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

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Date:	Fri, 11 Nov 2011 14:50:29 +0100
From:	Clarinet <clarinet@...as.cz>
To:	Jonathan Nieder <jrnieder@...il.com>
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 all,

> 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.

Yes, this is exactly how it works. Something happens when kernel shuts 
down. Not when kernel reboots.

> 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.

I have been able to narrow the interval manually a little bit from the 
"top" (the bad side) and I will go on from the bottom now. However, 
there seems to be a large area where kernels are unbootable for me - 
they mostly stop when init is called and I do not know why.

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

Please! I have spent more than a month trying to resolve it. I cannot 
revert back to 2.6.37 kernels and I cannot live with SMT changing on 
every shutdown - I have too many servers to allow such unusual behavior ...

Thank you,

Jiri Polach

> Thanks and hope that helps,
> Jonathan

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