kern/145385: [cpu] Logical processor cannot be disabled for some
SMT-enabled Intel procs
Jeff Roberson
jroberson at jroberson.net
Wed Aug 25 05:00:24 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 18:53:25 -1000 (HST)
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On Tue, 24 Aug 2010, Garrett Cooper wrote:
> 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> 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:
>>>
>>> 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).
>>>
>>> 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=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.
>>>
>>> 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?
>>>
>>> I don't think the bug is ok. We probably shouldn't have sysctls which
>>> readily break the kernel. As I said we should instead have the sysctl
>>> backend to cpuset. It shouldn't take more than an hour to code and test.
>>
>> 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.
>
> 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)?
Those are the actual idle functions that the scheduler uses. Those are
safe.
Thanks,
Jeff
> Thanks,
> -Garrett
>
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