The following reply was made to PR kern/145385; it has been noted by GNATS.
From: Garrett Cooper <gcoo...@freebsd.org>
To: Garrett Cooper <gcoo...@freebsd.org>
Cc: Jeff Roberson <jrober...@jroberson.net>, bug-follo...@freebsd.org,
j...@freebsd.org,
Attilio Rao <atti...@freebsd.org>, j...@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 <gcoo...@freebsd.org> wrote=
:
> On Tue, Aug 24, 2010 at 2:51 PM, Garrett Cooper <yaneg...@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 <jrober...@jroberson.net=
>
>> wrote:
>>
>> On Tue, 24 Aug 2010, Garrett Cooper wrote:
>>
>> On Mon, Aug 23, 2010 at 6:33 AM, John Baldwin <j...@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
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