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: Garrett Cooper <gcooper at freebsd.org>
Cc: Jeff Roberson <jroberson at jroberson.net>, 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 21:50:20 -0700

 On Tue, Aug 24, 2010 at 3:45 PM, Garrett Cooper <gcooper at freebsd.org> wrote=
 :
 > On Tue, Aug 24, 2010 at 2:51 PM, Garrett Cooper <yanegomi at gmail.com> wrot=
 e:
 >> 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 proc=
 essor;
 >>
 >> 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 defaul=
 t
 >>
 >> system set rather than specifically halting those cpus. =A0There are a n=
 umber
 >>
 >> of loops in the kernel that iterate over all cpus and attempt to bind an=
 d
 >>
 >> perform some task. =A0I think there are a number of other reasons to pre=
 fer 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 te=
 st.
 >
 > =A0 =A0Ok.. 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.
 
     BTW... there's a lot of code in machdep.c that does the same thing
 to idle the CPU, for instance, cpu_idle_hlt, cpu_idle_acpi,
 cpu_idle_amdc1e (on amd64). What should be done about those cases
 (same thing, or different)?
 Thanks,
 -Garrett