linux-kernel - Re: How to use a different sched

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Date:	Thu, 07 May 2009 12:34:50 +1200
From:	Ryan Mallon <ryan@...ewatersys.com>
To:	Tim Bird <tim.bird@...sony.com>
CC:	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	linux kernel <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.arm.linux.org.uk>
Subject: Re: How to use a different sched_clock() for ftrace on omap?

Tim Bird wrote:
> Hi all,
> 
> I've worked up a replacement sched_clock for ftrace on my omap platform.
> The current sched_clock, based on the 32K timer, has low resolution and
> doesn't provide very useful results.
> 
> Unfortunately, I'm not sure the best way to use my special one, in place
> of a common one in arch/arm/plat-omap/common.c
> 
> I thought the __attribute__ ((weak)) would be enough to have the
> sched_clock() in common.c get out of the way, and allow my board-specific
> sched_clock() to be used.  But that didn't work. 

Not sure that will work correctly since sched_clock in kernel/sched.c is
already defined as weak. If you have two weak versions of a function I
think the one which gets used is based on link order (if your board's
sched_clock is not being used).

> What is the recommended
> way to specify a board-specific function at compile time?

You could look at arch/x86/kernel/tsc_32.c, I think it does what you
want by using the function called native_sched_clock for the default
sched_clock implementation and using the alias attribute if no board
specific implementation is given, ie:

#ifdef CONFIG_SOME_BOARD
unsigned long long sched_clock(void)
{
	some_board_sched_clock();
}
#else
unsigned long long sched_clock(void)
	__attribute__((alias("native_sched_clock")));
#endif

You could make it more generic by have some_board_sched_clock be a
#define (__board_sched_clock or something), so if a board defines it
(will need to be in a platform wide header) then you use that, otherwise
you use native_sched_clock.

~Ryan

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