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

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Date:	Mon, 27 Feb 2012 08:40:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>, a.p.zijlstra@...llo.nl,
	davem@...emloft.net, ddaney.cavm@...il.com,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] jump label: introduce very_[un]likely + cleanups +
 docs


* Jason Baron <jbaron@...hat.com> wrote:

> On Fri, Feb 24, 2012 at 10:08:11AM +0100, Ingo Molnar wrote:
> > * Jason Baron <jbaron@...hat.com> wrote:
> > 
> > > On Thu, Feb 23, 2012 at 06:18:42PM -0500, Mathieu Desnoyers wrote:
> > > > * Ingo Molnar (mingo@...e.hu) wrote:
> > > > > 
> > > > > * Ingo Molnar <mingo@...e.hu> wrote:
> > > > > 
> > > > > > So, a modified scheme would be:
> > > > > > 
> > > > > >         #include <linux/static_key.h>
> > > > > > 
> > > > > >         struct static_key key = STATIC_KEY_INIT_TRUE;
> > > > > > 
> > > > > >         if (static_key_false(&key))
> > > > > >                 do unlikely code
> > > > > >         else
> > > > > >                 do likely code
> > > > > > 
> > > > > > Or:
> > > > > > 
> > > > > >         if (static_key_true(&key))
> > > > > >                 do likely code
> > > > > >         else
> > > > > >                 do unlikely code
> > > > > > 
> > > > > > The static key is modified via:
> > > > > > 
> > > > > >         static_key_slow_inc(&key);
> > > > > >         ...
> > > > > >         static_key_slow_dec(&key);
> > > > > > 
> > > > > > Is that API fine? I'll rework the series to such an effect if 
> > > > > > everyone agrees.
> > > > > 
> > > > > I.e. something like the patch below on top of 
> > > > > tip:perf/jump-labels.
> > > > > 
> > > > > Untested - will test it and will refactor the series if 
> > > > > everyone's happy.
> > > > 
> > > > Hi Ingo,
> > > > 
> > > > Reading your documentation updates makes me realise that adding the
> > > > "inline" keyword in there would make the whole thing even clearer:
> > > > 
> > > > struct static_key key = STATIC_KEY_INLINE_TRUE_INIT;
> > > > struct static_key key = STATIC_KEY_INLINE_FALSE_INIT;
> > > > 
> > > > static_key_inline_true() / static_key_inline_false()
> > > > 
> > > > to show that the "true/false" in there does not mean that the key will
> > > > always be true or false (the key value can indeed by changed by calling
> > > > static_key_slow_inc/dec), but that the inlined path is either the true
> > > > of false branch.
> > > > 
> > > 
> > > Also, as part of the API, there is a test to check the branch 
> > > direction - which was 'jump_label_true(key)', but is now also 
> > > 'static_key_true(key)', [...]
> > 
> > Yeah, there is such an overlap - I've renamed it to 
> > static_key_enabled(), which makes sense anyway as the original 
> > was jump_label_enabled()..
> > 
> > Btw., shouldnt it be an inline function? Currently it's:
> > 
> 
> Yes. I've had thought that too. In fact, it is already 'static 
> inline' for the !JUMP_LABEL case. So we can probably just 
> remove the function from the .c and move the 'static inline' 
> such that its defined for all cases.

Yep. Mind sending a patch for that, against latest -tip?

Thanks,

	Ingo
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