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

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Message-ID: <8763gcrf4t.fsf@deeprootsystems.com>
Date:	Thu, 07 May 2009 10:34:26 -0700
From:	Kevin Hilman <khilman@...prootsystems.com>
To:	Tim Bird <tim.bird@...sony.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: How to use a different sched_clock() for ftrace on omap?

Tim Bird <tim.bird@...sony.com> writes:

> Kevin Hilman wrote:
>> Tim Bird <tim.bird@...sony.com> writes:
>> 
>>> 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
>> 
>> Hi Tim,
>> 
>> If you're comiling mach-omap1/time.c than you've enabled the
>> higher-resolution MPU timer with CONFIG_OMAP_MPU_TIMER, right?
>
> Yes.
>
>> In that case, you could make the one in plat-omap/common.c inside and
>> #ifndef CONFIG_OMAP_MPU_TIMER and put the new one in the MPU_TIMER
>> code.
>> 
>> To be complete, you should add the same to the mach-omap2/timer-gp.c
>> as well.
>
> I was trying to avoid using #ifdefs, but maybe in this case it
> makes sense.  There are tradeoffs in using the different timers
> (nicely described in plat-omap/Kconfig help entries), so IMHO it
> would be good to make this a config preference.

Yes, we have PM reasons for wanting to choose between the different
timers.

> I'll work up a patch in this style and send it along.

OK, please send it to linux-omap as well.

On a related note, and not your problem here but something that would
be nice to see is the default sched_clock implementation using the
clocksource instead of jiffies.

Then, platform code just registers its preferred clocksource and
sched_clock() gets the same resolution as the clocksource.  Not sure
how yet to handle clocksources being added and removed though...

Kevin
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