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

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Message-Id: <1200609581.18731.152.camel@imap.mvista.com>
Date:	Thu, 17 Jan 2008 14:39:40 -0800
From:	Daniel Walker <dwalker@...sta.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
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 Thu, 2008-01-17 at 23:34 +0100, 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.

Yeah, I wasn't suggesting we shouldn't prevent it .. However, there are
tons of way to kill your system by manual changing things ..

Daniel

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