linux-kernel - Re: Use of absolute timeouts for oneshot timers

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Date:	Sun, 11 Mar 2007 10:21:38 +0100
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Dan Hecht <dhecht@...are.com>, john stultz <johnstul@...ibm.com>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Use of absolute timeouts for oneshot timers

On Sat, 2007-03-10 at 16:42 -0800, Jeremy Fitzhardinge wrote:
> Thomas Gleixner wrote:
> > It's simply enforced in NO_HZ, HIGHRES mode as we operate in absolute
> > time, which is read back from the clocksource, even if we use a relative
> > value for real hardware clock event devices to program the next event.
> > We calculate the delta between the absolute event and now. So we never
> > get an accumulating error.
> >
> > What problem are you observing ?
> 
> Actually, two things.  There was the unexpected pauses during boot,
> which is trivially fixable by not using the Xen periodic timer, and
> using the single-shot fallback.
> 
> But I'm making the more general observation that if you use an absolute
> rather than relative time to set the single-shot timeout, then you have
> to deal with a long-term cumulative drift between the kernel's monotonic
> time and the hypervisor's monotonic time.  This can happen even if your
> clocksource is derived directly from the hypervisor monotonic time,
> because running ntp will warp the kernel's time, and so it will drift
> with respect to the hypervisor clock.  You can only avoid this by 1) not
> allowing adjtime, or 2) making those same adjtime warps to the
> hypervisor time.  Neither of these is a good general solution.

Sigh, yes. Using a relative time for the next event is probably the
least ugly solution

	tglx



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