netdev - Re: GRO after RPS?

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Message-Id: <20100425.181757.10149105.davem@davemloft.net>
Date:	Sun, 25 Apr 2010 18:17:57 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	herbert@...dor.apana.org.au
Cc:	netdev@...r.kernel.org
Subject: Re: GRO after RPS?

From: Herbert Xu <herbert@...dor.apana.org.au>
Date: Mon, 26 Apr 2010 08:49:34 +0800

> On Sun, Apr 25, 2010 at 05:09:33PM -0700, David Miller wrote:
>> 
>> Herbert, after thinking about some ideas we've been discussing and
>> some suggestions from folks like Tom Herbert, I'm thinking of changing
>> it such that we do GRO after RPS sends the packet to a remove cpu.
> 
> Actually, I'd've thought that doing GRO before RPS would be a
> better solution as it means that RPS would have a lot less work
> to do.

Either GRO walks the skb->rxhash values one by one, or GRO does, the
cost is going to be roughly equivalent I think.

And when the packets do match, you can't just trust the ->rxhash in
the GRO code, since a hash match only indicates that the packets might
have the same flow.  So you're going to have to touch the packet
headers in that case on the pre-RPS cpu.

The goal is to eliminate all packet header references from the pre-RPS
path, and let the post-RPS cpu do it.
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