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

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Date: Mon, 18 Mar 2024 15:33:05 +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 v2] net: cache for same cpu skb_attempt_defer_free

On Mon, Mar 18, 2024 at 12:43 PM Pavel Begunkov <asml.silence@...il.com> wrote:
>
> On 3/18/24 10:11, Eric Dumazet wrote:
> > On Mon, Mar 18, 2024 at 1:46 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'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: 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);
> >> +       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
> >>
> >
> > 1) net-next is currently closed.
>
> Ok
>
> > 2) No NUMA awareness. SLUB does not guarantee the sk_buff was on the
> > correct node.
>
> Let me see if I read you right. You're saying that SLUB can
> allocate an skb from a different node, so skb->alloc_cpu
> might be not indicative of the node, and so we might locally
> cache an skb of a foreign numa node?
>
> If that's the case I don't see how it's different from the
> cpu != raw_smp_processor_id() path, which will transfer the
> skb to another cpu and still put it in the local cache in
> softirq.

The big win for skb_attempt_defer_free() is for the many pages that are attached
to TCP incoming GRO packets.

A typical GRO packet can have 45 page fragments.

Pages are not recycled by a NIC if the NUMA node does not match.

If the win was only for sk_buff itself, I think we should have asked
SLUB maintainers
why SLUB needs another cache to optimize SLUB fast cache !


>
>
> > 3) Given that many skbs (like TCP ACK) are freed using __kfree_skb(),  I wonder
> > why trying to cache the sk_buff in this particular path is needed.
> >
> > Why not change __kfree_skb() instead ?
>
> IIRC kfree_skb() can be called from any context including irqoff,
> it's convenient to have a function that just does the job without
> too much of extra care. Theoretically it can have a separate path
> inside based on irqs_disabled(), but that would be ugly.

Well, adding one case here, another here, and another here is also ugly.