netdev - Re: [PATCH RESEND net-next v3] net: cache for same cpu skb_attempt_defer

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Message-ID: <d475498f-f6db-476c-8c33-66c9f6685acf@gmail.com>
Date: Fri, 5 Apr 2024 12:55:58 +0100
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 RESEND net-next v3] net: cache for same cpu
 skb_attempt_defer_free

On 4/5/24 09:46, Eric Dumazet wrote:
> On Fri, Apr 5, 2024 at 1:38 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>
>> ---
>>
>> v3: rebased, no changes otherwise
>>
>> 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 2a5ce6667bbb..c4d36e462a9a 100644
>> --- a/net/core/skbuff.c
>> +++ b/net/core/skbuff.c
>> @@ -6968,6 +6968,19 @@ 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);
> 
> This needs to be SKB_CONSUMED

Net folks and yourself were previously strictly insisting that
every patch should do only one thing at a time without introducing
unrelated changes. Considering it replaces __kfree_skb, which
passes SKB_DROP_REASON_NOT_SPECIFIED, that should rather be a
separate patch.

-- 
Pavel Begunkov