linux-kernel - Re: [RFC] per-cpu preempt

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Message-ID: <20130813155637.GA5280@gmail.com>
Date:	Tue, 13 Aug 2013 17:56:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <bitbucket@...ine.de>,
	Peter Anvin <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] per-cpu preempt_count


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, Aug 13, 2013 at 5:26 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > So we're now down to something like:
> >
> >   decl fs:preempt_count
> >   cmpl PREEMPT_NEED_RESCHED,fs:preempt_count
> >   jnz 1f
> 
> Well, this isn't worth doing unless you can make PREEMPT_NEED_RESCHED be 
> the high bit, and we can combine it into just "decl+jns". Otherwise we'd 
> be better off with the simpler two separate adjacent variables.

Definitely, the cmpl should be avoided.

PREEMPT_NEED_RESCHED could be made the high bit - or maybe an even simpler 
solution is to invert its meaning: making '0' the "it needs to resched!" 
case, so the check would be decl+jz?

Thanks,

	Ingo
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