Re: CPU hyperthreading turned on after soft power-cycle

[Clarinet]

On 10/30/2011 4:25 PM, Ben Hutchings wrote:
> 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.

Yes, there are reasons that support my suspicion that BIOS is not doing 
its work properly. But I cannot prove it until it is clear what has been 
changed in the kernel.

> > 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.

I do not know. I have been using mostly "halt" to shutdown the system 
and turn the server off and I tried "shutdown -h" only several times to 
see if there is any difference. Both commands have turned the computer 
off, but I did not do any special "shutdown -h" configuration.

> > 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)?

OK, after another day of testing it seems that the problem appears in 
2.6.38.1, because

* linux 2.6.37.6 - custom compiled from source - ok
* linux 2.6.38.1 - custom compiled from source - not ok

Best regards,

Jiri Polach

> 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@xxxxxxxxxxxxx) (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
> [...]

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