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

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Date:	Wed, 09 Jan 2008 16:19:17 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, 2008-01-09 at 18:29 -0500, Steven Rostedt wrote:
> plain text document attachment (rt-time-starvation-fix.patch)
> Handle accurate time even if there's a long delay between
> accumulated clock cycles.
> 
> Signed-off-by: John Stultz <johnstul@...ibm.com>
> Signed-off-by: Steven Rostedt <srostedt@...hat.com>

Hrmm.. One more item I just noticed.

> 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.

> +	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
-john


Fixup merge between xtime_cache and timekeeping starvation fix.

Signed-off-by: John Stultz <johnstul@...ibm.com>

Index: 2.6/kernel/time/timekeeping.c
===================================================================
--- 2.6.orig/kernel/time/timekeeping.c	2008-01-09 16:12:49.000000000 -0800
+++ 2.6/kernel/time/timekeeping.c	2008-01-09 16:13:18.000000000 -0800
@@ -449,7 +449,7 @@ static void clocksource_adjust(s64 offse
  */
 void update_wall_time(void)
 {
-	cycle_t cycle_now, offset;
+	cycle_t cycle_now;
 
 	/* Make sure we're fully resumed: */
 	if (unlikely(timekeeping_suspended))
@@ -460,7 +460,6 @@ void update_wall_time(void)
 #else
 	cycle_now = clock->cycle_last + clock->cycle_interval;
 #endif
-	offset = (cycle_now - clock->cycle_last) & clock->mask;
 	clocksource_accumulate(clock, cycle_now);
 
 	clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
@@ -491,7 +490,7 @@ void update_wall_time(void)
 	xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
 	clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
 
-	update_xtime_cache(cyc2ns(clock, offset));
+	update_xtime_cache(cyc2ns(clock, clock->cycle_accumulated));
 
 	/* check to see if there is a new clocksource to use */
 	change_clocksource();


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