Subject: Re: [RFC] HZ free ntp
From: john stultz <johnstul@us.ibm.com>
To: Roman Zippel <zippel@linux-m68k.org>
Cc: Ingo Molnar <mingo@elte.hu>, Thomas Gleixner <tglx@linutronix.de>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
In-Reply-To: <200701011929.28546.zippel@linux-m68k.org>
References: <20061204204024.2401148d.akpm@osdl.org>
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	 <200701011929.28546.zippel@linux-m68k.org>
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Date: Tue, 02 Jan 2007 11:46:24 -0800
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On Mon, 2007-01-01 at 19:29 +0100, Roman Zippel wrote:
> On Wednesday 20 December 2006 02:54, john stultz wrote:
> 
> > And here would be the follow on patch (again *untested*) for
> > CONFIG_NO_HZ slowing the time accumulation down to once per second.
> 
> Changing it to one creates a potential problem with calling second_overflow().
> It should be called every NTP_INTERVAL_FREQ times, but occasionally it's off
> by one (when xtime is close to a full second and the tick length is different
> from 1sec). At a higher frequency that's not much of a problem, but at one it
> means second_overflow() is occasionally called twice a second or skipped for
> a second. Usually the error should be quite small, but sometimes it can be
> significant.
> So in this case the loop in update_wall_time() should rather look like this:
> 
> 	while (offset >= clock->cycle_interval) {
> 		...
> 		second_overflow();
> 		while (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
> 			clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
> 			xtime.tv_sec++;
> 		}
> 		...
> 	}
> 
> (Also note the change from "if" to "while".)

Ah, good catch! Thank you, I'll add that, retest and send it to akpm.

thanks
-john


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