linux-kernel - Re: setting jiffies as the clocksource stops time

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Date:	Fri, 18 Jan 2008 11:41:55 +0530
From:	Balaji Rao <balajirrao@...il.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
Cc:	Daniel Walker <dwalker@...sta.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: setting jiffies as the clocksource stops time

On Friday 18 January 2008 04:04:33 am Jan Engelhardt wrote:
> 
> On Jan 16 2008 13:20, Daniel Walker wrote:
> >On Thu, 2008-01-17 at 02:09 +0530, Balaji Rao wrote:
> >> Hi,
> >> 
> >> When i set jiffies as the current_clocksource, date(1) tells me
> >> that wallclock time has stopped, and soon after that, the system
> >> becomes unresponsive. This is not seen with CONFIG_NO_HZ disabled.
> >> 
> >> I wonder how can jiffies be used as a clocksource.. Its value
> >> depends on the tick and when we turn off ticks, we would stop
> >> incrementing jiffies and when we come come out of idle, we update
> >> the jiffies by reading the current_clocksource which now is
> >> 'jiffies', and hence jiffies wouldn't get updated. Could this be
> >> the explanation ?
> >
> >Your right, It can't be used as a clocksource with nohz , and the system
> >will refuse to automatically switch to it ..
> 
> I think that manually changing to jiffies by echoing into sysfs
> should also be prohibited.
> 
Yea, right. But why not unregister jiffies as a clocksource itself when we get into NO_HZ ? I think it's much cleaner 
provided it has no other consequences.

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