linux-kernel - Re: [RFC PATCH] Flipped jump labels

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Message-ID: <20140810160756.GB9490@pd.tnic>
Date:	Sun, 10 Aug 2014 18:07:56 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	x86-ml <x86@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jason Baron <jbaron@...mai.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] Flipped jump labels

On Sun, Aug 10, 2014 at 05:45:15PM +0200, Ingo Molnar wrote:
> Indeed - but could we use that interface to cleanly expose the
> arch_static_branch_active() code you've written, or do we need new
> variants?

We could probably.

The thing is, if we want to do the _active thing, the whole jump labels
infrastructure would need to know about those, let's call them "new
types" because they'd need different handling when enabling - see
__jump_label_transform() in arch/x86/kernel/jump_label.c with all the
NOP checks.

Which begs the more important question: is adding those just to save us
a JMP penalty justify the additional code complexity. Frankly, I'm still
on the fence here and I'd rather do some perf measurements of a kernel
build with and without the JMP in native_sched_clock() to see whether it
is even noticeable or it disappears in the noise.

Because if it does disappear, the whole trouble is just for nothing.

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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