netdev - Re: [PATCH RFC] act_cpu: packet distributing

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Message-ID: <1279081804.2444.117.camel@edumazet-laptop>
Date:	Wed, 14 Jul 2010 06:30:04 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Jamal Hadi Salim <hadi@...erus.ca>,
	"David S. Miller" <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>,
	Tom Herbert <therbert@...gle.com>, netdev@...r.kernel.org
Subject: Re: [PATCH RFC] act_cpu: packet distributing

Le mercredi 14 juillet 2010 à 12:17 +0800, Changli Gao a écrit :
> On Wed, Jul 14, 2010 at 11:41 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:

> >
> > Why doing the search again, in case skb->sk already set by another
> > module before you, like tproxy ?
> >
> 
> Although it is unlikely skb->sk is non null, as tc is before
> netfilter, I will handle this case. Thanks.
> 

In this case, provide the skb->sk already set case on tproxy, not in
act_cpu. But doing it on both will give a hint for future modules...

> >> +static int tcf_cpu(struct sk_buff *skb, struct tc_action *a,
> >> +                struct tcf_result *res)
> >> +{
> >> +     struct tcf_cpu *p = a->priv;
> >> +     u32 type;
> >> +     u32 value;
> >> +     int cpu, action;
> >> +     struct sk_buff *nskb;
> >> +     unsigned int qtail;
> >> +
> >> +     spin_lock(&p->tcf_lock);
> >
> > Ok, the big lock...
> >
> > We have a lockless TCP/UDP input path, and this modules adds a lock
> > again.
> >
> >> +     p->tcf_tm.lastuse = jiffies;
> >> +     p->tcf_bstats.bytes += qdisc_pkt_len(skb);
> >> +     p->tcf_bstats.packets++;
> >> +     type = p->type;
> >> +     value = p->value;
> >> +     action = p->tcf_action;
> >> +     spin_unlock(&p->tcf_lock);
> >> +
> >
> > Please change all this crap  (legacy crap, copied from other tc
> > modules), by modern one, using RCU and no locking in hot path.
> >
> 
> Thanks, I'll try. It is a write critical section, and for me it is
> difficult to convert this lock to RCU. Could you show me some
> examples?

We can convert bytes/packets stats to percpu stats for example.
That might need a generic change.

Then, a normal RCU protection should be enough to fetch
type/value/action fields.



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