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

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Message-ID: <CAL+tcoA=3KNFGNv4DSqnWcUu4LTY3Pz5ex+fRr4LkyS8ZNNKwQ@mail.gmail.com>
Date: Mon, 18 Mar 2024 10:44:05 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Pavel Begunkov <asml.silence@...il.com>
Cc: netdev@...r.kernel.org, edumazet@...gle.com, davem@...emloft.net, 
	dsahern@...nel.org, pabeni@...hat.com, kuba@...nel.org
Subject: Re: [PATCH net-next v2] net: cache for same cpu skb_attempt_defer_free

On Mon, Mar 18, 2024 at 8:46 AM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> Optimise skb_attempt_defer_free() when run by the same 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.
>
> CPU bound TCP ping pong style benchmarking (i.e. netbench) showed a 1%
> throughput increase (392.2 -> 396.4 Krps). Cross checking with profiles,
> the total CPU share of skb_attempt_defer_free() dropped by 0.6%. Note,

I suspect that we can stably gain this improvement. The reason why I
ask is because it might be caused by some factor of chance.

> 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.

I wonder how you did this test? Could you tell us more, please.

>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>
> v2: pass @napi_safe=true by using __napi_kfree_skb()
>
>  net/core/skbuff.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index b99127712e67..35d37ae70a3d 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -6995,6 +6995,19 @@ 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();
> +       __napi_kfree_skb(skb, SKB_DROP_REASON_NOT_SPECIFIED);

__napi_kfree_skb() doesn't care much about why we drop in the rx path,
I think. How about replacing it with SKB_CONSUMED like
napi_skb_finish() does?

Thanks,
Jason

> +       local_bh_enable();
> +}
> +
>  /**
>   * skb_attempt_defer_free - queue skb for remote freeing
>   * @skb: buffer
> @@ -7013,7 +7026,7 @@ 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;
>         }
>
> --
> 2.44.0
>
>