netdev - Re: [patch net-next-2.6] net: convert bonding to use rx

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Message-ID: <20110218192856.GB2602@psychotron.redhat.com>
Date:	Fri, 18 Feb 2011 20:28:57 +0100
From:	Jiri Pirko <jpirko@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	davem@...emloft.net, shemminger@...ux-foundation.org,
	fubar@...ibm.com, nicolas.2p.debian@...il.com, andy@...yhouse.net
Subject: Re: [patch net-next-2.6] net: convert bonding to use rx_handler

Fri, Feb 18, 2011 at 08:17:37PM CET, eric.dumazet@...il.com wrote:
>Le vendredi 18 février 2011 à 19:47 +0100, Jiri Pirko a écrit :
>> Fri, Feb 18, 2011 at 05:14:30PM CET, eric.dumazet@...il.com wrote:
>> >Le vendredi 18 février 2011 à 16:50 +0100, Patrick McHardy a écrit :
>> >> On 18.02.2011 15:58, Jiri Pirko wrote:
>> >> > Fri, Feb 18, 2011 at 03:46:45PM CET, kaber@...sh.net wrote:
>> >> >> Am 18.02.2011 15:27, schrieb Eric Dumazet:
>> >> >>> Le vendredi 18 février 2011 à 15:14 +0100, Jiri Pirko a écrit :
>> >> >>>
>> >> >>>> Do not know how to do it better. As for percpu variable, not only
>> >> >>>> origdev would have to be remembered but also probably skb pointer to
>> >> >>>> know if it's the first run on the skb or not. Can't really figure out a
>> >> >>>> better solution. Can you?
>> >> >>>
>> >> >>> I'll try and let you know.
>> >> >>
>> >> >> Why not simply do a lookup on skb->iif?
>> >> > 
>> >> > Well I was trying to avoid iterating over list of devices for each
>> >> > incoming frame.
>> >> > 
>> >> 
>> >> Well, there are a couple of holes on 64 bit, perhaps you can rearrange
>> >> things and eliminate either iif or input_dev without increasing size
>> >> since they appear to be redundant.
>> >
>> >Jiri
>> >
>> >I dont understand why netif_rx() is needed in your patch.
>> 
>> I used netif_rx() because bridge and macvlan does that too. I did not see
>> a reason to not to do the same.
>> 
>> >
>> >Can we stack 10 bond devices or so ???
>> >
>> >If we avoid this stage and call the real thing (netif_receive_skb()),
>> >then we dont need adding a field in each skb, since it can be carried by
>> >a global variable (per cpu of course)
>> >
>> I'm probably missing something. How do netif_receive_skb() and
>> netif_rx() differ in this point of view, since both are calling:
>> "ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);"
>> ?
>> 
>> Still I see a problem with the percpu global variable. We would have to
>> store skb pointer there as well and in each __netif_receive_skb() call it
>> would have to be checked if it's different from the current one.
>> In that case store new skb and orig_Dev.
>> 
>> Leaving aside that global variables are evil in general, I still think
>> this is not nicer solution then to add skb->input_dev (although I
>> understand your arguments).
>
>Really I must miss something about "global variables" thing/fear.
>
>Kernel is full of global variables, they are not evil if properly used.

I know. But that doesn't mean it's ok. But I see your point.

>
>Take a look at net/core/dev.c :
>
>static DEFINE_PER_CPU(int, xmit_recursion);
>
>For an example of what I have in mind.

Yes I saw this. We would have to do something like:

struct skb_rx_context {
	struct sk_buff *skb;
	struct net_device *orig_dev;
};

static DEFINE_PER_CPU(struct skb_rx_context, skb_rx_context);

and then in __netif_receive_skb():

struct skb_rx_context *cont = __this_cpu_read(skb_rx_context);

if (cont->skb != skb) {
	cont->skb = skb;
	orig_dev = cont->orig_dev = skb->dev;
} else {
	orig_dev = cont->orig_dev;
}


Does this make sense?

>
>
>
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