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

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Message-ID: <CANn89iKyg6ryhCnUxsGdjjRrpqWzP9MpZtzttjNXXpB1jXn5sA@mail.gmail.com>
Date: Wed, 7 Feb 2024 16:56:07 +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 4:50 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 2/7/24 15:26, Eric Dumazet wrote:
> > 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...
>
> I'll add the change, thanks
>
> >> +               __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.
>
> I played with some naive batching approaches there before but couldn't
> get anything out of it. From my observations,  skb_attempt_defer_free was
> rarely getting SKBs targeting the same CPU, but there are probably irq
> affinity configurations where it'd make more sense.

Well, you mentioned a high cost in cpu profiles for skb_attempt_defer_free()

This is what my patch was trying to reduce. Reducing false sharing
(acquiring remote spinlocks) was the goal.


>
> Just to note that this patch is targeting cases with perfect affinity, so
> it's orthogonal or complimentary to defer batching.
>
> --
> Pavel Begunkov