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

[Garrett Cooper]

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

From: Garrett Cooper <gcooper at FreeBSD.org>
To: Jeff Roberson <jroberson at jroberson.net>
Cc: bug-followup at freebsd.org, jkim at freebsd.org, 
	Attilio Rao <attilio at freebsd.org>, jeff at freebsd.org
Subject: Re: kern/145385: [cpu] Logical processor cannot be disabled for some
 SMT-enabled Intel procs
Date: Tue, 24 Aug 2010 14:00:19 -0700

 On Tue, Aug 24, 2010 at 12:22 PM, Jeff Roberson <jroberson at jroberson.net> w=
 rote:
 > On Tue, 24 Aug 2010, Garrett Cooper wrote:
 >
 >> On Mon, Aug 23, 2010 at 6:33 AM, John Baldwin <jhb at freebsd.org> wrote:
 >>>
 >>> On Sunday, August 22, 2010 4:17:37 am Garrett Cooper wrote:
 >>>>
 >>>> =A0 =A0 =A0 The following trivial patch fixes the issue on my W3520 pr=
 ocessor;
 >>>> 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 recentl=
 y
 >>> 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=3D1 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. =A0There are a nu=
 mber
 > of loops in the kernel that iterate over all cpus and attempt to bind and
 > perform some task. =A0I think there are a number of other reasons to pref=
 er 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?
 Thanks!
 -Garrett