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

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Date:	Sat, 22 Mar 2008 14:34:49 +0100
From:	Gabriel C <crazy@...galware.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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

Thomas Gleixner wrote:
> On Fri, 21 Mar 2008, Thomas Gleixner wrote:
>>> | 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.
> 
> Ok. Here is what's happening:
> 
> CPU0 runs the watchdog timer and schedules it on CPU1.
> 
> With NO_HZ enabled CPU1 is in a long idle sleep. At this point of the
> boot process there is probably no timer pending on CPU1, which means
> the idle sleep is infinite.
> 
> Now some time later CPU1 gets woken by an interrupt/IPI and runs the
> timer wheel. At this point the pm_timer which is the reference clock
> has already wrapped around, so the watchdog thinks that there is a
> huge time difference and marks the TSC unstable.
> 
> Aside of that watchdog issue this also affects the other users of
> add_timer_on(): e.g. queue_delayed_work_on().
> 
> Can you please apply the patch below and verify it with Andi's
> watchdog patch applied ? 


Did that , git head , Andi's + your patch but TSC is still marked unstable.

> 
> Thanks,
> 
> 	tglx
> 


Gabriel
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