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Date: Wed, 04 Jun 2014 13:50:11 -0400
From: Jung-uk Kim <jkim@FreeBSD.org>
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To: Chris Torek <torek@torek.net>
Subject: Re: kern.timecounter.smp_tsc_adjust
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Cc: freebsd-hackers@freebsd.org, Alexander Motin <mav@freebsd.org>
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On 2014-06-04 13:32:13 -0400, Chris Torek wrote:
>> In any case, at least one, and maybe two, routines need their 
>> types changed (and maybe the TSC_READ macro as well).  Is it OK 
>> to just assume the upper 32 bits are in sync?
> 
> For whatever it may be worth, the following patch (to change 
> everything to 64 bits) is boot-tested and no longer crashes the 
> system that did crash with the unpatched version.  It also 
> successfully synchronized TSCs at least once (Intel based board 
> with the "invariant TSC" bit set, 40 CPUs).
> 
> Chris
> 
> x86/tsc: fix SMP TSC adjustment code
> 
> On SMP systems, if kern.timecounter.smp_tsc is set, the kernel 
> attempts to determine whether the TSCs on the processors are 
> sufficiently in-sync to be used for time counting.  If not, and 
> kern.timecounter.smp_tsc_adjust is set, we then attempt to adjust
> all the TSCs.
> 
> The adjustment code assumed we kept a full 64-bit value for each 
> TSC, but the data-gathering and comparison code used 32-bit values.
> As a result, if the timecounters were out of sync, the adjustment
> code could crash (depending on the number of CPUs).
> 
> This converts everything to full 64-bit values.
...

It was done on head almost two years ago:

http://svnweb.freebsd.org/changeset/base/239133

I guess mav forgot to MFC r239133 before r250772 (CC'ed):

http://svnweb.freebsd.org/changeset/base/250772

Jung-uk Kim
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