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

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Date:	Thu, 17 Jan 2008 23:34:33 +0100 (CET)
From:	Jan Engelhardt <jengelh@...putergmbh.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	Balaji Rao <balajirrao@...il.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 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.
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