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: Garrett Cooper <gcooper at freebsd.org>, 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 15:45:15 -0700

 On Tue, Aug 24, 2010 at 2:51 PM, Garrett Cooper <yanegomi at gmail.com> wrote:
 > On Aug 24, 2010, at 2:03 PM, Jeff Roberson wrote:
 >
 >
 > On Tue, 24 Aug 2010, Garrett Cooper wrote:
 >
 > On Tue, Aug 24, 2010 at 12:22 PM, Jeff Roberson <jroberson at jroberson.net>
 > wrote:
 >
 > 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 proce=
 ssor;
 >
 > 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=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.
 >
 > =A0=A0Ok... 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. =A0We probably shouldn't have sysctls which
 > readily break the kernel. =A0As I said we should instead have the sysctl
 > backend to cpuset. =A0It shouldn't take more than an hour to code and tes=
 t.
 
     Ok.. I'll look at this once I have my other system back online so
 I can actively break something until I get it to work.
 Thanks,
 -Garrett