linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Message-ID: <CA+55aFwRHpL-3YObZfw6NxxDxMqQbiwp0=NnpotGj2RHDNF3iw@mail.gmail.com>
Date:	Mon, 5 Aug 2013 11:20:29 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On Mon, Aug 5, 2013 at 10:55 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>
> My main concern is with tracepoints. Which on 90% (or more) of systems
> running Linux, is completely off, and basically just dead code, until
> someone wants to see what's happening and enables them.

The static_key_false() approach with minimal inlining sounds like a
much better approach overall. Sure, it might add a call/ret, but it
adds it to just the unlikely tracepoint taken path.

Of course, it would be good to optimize static_key_false() itself -
right now those static key jumps are always five bytes, and while they
get nopped out, it would still be nice if there was some way to have
just a two-byte nop (turning into a short branch) *if* we can reach
another jump that way..For small functions that would be lovely. Oh
well.

                Linus
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