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

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Message-ID: <20081009090605.GA21798@elte.hu>
Date:	Thu, 9 Oct 2008 11:06:05 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dave Kleikamp <shaggy@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched_clock: prevent scd->clock from moving backwards


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> 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>

applied to tip/sched/core, thanks!

	Ingo
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