netdev - Re: [PATCH net-next] net: cache for same cpu skb_attempt_defer

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Date: Wed, 7 Feb 2024 16:26:08 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, dsahern@...nel.org, 
	pabeni@...hat.com, kuba@...nel.org
Subject: Re: [PATCH net-next] net: cache for same cpu skb_attempt_defer_free

On Wed, Feb 7, 2024 at 3:42 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> Optimise skb_attempt_defer_free() executed by the CPU the skb was
> allocated on. Instead of __kfree_skb() -> kmem_cache_free() we can
> disable softirqs and put the buffer into cpu local caches.
>
> Trying it with a TCP CPU bound ping pong benchmark (i.e. netbench), it
> showed a 1% throughput improvement (392.2 -> 396.4 Krps). Cross checking
> with profiles, the total CPU share of skb_attempt_defer_free() dropped by
> 0.6%. Note, I'd expect the win doubled with rx only benchmarks, as the
> optimisation is for the receive path, but the test spends >55% of CPU
> doing writes.
>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  net/core/skbuff.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index edbbef563d4d..5ac3c353c8a4 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6877,6 +6877,20 @@ void __skb_ext_put(struct skb_ext *ext)
>  EXPORT_SYMBOL(__skb_ext_put);
>  #endif /* CONFIG_SKB_EXTENSIONS */
>
> +static void kfree_skb_napi_cache(struct sk_buff *skb)
> +{
> +       /* if SKB is a clone, don't handle this case */
> +       if (skb->fclone != SKB_FCLONE_UNAVAILABLE || in_hardirq()) {

skb_attempt_defer_free() can not run from hard irq, please do not add
code suggesting otherwise...

> +               __kfree_skb(skb);
> +               return;
> +       }
> +
> +       local_bh_disable();
> +       skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED, false);
> +       napi_skb_cache_put(skb);
> +       local_bh_enable();
> +}
> +

I had a patch adding local per-cpu caches of ~8 skbs, to batch
sd->defer_lock acquisitions,
it seems I forgot to finish it.