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

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Date:	Thu, 23 Feb 2012 21:25:13 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, mingo@...e.hu
Cc:	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

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)', so
we are going to have to change either the branch site or the test for
true/false name. The above 'static_key_inline_true/false' solves that.

Also, I do like having a consistent 'static_key_*' prefix for the entire API -
definitely an improvement for me.

Thanks,

-Jason

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