netdev - Re: [RFC] Kernel unaligned access at __skb_flow

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Date:	Fri, 29 Jan 2016 14:08:41 -0800
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Sowmini Varadhan <sowmini.varadhan@...cle.com>,
	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>
Subject: Re: [RFC] Kernel unaligned access at __skb_flow_dissect

On Fri, Jan 29, 2016 at 1:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2016-01-29 at 13:16 -0800, Alexander Duyck wrote:
>
>> It has to be something recent.  I know back when I wrote the code I
>> tested it on a few different architectures and had to add a few bits
>> in __skb_get_poff so that it would read doff as a u8 instead of
>> bitfield in a u32.
>>
>> Looking at the code it seems like this should be an easy fix.  Just
>> swap the two lines that acquire and test the flow label with the check
>> for dissector_uses_key(... _KEY_FLOW_LABEL).  Then ixgbe will stop
>> trying to grab flow labels.
>
> Note that if we properly align headers, we also align TCP/UDP payload.
>
> Meaning that copying data in recvmsg() is likely faster in some cpus.
> (The ones having NET_IP_ALIGN set to 2 presumably)

It also means DMA becomes dramatically slower as it introduces a
partial write access for the start of every frame.  It is why we had
set NET_IP_ALIGN to 0 on x86 since DMA was becoming more expensive
when unaligned then reading IP unaligned headers.

The gain on recvmsg would probably be minimal.  The only time I have
seen any significant speed-up for copying is if you can get both ends
aligned to something like 16B.

- Alex