netdev - Re: [PATCH net-next v2 3/3] udp: try to avoid 2 cache miss on dequeue

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Message-Id: <20170607.101056.833234534371053047.davem@davemloft.net>
Date:   Wed, 07 Jun 2017 10:10:56 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     netdev@...r.kernel.org, edumazet@...gle.com
Subject: Re: [PATCH net-next v2 3/3] udp: try to avoid 2 cache miss on
 dequeue

From: Paolo Abeni <pabeni@...hat.com>
Date: Wed, 07 Jun 2017 09:56:45 +0200

> Hi David,
> 
> On Tue, 2017-06-06 at 16:23 +0200, Paolo Abeni wrote:
>> when udp_recvmsg() is executed, on x86_64 and other archs, most skb
>> fields are on cold cachelines.
>> If the skb are linear and the kernel don't need to compute the udp
>> csum, only a handful of skb fields are required by udp_recvmsg().
>> Since we already use skb->dev_scratch to cache hot data, and
>> there are 32 bits unused on 64 bit archs, use such field to cache
>> as much data as we can, and try to prefetch on dequeue the relevant
>> fields that are left out.
>> 
>> This can save up to 2 cache miss per packet.
>> 
>> v1 -> v2:
>>   - changed udp_dev_scratch fields types to u{32,16} variant,
>>     replaced bitfield with bool
>> 
>> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> 
> Can you please keep on-hold this series a little time? the lkp-robot
> just reported a performance regression on v1 which I have still to
> investigate. I can't look at it really soon, but I expect the same
> should apply to v2.
> 
> It sounds quite weird to me, since the bisected patch touches the UDP
> code only and the regression is on apachebench.

Hmmm, DNS lookups?

Thanks for looking into this.