netdev - Re: Tx queue selection

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Date:	Tue, 27 Jul 2010 13:57:52 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: Tx queue selection

Le mardi 27 juillet 2010 à 20:51 +1000, Benjamin Herrenschmidt a écrit :
> Hi folks !
> 
> I'm putting my newbie hat on ... :-)
> 
> While looking at our ehea driver (and in fact another upcoming driver
> I'm helping with), I noticed it's using the "old style" multiqueue. IE.
> It doesn't use the alloc_netdev_mq() variant, creates one queue on the
> linux side, an makes its own selection of HW queue in start_xmit.
> 
> This had many drawbacks, obviously, such as not getting per-queue locks
> etc...
> 
> Now, the mechanics of converting that to the new scheme are easy enough
> to figure out by reading the code. However, where my lack of networking
> background fails me is when it comes to the policy of choosing a Tx
> queue.
> 
> ehea uses its own hash of the header, different from the "default" queue
> selection in the net core. Looking at other drivers such as ixgbe, I see
> that it can chose to use smp_processor_id() when a flag is set for which
> I don't totally understand the meaning or default to the core algorithm.
> 
> Now, while I can understand why it's a good idea to use the current
> processor, in order to limit cache ping pong etc... I'm not really
> confident I understand the pro/cons of using the hashing for tx. I
> understand that the net core can play interesting games with associating
> sockets with queues etc... but I'm a bit at a loss when it comes to
> deciding what's best for this driver. I suppose I could start by
> implementing my own queue selection based on what ehea does today but I
> have the nasty feeling that's going to be sub-optimal :-)
> 
> So I would very much appreciate (and reward with free beer at the next
> conference) if somebody could give me a bit of a heads up on how things
> are expected to be done there, pro/cons, perf impact etc...

I am not sure ndo_select_queue() is really needed these days. It was
done before core network was able to use a socket provided hash.

tx queue selection done by default (skb_tx_hash()) should be fine.

bnx2 for example doesnt provide a ndo_select_queue()



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