linux-kernel - Re: [PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state machine

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Message-ID: <20130907185710.GF3966@linux.vnet.ibm.com>
Date:	Sat, 7 Sep 2013 11:57:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Yann E. MORIN" <yann.morin.1998@...e.fr>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, laijs@...fujitsu.com,
	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	josh@...htriplett.org, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>, edumazet@...gle.com,
	darren@...art.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	sbw@....edu, Linux-Arch <linux-arch@...r.kernel.org>,
	linux-kbuild <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state
 machine

On Sat, Sep 07, 2013 at 11:13:48AM +0200, Yann E. MORIN wrote:
> Paul, All,
> 
> On 2013-09-06 10:30 -0700, Paul E. McKenney spake thusly:
> [--SNIP--]
> > I also tried using Kconfig "if":
> > 
> > if SMP
> > config NR_CPUS_REALLY
> > 	int "Fixed version of NR_CPUS"
> > 	default NR_CPUS
> > endif
> > if !SMP
> > config NR_CPUS_REALLY
> > 	int "Fixed version of NR_CPUS"
> > 	default 1 if !SMP
> 
> The 'if !SMP' here is unneeded, you're already in a 'if !SMP' if-block.

Agreed, though I get the same result even without the !SMP.

> > endif
> > 
> > However, Kconfig complained about the use of NR_CPUS even though this
> > was under an "if" whose condition was not set.  Perhaps someone with
> > better Kconfig-fu than I have can come up with something.
> 
> That's because the 'if' condition is added to the dependency list of the
> symbol(s) that is(are) enclosed in the if.
> 
> 'if' in Kconfig behaves the same way as an 'if' in C. What you expected
> (I believe) was the behaviour of '#ifdef', which is not the case. From
> Documentation/kbuild/kconfig-language.txt:
> 
>     ---8<---
>     if:
> 
>             "if" <expr>
>             <if block>
>             "endif"
> 
>     This defines an if block. The dependency expression <expr> is appended
>     to all enclosed menu entries.
>     ---8<---

OK, I did read this, but misunderstood it.

> There's no equivlaent to '#ifdef' in Kconfig.
> 
> I'll see if I can come up with a meaningfull construct that fixes your
> use-case. Don't hold your breath, though! ;-)

If not, we need to add NR_CPUS to the architectures lacking them...

								Thanx, Paul

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