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SubjectRe: [PATCH v4 net-next 08/11] skbuff: introduce {,__}napi_build_skb() which reuses NAPI cache heads
On Wed, 10 Feb 2021 16:30:23 +0000
Alexander Lobakin <alobakin@pm.me> 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 <ecree.xilinx@gmail.com> # Divide on two halves
> Suggested-by: Eric Dumazet <edumazet@google.com> # KASAN poisoning
> Cc: Dmitry Vyukov <dvyukov@google.com> # Help with KASAN
> Cc: Paolo Abeni <pabeni@redhat.com> # Reduced batch size
> Signed-off-by: Alexander Lobakin <alobakin@pm.me>
> ---
> 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
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