CPU hyperthreading turned on after soft power-cycle

[Ben Hutchings <ben@decadent.org.uk>]

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On Sun, 2011-10-30 at 07:05 -0400, Jiri Polach wrote:
> Package: linux-2.6
> Version: 2.6.39-3~bpo60+1
> Severity: normal
> 
> 
> When the computer is turned off using "shutdown -h" or "halt" command,
> the hypertherading BIOS setting is changed - even if hypertherading is
> disabled in BIOS, the kernel detects twice as many "processors" on
> next boot as if hyperthreading was enabled. Please see details below.
> 
> I have observed the problem on several Supermicro platforms with
> various Intel Xeon processors. The particular case I report was
> observed on Supermicro X8DTT-F mainboard with two Intel Xeon E5645
> processors (6core). The problem can be reproduced the following way:

By my understanding of how hyperthreading is controlled, this has to be
a BIOS bug, as you seem to have suspected.  But if the BIOS behaviour is
kernel version-dependent, then presumably there is something the kernel
can do to work around it.

> 1. Turn on the computer, go to BIOS setup and turn "Simultaneous
> multithreading" to "Disabled". Boot Debian.
> 
> 2. Check with "cat /proc/cpuinfo" that the system reports 12 CPUs (2 x
> six-core processor).
> 
> 3. (optionally) Reboot the system (shutdown -r) and check that there
> are still 12 CPUs detected and reported.
> 
> 4. Halt the system using "shutdown -h" or "halt", turn it on again,
> and boot Debian.

I assume from this that shutdown -h is configured to turn the system
off.

> 5. Check the number of CPUs reported - it will show you that there are
> 24 CPUs as if hyperthreading was enabled.
> 
> 6. Reboot and go to BIOS setup - it still shows that "Simultaneous
> multithreading" is set to "Disabled". Do not change anythig, just
> select "Save and Exit". Boot Debian and check the number of CPUs - it
> now shows 12 CPUs again.
> 
> I have tested several kernel versions and it seems that this behavior
> appeared for the first time somewhere between 2.6.35.7 and 2.6.38.6
> versions (ok = does not show the decribed behavior, not ok = does
> show):
> 
> * linux-image-2.6.32-5-amd64 official Debian - ok
> * linux-image-2.6.39-bpo.2-amd64 official Debian from backports - not
> ok
> 
> * linux 2.6.35.7 - custom compiled from source - ok
> * linux 2.6.38.6 - custom compiled from source - not ok
> * linux 2.6.39.4 - custom compiled from source - not ok
> * linux 3.0.4 - custom compiled from source - not ok

That might be too large a range for developers to consider.  Can you
test some versions between 2.6.35.7 and 2.6.38.6 (bisection)?

Ben.

> I have exchnged many e-mails with Supermicro distributor who
> apparently is in direct contact with Supermicro technicians. They more
> or less deny any responsibility for this problem and repeatedly point
> to the fact that some (older) kernels do not exhibit this behavior so
> it must be a kernel problem. Their representative writes:
> 
> "I discussed this with supermicro and they informed me that the Kernel
> itself is causing the issue, that it may be sending the hyperthreading
> command code to the BIOS."
> 
> Although I do not completely agree with their arguments, my knowledge
> is not deep enough to recognize where exactly the core of the problem
> is so I report this as a bug in a hope that someone will know what
> happens when a kernel turns a computer off and what has changed in
> kernel somewhere between the versions I mention above. I have asked
> Supermicro distributor for more information on what they think happens
> there and what exactly they mean by "hyperhreading command code" and I
> am waiting for their response.
> 
> -- Package-specific info:
> ** Version:
> Linux version 2.6.39-bpo.2-amd64 (Debian 2.6.39-3~bpo60+1) (norbert@tretkowski.de) (gcc version 4.4.5 (Debian 4.4.5-8) ) #1 SMP Tue Jul 26 10:35:23 UTC 2011
[...]
> ** Model information
> sys_vendor: Supermicro
> product_name: X8DTT
> product_version: 1234567890
> chassis_vendor: Supermicro
> chassis_version: 1234567890
> bios_vendor: American Megatrends Inc.
> bios_version: 080016 
> board_vendor: Supermicro
> board_name: X8DTT
> board_version: 2.0       
[...]

-- 
Ben Hutchings
compatible: Gracefully accepts erroneous data from any source

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