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

Jeff Roberson jroberson at
Tue Aug 24 21:10:10 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 11:03:40 -1000 (HST)

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 On Tue, 24 Aug 2010, Garrett Cooper wrote:
 > On Tue, Aug 24, 2010 at 12:22 PM, Jeff Roberson <jroberson at> wrote:
 >> 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.
 >    Ok... in that event then the bug is ok, but maybe I should add
 > some code to the patch to warn the user about functional issues
 > associated with halting logical CPUs?
 I don't think the bug is ok.  We probably shouldn't have sysctls which 
 readily break the kernel.  As I said we should instead have the sysctl 
 backend to cpuset.  It shouldn't take more than an hour to code and test.
 > Thanks!
 > -Garrett

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