Date: Tue, 24 Aug 2010 21:10:08 GMT From: Garrett Cooper <gcooper@FreeBSD.org> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/145385: [cpu] Logical processor cannot be disabled for some SMT-enabled Intel procs Message-ID: <201008242110.o7OLA8dc033565@freefall.freebsd.org>
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The following reply was made to PR kern/145385; it has been noted by GNATS.
From: Garrett Cooper <gcooper@FreeBSD.org>
To: Jeff Roberson <jroberson@jroberson.net>
Cc: bug-followup@freebsd.org, jkim@freebsd.org,
Attilio Rao <attilio@freebsd.org>, jeff@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@jroberson.net> w=
rote:
> On Tue, 24 Aug 2010, Garrett Cooper wrote:
>
>> On Mon, Aug 23, 2010 at 6:33 AM, John Baldwin <jhb@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
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