linux-kernel - Re: [RFC] init.h: mark init functions hot instead of cold

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Message-ID: <20150509091739.GX13605@tassilo.jf.intel.com>
Date:	Sat, 9 May 2015 02:17:39 -0700
From:	Andi Kleen <ak@...ux.intel.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] init.h: mark init functions hot instead of cold

On Sat, May 09, 2015 at 12:45:01AM +0200, Rasmus Villemoes wrote:
> attribute((cold)) causes gcc to optimize the function for size rather
> than speed. But since __init functions will be discarded anyway, I
> don't see why memory should be a concern. On the contrary, everybody

It makes the bzImage smaller.
A lot of people on smaller systems are interested in flash size.

> wants their box to boot faster. Using the opposite attribute, hot,
> causes gcc to optimize the functions more aggressively, possibly at
> the expense of larger (.init).text. A completely unscientific test
> showed about 2% faster boot time: I booted a kernel in qemu with and
> without this patch five times each; the boot times were very stable in
> each case, so I think the 2% is ok, but of course only applies to that
> specific .config running in a virtual machine on my hardware.

2% on boot is basically noise.

-Andi
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