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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.
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