netdev - Re: [RFC] act_cpu: redirect skb receiving to a special CPU.

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Date:	Sat, 05 Jun 2010 09:54:05 -0400
From:	jamal <hadi@...erus.ca>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [RFC] act_cpu: redirect skb receiving to a special CPU.

On Sat, 2010-06-05 at 21:26 +0800, Changli Gao wrote:

> ingress doesn't have any qdisc, but a class tree. The ingress_queue
> will be sth. like this:
[..]

> Then we can classify skbs in tree  manner.
[..]
> > The cpuid should be sufficient to map to a remote cpu queue, no?
> 
> It should be sufficient, but it isn't efficient. With map option, we
> can use cls_flow to map traffic to classid, and use act_cpu map to map
> classid to cpuid.

I am missing something, I would see the flow as:
-->ethx/lo/etc->ingressqdisc->classify-->action(redirect to cpuidX)
Why/when do you need the tree variant? If you are thinking of maybe
rate limiting to a specific CPU, then would passing it to a policer
first not be sufficient?  IOW, classid is not very useful.

> I won't implement a new netdevice, but reuse the softnet. Even, I'll
> reuse the enqueue_to_backlog() introduced by RPS, and of course, use
> IPIs as RPS. Is there another way to trigger an IRQ of the remote CPU?

I would look at it as "messaging of remote CPU" which may not result
in an IRQ. I am pretty sure if you tried hard you could use HT in AMD
hardware - the remote cpu may have an IRQ triggered but it wont be as
expensive as IPI.

cheers,
jamal

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