netdev - Re: [PATCH v2] net: cache for same cpu skb_attempt_defer

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Message-ID: <CANn89iJBQLv7JKq5OUYu7gv2y9nh4HOFmG_N7g1S1fVfbn=-uA@mail.gmail.com>
Date: Tue, 13 Feb 2024 14:53:19 +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 v2] net: cache for same cpu skb_attempt_defer_free

On Tue, Feb 13, 2024 at 2:42 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>
> ---
>
> v2: remove in_hardirq()
>
>  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 9b790994da0c..f32f358ef1d8 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6947,6 +6947,20 @@ 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) {
> +               __kfree_skb(skb);
> +               return;
> +       }
> +
> +       local_bh_disable();
> +       skb_release_all(skb, SKB_DROP_REASON_NOT_SPECIFIED, false);

I am trying to understand why we use false instead of true here ?
Or if you prefer:
local_bh_disable();
__napi_kfree_skb(skb, SKB_DROP_REASON_NOT_SPECIFIED);
local_bh_enable();


> +       napi_skb_cache_put(skb);
> +       local_bh_enable();
> +}
> +
>  /**
>   * skb_attempt_defer_free - queue skb for remote freeing
>   * @skb: buffer
> @@ -6965,7 +6979,7 @@ void skb_attempt_defer_free(struct sk_buff *skb)
>         if (WARN_ON_ONCE(cpu >= nr_cpu_ids) ||
>             !cpu_online(cpu) ||
>             cpu == raw_smp_processor_id()) {
> -nodefer:       __kfree_skb(skb);
> +nodefer:       kfree_skb_napi_cache(skb);
>                 return;
>         }
>
> --
> 2.43.0
>