linux-kernel - Re: TSC not updating after resume: Bug or Feature?

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Message-Id: <200812230147.32340.rjw@sisk.pl>
Date:	Tue, 23 Dec 2008 01:47:31 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>, dsaxena@...xity.net,
	linux-kernel@...r.kernel.org,
	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Subject: Re: TSC not updating after resume: Bug or Feature?

On Sunday, 21 of December 2008, Thomas Gleixner wrote:
> On Thu, 18 Dec 2008, Rafael J. Wysocki wrote:
> 
> > On Thursday, 18 of December 2008, Ingo Molnar wrote:
> > > 
> > > * Peter Zijlstra <peterz@...radead.org> wrote:
> > > 
> > > > Rafael, would something like this explain why we had to revert Shaggy's 
> > > > patch?
> > 
> > Well, I have yet to understand what the suspend-resume of the timekeeping code
> > actually does.
> 
> Thats rather simple:
> 
> suspend() saves the current time of the persistent clock (if
> available), forwards the timekeeping variables so they can be reused
> on resume, disables timekeeping activities and shuts down the clock
> events layer.
>  
> resume() estimates the suspend time via persistent clock (if
> available) and update xtime with the sleep length. After that it
> reactivates timekeeping and resumes clock events and high resolution
> timers.
> 
> So the sole purpose is:
>    - dis/enable timekeeping and clock event devices.
>    - keep track of the suspend time (if a persistent clock is available)
> 
> We reactivate clock event devices and hrtimers from timekeeping_resume
> because clock events depend on functional timekeeping.

Thanks for the explanation.  In fact, the reactivation of clock event devices
and hrtimers is the part I'm not familiar with.

> > The original description sounds worrisome to me, it looks like we've overlooked
> > something at least.
> 
> Care to explain ?

Well, the fact that in the resume code path the clocksource is only updated as
a result of executing pci_set_power_state() is worrisome.  I would be more
reliable to update it directly at one point.

Thanks,
Rafael
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