kern/145385: [cpu] Logical processor cannot be disabled for some
SMT-enabled Intel procs
jroberson at jroberson.net
Tue Aug 24 19:50:09 UTC 2010
The following reply was made to PR kern/145385; it has been noted by GNATS.
From: Jeff Roberson <jroberson at jroberson.net>
To: Garrett Cooper <gcooper at FreeBSD.org>
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 09:22:02 -1000 (HST)
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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:
>>> The following trivial patch fixes the issue on my W3520 processor; 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).
>> 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=1 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. There are a
number of loops in the kernel that iterate over all cpus and attempt to
bind and perform some task. I think there are a number of other reasons
to prefer 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.
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