linux-kernel - Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation

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Message-ID: <47A079BD.8020106@siemens.com>
Date:	Wed, 30 Jan 2008 14:21:01 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	John Stultz <johnstul@...ibm.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 02/22 -v7] Add basic support for gcc profiler instrumentation

Steven Rostedt wrote:

> --- linux-mcount.git.orig/arch/x86/kernel/entry_32.S	2008-01-29 16:59:15.000000000 -0500
> +++ linux-mcount.git/arch/x86/kernel/entry_32.S	2008-01-29 17:26:18.000000000 -0500
> @@ -75,6 +75,31 @@ DF_MASK		= 0x00000400 
>  NT_MASK		= 0x00004000
>  VM_MASK		= 0x00020000
>  
> +#ifdef CONFIG_MCOUNT
> +.globl mcount
> +mcount:
> +	/* unlikely(mcount_enabled) */
> +	cmpl $0, mcount_enabled
> +	jnz trace
> +	ret

(and the corresponding 64-bit version)

Is the impact of this change on the (already expensive) mcount_enabled
case negligible? I worried about use cases where we want to gain some
(relative) worst-case numbers via these instrumentations.

In my personal priority scheme, CONFIG_MCOUNT=y && !mcount_enabled comes
after mcount_enabled.

Jan

-- 
Siemens AG, Corporate Technology, CT SE 2
Corporate Competence Center Embedded Linux
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