linux-kernel - Re: [patch] make pit clocksource continuous

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Date:	Sun, 13 May 2007 16:54:53 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Stas Sergeev <stsp@...et.ru>
Cc:	Daniel Walker <dwalker@...sta.com>,
	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] make pit clocksource continuous

On Sun, 2007-05-13 at 13:04 +0400, Stas Sergeev wrote:
> Hello.
> 
> Thomas Gleixner wrote:
> > The PIT can only be used as a clock source, when it is in periodic mode,
> > but it is never continuous. If we miss one PIT interrupt the time
> > keeping is hosed.

> OK, thank you and Daniel for the explanations
> about the flag, but now to the problem.
> Before hrtimer_switch_to_hres() is called,
> the check for timekeeping_is_continuous() is
> performed in tick_ckeck_oneshot_change(), and
> that check fails (by adding a flag I compel it
> to succeed).

It succeeds, but it does not work correctly, because your time keeping
is unreliable.

>  With that check failing,
> hrtimer_switch_to_hres() is never called, and
> the hrtimers do not work. That's why I posted
> that patch, but what would be the right way
> of getting the hrtimers to work?
> /proc/timer_list lists only the pit timesource.
> If I enable lapic then it is also listed, yet
> it doesn't help at all.

You need either pm_timer or hpet in order to switch to highres /
dynticks. Are those enabled in the kernel config ?

	tglx



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