netdev - Re: [PATCH] NIU support for skb->rxhash

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Message-Id: <20100422.141922.39169749.davem@davemloft.net>
Date:	Thu, 22 Apr 2010 14:19:22 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH] NIU support for skb->rxhash

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 22 Apr 2010 13:43:18 +0200

> Then, our stack also touch all 256 bytes of skb structure itself.
> 
> offsetof(struct sk_buff, next)    =0x0
> offsetof(struct sk_buff, rxhash)  =0xa8
> offsetof(struct sk_buff, dev)     =0x20
> offsetof(struct sk_buff, len)     =0x68
> offsetof(struct sk_buff, protocol)=0x7e
> offsetof(struct sk_buff, network_header)=0xc0
> offsetof(struct sk_buff, data)    =0xd8
> offsetof(struct sk_buff, head)    =0xd0
> 
> Time for a reordering I guess ;)

Indeed.

Also I have some ideas about what we can do if we have
just the rxhash.  It seems we can avoid the type_trans
overhead on the interrupting cpu.

Things like eth_type_trans() become a netdev operation rather than
something drivers statically call by hand. ->ndo_type_trans or
similar.

SKB has a state bit saying whether ->ndo_type_trans has been invoked
yet on RX.

Drivers pass raw SKBs up into the stack.

We defer the ->ndo_type_trans as far as possible, for RPS when we have
->rxhash we can defer this all the way to the destination RPS cpu.

If we lack ->rxhash, the source cpu will need to invoke
->ndo_type_trans before it can begin parsing the packet.

Anyways, something like that.
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