netdev - Re: [PATCH] net: Check skb->rxhash in gro

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Date:	Fri, 10 Jan 2014 08:47:58 -0800
From:	Tom Herbert <therbert@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: Check skb->rxhash in gro_receive

On Thu, Jan 9, 2014 at 9:43 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2014-01-09 at 21:38 -0800, Eric Dumazet wrote:
>
>> Hmm... this idea was discussed in the past. I used it when attempting to
>> use a hash table instead of a gro_list last year.
>>
>> Unfortunately this added lot of cycles when rxhash is not provided by
>> hardware, and my tests found it was not a win.
>>
>> Remember : in most cases, gro_list contains one flow, so this test does
>> nothing special but adds overhead.
>
> Additional point : For napi_gro_frag() users, skb->head do not contains
> headers, so flow dissector enters slow path to fetch the headers from
> the fragment, one by one.
>
skb_get_hash is only called for the new skb, we compare against
skb->rxhash for packets on the gro_list. Also, I still intend to
change skb_get_hash to not repeatedly compute the hash when there's no
L4 hash to be found.

>
>
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