linux-kernel - Re: [PATCH] sched_clock: prevent scd->clock from moving backwards

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Date:	Thu, 09 Oct 2008 01:05:04 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_clock: prevent scd->clock from moving backwards

On Wed, 2008-10-08 at 08:00 -0500, Dave Kleikamp wrote:
> sched_clock: prevent scd->clock from moving backwards
> 
> When sched_clock_cpu() couples the clocks between two cpus, it may
> increment scd->clock beyond the GTOD tick window that __update_sched_clock()
> uses to clamp the clock.  A later call to __update_sched_clock() may move
> the clock back to scd->tick_gtod + TICK_NSEC, violating the clock's
> monotonic property.
> 
> This patch ensures that scd->clock will not be set backward.

Ah, yes indeed, this comes from the tick not happening at the same time
on different cpus, so if we use a local timestamp to move a remote clock
forward, this scenario could indeed happen.

The fix looks good to me, good catch, thanks shaggy!

A related 'fix' which I'm still not quite sure about is making the
window 'tick_gtod + 2*TICK_NSEC'. That increases the max observed
difference to 4 jiffies, but allows ticks to be 'late' a bit without
first holding back time and then jumping ahead again.

> Signed-off-by: Dave Kleikamp <shaggy@...ux.vnet.ibm.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> 
> diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
> index e8ab096..a989d64 100644
> --- a/kernel/sched_clock.c
> +++ b/kernel/sched_clock.c
> @@ -118,13 +118,13 @@ static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
>  
>  	/*
>  	 * scd->clock = clamp(scd->tick_gtod + delta,
> -	 * 		      max(scd->tick_gtod, scd->clock),
> -	 * 		      scd->tick_gtod + TICK_NSEC);
> +	 *		      max(scd->tick_gtod, scd->clock),
> +	 *		      min(scd->clock, scd->tick_gtod + TICK_NSEC));
>  	 */
>  
>  	clock = scd->tick_gtod + delta;
>  	min_clock = wrap_max(scd->tick_gtod, scd->clock);
> -	max_clock = scd->tick_gtod + TICK_NSEC;
> +	max_clock = wrap_min(scd->clock, scd->tick_gtod + TICK_NSEC);
>  
>  	clock = wrap_max(clock, min_clock);
>  	clock = wrap_min(clock, max_clock);
> 
> 

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