linux-kernel - Re: [RFC][PATCH 3/2] x86/jump labels: Count and display the short jumps used

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Date:	Wed, 07 Aug 2013 17:37:27 -0400
From:	Jason Baron <jbaron@...mai.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC][PATCH 3/2] x86/jump labels: Count and display the short
 jumps used

On 08/07/2013 04:47 PM, Linus Torvalds wrote:
> On Wed, Aug 7, 2013 at 1:19 PM, Jason Baron <jbaron@...mai.com> wrote:
>> The whole point of the thread started with wanting to move the default
>> 'disabled' branch further out-of-line.
> Yeah, but I always disagreed with that.
>
> Putting the unusual code out-of-line (as in "at the end of the
> function") is a good idea, but putting it *far* out of line (as in "in
> a different section") likely makes little sense.
>
> Now, the tracing code is admittedly specialized enough that we could
> have some "really cold" attribute and move it to that kind of "even
> further away" model, but most of the other uses of the static keys are
> not necessarily of the kind where the non-default case is completely
> or utterly unlikely - they want to use the static keys not because
> some codepath is basically never taken, but because the code-path is
> so critical that loading and testing a value from memory is considered
> to be excessive for when the feature is turned off (ie scheduler
> statistics etc).
>
> So the code may not even be all that cold - some people may well run
> with statistics enabled all the time - it's just that the non-enabled
> case really *really* doesn't want to have the overhead of even
> bothering to test for this event.
>
>

ok - I can see 2 variants here as you mentioned:

1) 'Unbiased' - we want to treat both branches equally but don't want
the load/test/jmp sequence. For things like the scheduler stats.

2) 'Biased' - where the unlikely path is moved completely out-of-line.
And we have a strong 'bias' to optimize the default path.

If we can get the completely out-of-line thing working, we could make
this distinction.

Thanks,

-Jason


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