linux-kernel - Re: perf hw in kexeced kernel broken in tip

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Date:	Wed, 1 Dec 2010 11:06:40 -0500
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Jason Wessel <jason.wessel@...driver.com>,
	Don Zickus <dzickus@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haren Myneni <hbabu@...ibm.com>
Subject: Re: perf hw  in kexeced kernel broken in tip

On Wed, Dec 01, 2010 at 12:27:47PM +0100, Peter Zijlstra wrote:
> On Wed, 2010-12-01 at 00:00 -0800, Yinghai Lu wrote:
> > First kernel:
> > [    1.139418] calling  init_hw_perf_events+0x0/0xb77 @ 1
> > [    1.159111] Performance Events: PEBS fmt1+, Nehalem events, Intel PMU
> > driver.
> > [    1.159567] ... version:                3
> > [    1.179121] ... bit width:              48
> > [    1.179353] ... generic registers:      4
> > [    1.179593] ... value mask:             0000ffffffffffff
> > [    1.199211] ... max period:             000000007fffffff
> > [    1.199554] ... fixed-purpose events:   3
> > [    1.219108] ... event mask:             000000070000000f
> > [    1.219454] initcall init_hw_perf_events+0x0/0xb77 returned 0 after
> > 11719 usecs
> > 
> > .....
> > [   20.220997] checking TSC synchronization [CPU#0 -> CPU#11]: passed.
> > [   20.260818] NMI watchdog enabled, takes one hw-pmu counter.
> > 
> > kexeced kernel.
> > 
> > 
> > [    1.169470] calling  init_hw_perf_events+0x0/0xb77 @ 1
> > [    1.189265] Performance Events: PEBS fmt1+, Nehalem events, Broken
> > PMU hardware detected, software events only.
> > ...
> > [   21.010407] NMI watchdog failed to create perf event on cpu14:
> > fffffffffffffffe
> > 
> > caused by:
> 
> *sigh*, and people ask me why kexec/kdump are such bad ideas..
> 
> apparently kexec doesn't properly shut down the first kernel and leaves
> a counter running, then when we write and read the counter value they
> don't match because its still running and voila, crap happens.
> 
> I've CC'ed the kexec people, maybe they got clue as to how to sort this.

So we can shutdown counters while first kernel is going down. Is there a
simple function already which I can call?

Thanks
Vivek
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