linux-kernel - Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long delays

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Date:	Wed, 9 Jan 2008 19:25:00 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	john stultz <johnstul@...ibm.com>
cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 13/22 -v2] handle accurate time keeping over long
 delays


On Wed, 9 Jan 2008, john stultz wrote:
> > Index: linux-compile-i386.git/kernel/time/timekeeping.c
> > ===================================================================
> > --- linux-compile-i386.git.orig/kernel/time/timekeeping.c	2008-01-09 14:07:34.000000000 -0500
> > +++ linux-compile-i386.git/kernel/time/timekeeping.c	2008-01-09 15:17:31.000000000 -0500
> > @@ -448,27 +449,29 @@ static void clocksource_adjust(s64 offse
> >   */
> >  void update_wall_time(void)
> >  {
> > -	cycle_t offset;
> > +	cycle_t cycle_now, offset;
> >
> >  	/* Make sure we're fully resumed: */
> >  	if (unlikely(timekeeping_suspended))
> >  		return;
> >
> >  #ifdef CONFIG_GENERIC_TIME
> > -	offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
> > +	cycle_now = clocksource_read(clock);
> >  #else
> > -	offset = clock->cycle_interval;
> > +	cycle_now = clock->cycle_last + clock->cycle_interval;
> >  #endif
> > +	offset = (cycle_now - clock->cycle_last) & clock->mask;
>
> It seems this offset addition was to merge against the colliding
> xtime_cache changes in mainline. However, I don't think its quite right,
> and might be causing incorrect time() or vtime() results if NO_HZ is
> enabled.

Yeah, this had a bit of clashes in its life in the RT kernel.

>
> > +	clocksource_accumulate(clock, cycle_now);
> > +
> >  	clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
> >
> >  	/* normally this loop will run just once, however in the
> >  	 * case of lost or late ticks, it will accumulate correctly.
> >  	 */
> > -	while (offset >= clock->cycle_interval) {
> > +	while (clock->cycle_accumulated >= clock->cycle_interval) {
> >  		/* accumulate one interval */
> >  		clock->xtime_nsec += clock->xtime_interval;
> > -		clock->cycle_last += clock->cycle_interval;
> > -		offset -= clock->cycle_interval;
> > +		clock->cycle_accumulated -= clock->cycle_interval;
> >
> >  		if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
> >  			clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
> > @@ -482,7 +485,7 @@ void update_wall_time(void)
> >  	}
> >
> >  	/* correct the clock when NTP error is too big */
> > -	clocksource_adjust(offset);
> > +	clocksource_adjust(clock->cycle_accumulated);
>
>
> I suspect the following is needed, but haven't been able to test it yet.

Thanks, I'll pull it in and start testing it.

-- Steve

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