: [PATCH v4 net-next 08/11] skbuff: introduce {,__}napi_build_skb() which reuses NAPI cache heads

From: Jesper Dangaard Brouer <[email-masked]>
Date: Thu, 11 Feb 2021 13:54:59 +0100

On Wed, 10 Feb 2021 16:30:23 +0000
Alexander Lobakin <[email-masked]> wrote:

> Instead of just bulk-flushing skbuff_heads queued up through
> napi_consume_skb() or __kfree_skb_defer(), try to reuse them
> on allocation path.

Maybe you are already aware of this dynamics, but high speed NICs will
usually run the TX "cleanup" (opportunistic DMA-completion) in the napi
poll function call, and often before processing RX packets. Like
ixgbe_poll[1] calls ixgbe_clean_tx_irq() before ixgbe_clean_rx_irq().

If traffic is symmetric (or is routed-back same interface) then this
SKB recycle scheme will be highly efficient. (I had this part of my
initial patchset and tested it on ixgbe).

[1] https://elixir.bootlin.com/linux/v5.11-rc7/source/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c#L3149

> If the cache is empty on allocation, bulk-allocate the first
> 16 elements, which is more efficient than per-skb allocation.
> If the cache is full on freeing, bulk-wipe the second half of
> the cache (32 elements).
> This also includes custom KASAN poisoning/unpoisoning to be
> double sure there are no use-after-free cases.
> 
> To not change current behaviour, introduce a new function,
> napi_build_skb(), to optionally use a new approach later
> in drivers.
> 
> Note on selected bulk size, 16:
>  - this equals to XDP_BULK_QUEUE_SIZE, DEV_MAP_BULK_SIZE
>    and especially VETH_XDP_BATCH, which is also used to
>    bulk-allocate skbuff_heads and was tested on powerful
>    setups;
>  - this also showed the best performance in the actual
>    test series (from the array of {8, 16, 32}).
> 
> Suggested-by: Edward Cree <[email-masked]> # Divide on two halves
> Suggested-by: Eric Dumazet <[email-masked]>   # KASAN poisoning
> Cc: Dmitry Vyukov <[email-masked]>             # Help with KASAN
> Cc: Paolo Abeni <[email-masked]>                # Reduced batch size
> Signed-off-by: Alexander Lobakin <[email-masked]>
> ---
>  include/linux/skbuff.h |  2 +
>  net/core/skbuff.c      | 94 ++++++++++++++++++++++++++++++++++++------
>  2 files changed, 83 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 0e0707296098..906122eac82a 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -1087,6 +1087,8 @@ struct sk_buff *build_skb(void *data, unsigned int frag_size);
>  struct sk_buff *build_skb_around(struct sk_buff *skb,
>  				 void *data, unsigned int frag_size);
>  
> +struct sk_buff *napi_build_skb(void *data, unsigned int frag_size);
> +
>  /**
>   * alloc_skb - allocate a network buffer
>   * @size: size to allocate
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 860a9d4f752f..9e1a8ded4acc 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -120,6 +120,8 @@ static void skb_under_panic(struct sk_buff *skb, unsigned int sz, void *addr)
>  }
>  
>  #define NAPI_SKB_CACHE_SIZE	64
> +#define NAPI_SKB_CACHE_BULK	16
> +#define NAPI_SKB_CACHE_HALF	(NAPI_SKB_CACHE_SIZE / 2)
>  

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer