linux-kernel - Re: [PATCH] clockevent: on resume program the next oneshot tick with the next actual event

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Date:	Wed, 25 Mar 2009 10:19:06 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ian Campbell <Ian.Campbell@...rix.com>
CC:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"Alex.Zeffertt" <Alex.Zeffertt@...rix.com>, stable@...nel.org
Subject: Re: [PATCH] clockevent: on resume program the next oneshot tick with
 the next actual event

Ian Campbell wrote:
> When resuming a Xen domU we were seeing an issue where the timer ticks never
> seemed to start up again. This was with CONFIG_NO_HZ=y, the Xen clocksource
> has CLOCK_EVT_FEAT_ONESHOT but not CLOCK_EVT_FEAT_PERIODIC.
>
> The issue is that on resume tick_resume_oneshot() tries to program an event for
> "now", e.g.
> 	tick_program_event(ktime_get(), 1);
>
> However further down the call chain tick_dev_program_event() then compares that
>   
Where's that?  Do you mean in clockevents_program_event():

	delta = ktime_to_ns(ktime_sub(expires, now));

	if (delta <= 0)
		return -ETIME;


?

> expiry time with a second call to ktime_get() and discards the event if the
> timeout is negative -- which it always will be since some time must have passed
> since tick_program_event was called.
>
> Instead of asking for an immediate event on resume, instead ask for the next
> actual event.
>   

What if the next event is now anyway?  What timebase is it in anyway?

> With this fix I can successfully resume a Xen domain.
>   

That's good, but I think there's more going on here.

    J
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