kern/145385: [cpu] Logical processor cannot be disabled for some SMT-enabled Intel procs

Jeff Roberson jroberson at
Tue Aug 24 19:50:09 UTC 2010

The following reply was made to PR kern/145385; it has been noted by GNATS.

From: Jeff Roberson <jroberson at>
To: Garrett Cooper <gcooper at>
Cc: bug-followup at, jkim at, 
    Attilio Rao <attilio at>, jeff at
Subject: Re: kern/145385: [cpu] Logical processor cannot be disabled for some
 SMT-enabled Intel procs
Date: Tue, 24 Aug 2010 09:22:02 -1000 (HST)

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 On Tue, 24 Aug 2010, Garrett Cooper wrote:
 > On Mon, Aug 23, 2010 at 6:33 AM, John Baldwin <jhb at> wrote:
 >> On Sunday, August 22, 2010 4:17:37 am Garrett Cooper wrote:
 >>>       The following trivial patch fixes the issue on my W3520 processor; AFAICS
 >> it's what should be done after reading several of the specs because the
 >> logical count that's tracked with ebx is exactly what is needed for
 >> logical_cpus (it's an absolute quantity). I need to verify it with a multi-cpu
 >> topology at work (the two r710s I was testing with E-series Xeons on aren't
 >> available remotely right now).
 >>> Thanks!
 >>> -Garrett
 >> Jung-uk Kim and Attilio Rao have both been looking at this code recently and
 >> are in a better position to review the patch in the PR.
 > (Moving jhb@ to BCC, adding jeff@ for possible input on ULE)
 > The patch works as expected (it now properly detects the SMIT CPUs as
 > logical CPUs), but setting machdep.hlt_logical_cpus=1 causes other
 > problems with scheduling tasks because certain kernel threads get
 > stuck at boot when netbooting (in particular I've seen problems with
 > usbhub* and a few others bits), so in order for
 > machdep.hlt_logical_cpus to be fixed on SMT processors, it might
 > require some changes to the ULE scheduler to shuffle around the
 > threads to available cores/processors?
 hlt_logical_cpus should be rewritten to use cpusets to change the default 
 system set rather than specifically halting those cpus.  There are a 
 number of loops in the kernel that iterate over all cpus and attempt to 
 bind and perform some task.  I think there are a number of other reasons 
 to prefer a less aggressive approach to avoiding the logical cpus as well. 
 Simply preventing user thread schedule will achieve the intent of the 
 sysctl in any event.
 > Thanks!
 > -Garrett

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