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

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Message-ID: <CANn89iL4ViyMQ3gm32K6LqfLWEvTeGSn27j729d1x3vaDyCzXQ@mail.gmail.com>
Date: Tue, 13 Feb 2024 15:28:34 +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 3:17 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 2/13/24 13:53, Eric Dumazet wrote:
> > 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();
>
> Maybe it's my misunderstanding but disabled bh != "napi safe",
> e.g. the napi_struct we're interested in might be scheduled for
> another CPU. Which is also why "napi" prefix in percpu
> napi_alloc_cache sounds a bit misleading to me.

Indeed, this is very misleading.

napi_skb_cache_put() & napi_skb_cache_get() should be renamed eventually.

Thanks.