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

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Message-ID: <20120222081855.GB25318@elte.hu>
Date:	Wed, 22 Feb 2012 09:18:55 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	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

* H. Peter Anvin <hpa@...or.com> wrote:

> Stupid thought... do we have cases that matter where the bias 
> and default don't agree?

Yeah, that was one of my worries about the proposed original 
tongue twisters (see Jason's original series: "jump label: 
introduce default true branch").

For example could you tell *at a glance* what this does:

+       if (!static_branch_def_false(&perf_sched_events.key))

?

I certainly couldn't, I'd have to consider the '!', that it's a 
'static branch' and that it's either 'defined to false' or 
'default to false'.

Linguistic and visual barriers all around, and that's for code 
that I am intimately familar with ...

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 ;-)

Thanks,

	Ingo
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