Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping duringidle
From: Thomas Gleixner
Date: Wed May 27 2009 - 17:13:03 EST
On Wed, 27 May 2009, Alok Kataria wrote:
> On Wed, May 27, 2009 at 7:49 AM, Jon Hunter <jon-hunter@xxxxxx> 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.
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