linux-kernel - Re: 2.6.25-rc5-git6: Reported regressions from 2.6.24

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Date:	Fri, 21 Mar 2008 23:09:32 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Gabriel C <crazy@...galware.org>
cc:	Gabriel C <nix.or.die@...glemail.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Adrian Bunk <bunk@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Natalie Protasevich <protasnb@...il.com>,
	andi-bz@...stfloor.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: 2.6.25-rc5-git6: Reported regressions from 2.6.24

On Fri, 21 Mar 2008, Gabriel C wrote:
> > So the rotating watchdog triggers for a yet to figure out reason.
> > 
> > Oh, now that the pm timer seems to work again, can you try the following:
> > 
> > apply the reverted patch again and let the box boot. At some point the
> > TSC is marked unstable and is replaced by acpi_pm clocksource.
> > 
> > What result does timewarp.c show in that situation ?
> 
> Here it is , same kernel + Andi's patch :
> 
> ./time-warp-test
> 4 CPUs, running 4 parallel test-tasks.
> checking for time-warps via:
> - read time stamp counter (RDTSC) instruction (cycle resolution)
> - gettimeofday (TOD) syscall (usec resolution)
> - clock_gettime(CLOCK_MONOTONIC) syscall (nsec resolution)
> 
> | 1.78 us, TSC-warps:0 | 19.27 us, TOD-warps:0 | 19.37 us, CLOCK-warps:0

Ok. So the watchdog trigger is a false positive. 

Thinking more about it, it looks like Andi's change triggers some
hidden bug in the combination of NO_HZ and add_timer_on(), where the
CPU on which the timer is added is likely in a long idle sleep. I look
into this tomorrow.

Thanks for testing

       tglx
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