linux-kernel - Re: jump_label defaults (was Re: [patch 00/17] CFS Bandwidth Control v7.1)

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Message-ID: <20110708154757.GB2438@redhat.com>
Date:	Fri, 8 Jul 2011 11:47:57 -0400
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	linux-kernel@...r.kernel.org,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Hu Tao <hutao@...fujitsu.com>, Mike Galbraith <efault@....de>
Subject: Re: jump_label defaults (was Re: [patch 00/17] CFS Bandwidth Control
 v7.1)

On Fri, Jul 08, 2011 at 11:20:32AM +0200, Peter Zijlstra wrote:
> On Thu, 2011-07-07 at 22:36 +0200, Peter Zijlstra wrote:
> > Hrm,. I can't seem to make that work, damn CPP for not being
> > recursive.
> 
> Ha! the wonders of sleep, ok I can make this part work.
> 
> So how do we write this static_branch_true() thing?
> 
> But then I realized that if we do something like:
> 
> static __always_inline bool arch_static_branch(struct jump_label_key *key)
> {
>         asm goto("1:"
>                 JUMP_LABEL_INITIAL_NOP
>                 ".pushsection __jump_table,  \"aw\" \n\t"
>                 _ASM_ALIGN "\n\t"
>                 _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
>                 ".popsection \n\t"
>                 : :  "i" (key) : : l_yes);
>         return false;
> l_yes:
>         return true;
> }
> 
> Simply flipping the true and false in there isn't going to work, because
> then its similar to !static_branch() and jump_label_inc() is going to
> disable it.

right, you'd also have to take into account the original state of branch
in deciding whether or not to call jump_label_inc()/dec()


thanks,

-Jason
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