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

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Date:	Sat, 5 Jun 2010 22:15:01 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	hadi@...erus.ca
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, Jun 5, 2010 at 9:54 PM, jamal <hadi@...erus.ca> wrote:
> 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.

For instance: there are 4 CPUs. I want redirect traffic to CPU 1-3
evenly. If the qdisc is linear the rules as

flow classify(flow classid ffff:2-4) | tc_index 2 action cpu 1 |
tc_index 3 action cpu 2 | tc_index 4 action cpu3

a tree variant:

class ffff:1 : flow classify(flow classid ffff:2-4)
class ffff:2 parent ffff:1 : action cpu 1
class ffff:3 parent ffff:1 : action cpu 2
class ffff:4 parent ffff:1 : action cpu 3

ingress_classify: use flow classify to get the subclass ID, then find
the corresponding class and exec action.

When there are lots of CPUs, tree is more efficient.

>
>> 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.
>

It seems AMD specific. Why do the AMD guys use this to implement async
smp_call_function() if it is useful as you said?

-- 
Regards,
Changli Gao(xiaosuo@...il.com)
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