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

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Date: Mon, 18 Mar 2024 11:41:26 +0000
From: Pavel Begunkov <asml.silence@...il.com>
To: Eric Dumazet <edumazet@...gle.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 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.


> 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.

-- 
Pavel Begunkov