linux-kernel - Re: clocksource changes in 2.6.31

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Date:	Mon, 17 Aug 2009 19:46:49 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stephen Hemminger <shemminger@...tta.com>
cc:	john stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: clocksource changes in 2.6.31 - possible regression

On Mon, 17 Aug 2009, Stephen Hemminger wrote:
> The following commit causes a change for kernels built with HRT but
> not actually using HRT.  I typically use the generic kernel we ship
> on test machines, and that kernel has NOHZ and HRT (for power savings/virt
> and HRT for QoS), but I want to be able to enable TSC as a clock source
> when doing performance tests with pktgen.
> 
> The machine in question is a several year old Opteron box, that
> normally reports clocksources: acpi_pm jiffies tsc
> but now with 2.6.31-rc6, it only has acpi_pm.
> 
> Since HRT/NOHZ is not really runtime configurable, I think the
> proper behavior is:

Isn't highres=off resp. nohz=off on the kernel command line not good
enough ?
 
>   * kernel reports all possible clocksources and chooses the best
>     by default
>   * if user demands a different clocksource, the kernel should use that
>     but degrade if necessary: ie. high-res timers have less (maybe even
>     only HZ accuracy), and nohz should be automatically disabled if
>     needed

We never implemented to back out from highres/nohz and I have no urge
to do so :)

Thanks,

	tglx
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