linux-kernel - Re: [PATCH 00/10] jump label: introduce very

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Message-ID: <20120223100205.GD24310@elte.hu>
Date:	Thu, 23 Feb 2012 11:02:05 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jason Baron <jbaron@...hat.com>, a.p.zijlstra@...llo.nl,
	mathieu.desnoyers@...icios.com, davem@...emloft.net,
	ddaney.cavm@...il.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups +
 docs


* Paul Mackerras <paulus@...ba.org> wrote:

> On Wed, Feb 22, 2012 at 09:18:55AM +0100, Ingo Molnar wrote:
> 
> > The problem with static_branch_def_false/def_true was that the 
> > very intuitively visible bias that we see with 
> > likely()/unlikely() is confused in jump label constructs through 
> > two layers of modifiers. And the fix is so easy, a simple rename 
> > in most cases ;-)
> > 
> > So instead of that, in this series we have:
> > 
> > +       if (very_unlikely(&perf_sched_events.key))
> > 
> > which is a heck of an improvement IMO. I'd still up its 
> > readability a notch, by also signalling the overhead of the 
> > update path by making it:
> > 
> > +       if (very_unlikely(&perf_sched_events.slow_flag))
> > 
> > ... but I don't want to be that much of a readability nazi ;-)
> 
> I have to say I don't like the "very_unlikely" name.  It's 
> confusing because the condition being evaluated appears to be 
> the address of something, i.e. &perf_sched_events.key in your 
> example, and that looks to me to be very very likely to be 
> true, i.e. non-zero.  But the code is telling me that's very 
> *un*likely, which is confusing.

Having to take the address gives us type safety - i.e. it will 
not be possible to accidentally pass in a non-jump-label key and 
get it misinterpreted.

If some macro magic could be used to remove the address taking 
I'd be in favor of such a simplification, i.e.:

	if (very_unlikely(perf_sched_events.key))

which should address your observation.

Thanks,

	Ingo
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