netdev - Re: [RFC net-next] netpoll: use static branch

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Message-ID: <20120919130059.7b7ebc83@s6510.linuxnetplumber.net>
Date:	Wed, 19 Sep 2012 13:00:59 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Cong Wang <amwang@...hat.com>
Cc:	David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>, netdev@...r.kernel.org
Subject: Re: [RFC net-next] netpoll: use static branch

On Wed, 19 Sep 2012 12:50:10 +0800
Cong Wang <amwang@...hat.com> wrote:

> On Tue, 2012-09-18 at 14:10 -0700, Stephen Hemminger wrote:
> > This is an attempt to optimize netpoll when not used.
> > 
> > Since distro's enable everything and netpoll is only occasionally
> > used, improve performance by getting netpoll condition check
> > out of the Rx fastpath.
> > 
> > Compile tested only, I have no real use for netpoll.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
> > 
> > 
> > ---
> >  include/linux/netpoll.h |   28 ++++++++++++++++++++--------
> >  net/core/netpoll.c      |    8 +++++++-
> >  2 files changed, 27 insertions(+), 9 deletions(-)
> > 
> > --- a/include/linux/netpoll.h	2012-09-18 13:25:15.575750004 -0700
> > +++ b/include/linux/netpoll.h	2012-09-18 13:29:16.245323347 -0700
> > @@ -66,10 +66,16 @@ static inline void netpoll_send_skb(stru
> >  
> > 
> >  #ifdef CONFIG_NETPOLL
> > +extern struct static_key netpoll_needed;
> > +
> >  static inline bool netpoll_rx_on(struct sk_buff *skb)
> >  {
> > -	struct netpoll_info *npinfo = rcu_dereference_bh(skb->dev->npinfo);
> > +	struct netpoll_info *npinfo;
> > +
> > +	if (static_key_true(&netpoll_needed))
> > +		return false;
> >  
> 
> I think we should use static_key_false() here, as netpoll is an
> "unlikely" code path.
> 
> Using static branch is a good idea though.
> 
> Thanks.

But static_key_true is just a wrapper around !static_key_false()
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