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

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Date:	Thu, 23 Feb 2012 08:33:44 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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.

In other words I can't think about "very_unlikely" as being similar to
"unlikely".  Clearly very_unlikely isn't just passing through the
true/falseness of its argument to the surrounding if, the way that
unlikely does; it (very_unlikely) is a real function that is doing
something more complicated with its argument -- dereferencing it,
conceptually at least, for a start.

Just from a readability perspective, I like the static_branch name,
and if we want to add information about the execution bias, I'd
suggest we do:

	if (likely(static_branch(&x))) ...

	if (unlikely(static_branch(&x))) ...

if that can be made to work, or else have variants like this:

   	if (likely_static_branch(&x)) ...

	if (unlikely_static_branch(&x)) ...

Paul.
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