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

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Message-ID: <4ED4CF7C.8020808@atlas.cz>
Date:	Tue, 29 Nov 2011 13:26:36 +0100
From:	Clarinet <clarinet@...as.cz>
To:	John Stultz <john.stultz@...aro.org>
CC:	Jiri Polach <clarinet@...as.cz>, 647095@...s.debian.org,
	Jonathan Nieder <jrnieder@...il.com>,
	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


> Using an older "known-good" kernel, could you build and run the test
> case at the end of Documentation/rtc.txt a few times and see if it
> triggers the same problem?
>
> I'm suspicious that the setting the alarm is whats tripping the BIOS
> into enabling the HT bit. Because with older kernels, we used PIE mode
> irqs which hwclock usually uses at boot, but with newer kernels, we
> emulate PIE via AIE alarm mode. So if the BIOS was broken before, you
> wouldn't have noticed unless you tried to use AIE irqs.
>
> If this doesn't work, I'll get some patches to both 2.6.27 and 2.6.28
> kernels to debug the exact flow of how we're touching the hardware and
> then we can further narrow it down.

I ran the tests the following way:

- boot 2.6.37.6 - check /proc/cpuinfo - 12 processors
- halt
- boot 2.6.37.6 - check /proc/cpuinfo - 12 processors
- run rtctest
- reboot
- boot 2.6.37.6 - check /proc/cpuinfo - 12 processors
- halt
- boot 2.6.37.6 - check /proc/cpuinfo - 12 processors
- run rtctest
- halt
- boot 2.6.37.6 - check /proc/cpuinfo - 24 processors

So the conclusion is that only if rtctest is run and the machine is 
halted, it triggers the HT problem. Reboot seems to "neutralize" 
whatever rtctest did.

Jiri
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