linux-kernel - Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

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Date:	Thu, 06 Oct 2011 15:10:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Richard Henderson <rth@...hat.com>,
	Jason Baron <jbaron@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	peterz@...radead.org
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized,
 don't nop it out

On 10/06/2011 03:06 PM, Steven Rostedt wrote:
> But it only speeds up the tracing case. The non-tracing case is a nop
> and 5bytes is 5bytes regardless.
>
> Did you see a 5% speed up while tracing was happening? How did you do
> your test. I find a 5 byte compared to a 2 byte jump being negligible
> with the rest of the overhead of tracing, but I could be wrong.

You're right, this was a completely artificial microbenchmark.  In
practice the improvement would be a much smaller effect.

But bear in mind, I'm not using jump-label for tracing.  While its
important for the "disabled" state to be quick, performance of the
"enabled" state is also important.

Thanks,
    J
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