linux-kernel - Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle

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Date:	Wed, 27 May 2009 23:12:35 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Alok Kataria <alokkataria1@...il.com>
cc:	Jon Hunter <jon-hunter@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	john stultz <johnstul@...ibm.com>, Ingo Molnar <mingo@...e.hu>,
	akataria@...are.com
Subject: Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during
 idle

On Wed, 27 May 2009, Alok Kataria wrote:

> On Wed, May 27, 2009 at 7:49 AM, Jon Hunter <jon-hunter@...com> wrote:
> >
> > The dynamic tick allows the kernel to sleep for periods longer than a single
> > tick. This patch prevents that the kernel from sleeping for a period longer
> > than the maximum time that the current clocksource can count. This ensures
> > that the kernel will not lose track of time. This patch adds a new function
> > called "timekeeping_max_deferment()" that calculates the maximum time the
> > kernel can sleep for a given clocksource.
> >
> 
> >From the patch description I understand that this will avoid wrapping
> around for only the *current* clocksource. What happens if, say, TSC
> is the clocksource and ACPI_PM is being used as the
> watchdog_clocksource, in that case the timekeeping_max_deferement will
> give TSC' max allowed sleep value (which is greater than ACPI_PMs).
> i.e. We could still sleep beyond ACPI_PM's wrap around threshold which
> may result in us marking TSC as unsuable as a clocksource.
> That could still result in incorrect timekeeping right ?

No, because the watchdog timer takes care of that. It wakes up at time.

Thanks,

	tglx
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