Date: Tue, 24 Aug 2010 19:50:08 GMT From: Jeff Roberson <jroberson@jroberson.net> To: freebsd-bugs@FreeBSD.org Subject: Re: kern/145385: [cpu] Logical processor cannot be disabled for some SMT-enabled Intel procs Message-ID: <201008241950.o7OJo8tV052642@freefall.freebsd.org>
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The following reply was made to PR kern/145385; it has been noted by GNATS. From: Jeff Roberson <jroberson@jroberson.net> To: Garrett Cooper <gcooper@FreeBSD.org> 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 09:22:02 -1000 (HST) This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --2547152148-1715665896-1282677727=:1407 Content-Type: TEXT/PLAIN; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 8BIT 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: >>> 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. Thanks, Jeff > Thanks! > -Garrett > --2547152148-1715665896-1282677727=:1407--
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