linux-kernel - Re: CPU hyperthreading turned on after soft power-cycle

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Date:	Wed, 9 Nov 2011 19:52:12 -0600
From:	Jonathan Nieder <jrnieder@...il.com>
To:	Jiri Polach <jiri.polach@...nam.cz>
Cc:	647095@...s.debian.org, Ben Hutchings <ben@...adent.org.uk>,
	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org
Subject: Re: CPU hyperthreading turned on after soft power-cycle

Hi Jiri,

Jiri Polach wrote:

> On Ben's advice I am trying to locate the commit that causes the problem to
> appear more precisely using 'git bisect'. However, too many of generated
> revisions are unbootable so I have to use 'bisect skip' frequently.

Ok, so I've looked over the log at <http://bugs.debian.org/647095>, and
this seems totally weird.  Have I described the symptoms correctly below?
(Warning: I am making some guesses, especially at step 5.  In case of
doubt, see the bug log just mentioned.)

	1. Disable SMT in the BIOS.

	2. Boot a bad kernel.  /proc/cpuinfo (correctly) shows one entry
	   per core.

	3. "shutdown -h now".  Enter BIOS.  SMT is still disabled.
	   Don't save.

	4. Boot any kernel.  /proc/cpuinfo shows two entries per core.

	5. "shutdown -h now".  Boot any kernel.  /proc/cpuinfo still shows
	   two entries per core.

	6. "shutdown -h now".  Enter BIOS.  SMT is still disabled.  Save.
	   Now /proc/cpuinfo will (correctly) shows one entry per core.

Reproducible for Jiri with v3.0.4.

Result of bisecting: v2.6.38-rc1 exhibits the problem.  v2.6.37 and
many of the topic branches merged in the 2.6.38 merge window work ok.
Some other topic branches do not boot at all.

Jiri: if you have gitk installed, then "git bisect visualize" can help
get a sense of what's in the middle of the regression range.
"gitk --bisect --first-parent v2.6.37..v2.6.38-rc1" might be a good way
to find mainline commits to test before finding a topic branch to delve
into.

x86 people: do the symptoms seem familiar?  Any hints for tracking it
down?

Thanks and hope that helps,
Jonathan
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