Relevant commits are:


commit 09ec54429c6d10f87d1f084de53ae2c1c3a81108
Author: Thomas Gleixner <tglx>
Date:   Wed Jul 16 21:05:12 2014 +0000

    clocksource: Move cycle_last validation to core code
    
    The only user of the cycle_last validation is the x86 TSC. In order to
    provide NMI safe accessor functions for clock monotonic and
    monotonic_raw we need to do that in the core.
    
    We can't do the TSC specific
    
        if (now < cycle_last)
                    now = cycle_last;
    
    for the other wrapping around clocksources, but TSC has
    CLOCKSOURCE_MASK(64) which actually does not mask out anything so if
    now is less than cycle_last the subtraction will give a negative
    result. So we can check for that in clocksource_delta() and return 0
    for that case.
    
    Implement and enable it for x86
    
    Signed-off-by: Thomas Gleixner <tglx>
    Signed-off-by: John Stultz <john.stultz>

commit 3a97837784acbf9fed699fc04d1799b0eb742fdf
Author: Thomas Gleixner <tglx>
Date:   Wed Jul 16 21:05:10 2014 +0000

    clocksource: Make delta calculation a function
    
    We want to move the TSC sanity check into core code to make NMI safe
    accessors to clock monotonic[_raw] possible. For this we need to
    sanity check the delta calculation. Create a helper function and
    convert all sites to use it.
    
    [ Build fix from jstultz ]
    
    Signed-off-by: Thomas Gleixner <tglx>
    Signed-off-by: John Stultz <john.stultz>

For the record, the problem was observed on systems with AMD erratum #759 (see
http://support.amd.com/TechDocs/48063_15h_Mod_00h-0Fh_Rev_Guide.pdf, p. 90):

759 One Core May Observe a Time Stamp Counter Skew
==================================================

Description
-----------

During a P-state change or following a C-state change, the processor core may
synchronize an internal copy of the time stamp counter (TSC) incorrectly. The
processor may then observe TSC values (e.g., RDTSC, RDTSCP and RDMSR 0000_0010h
instructions) or MPERF (MSR0000_000E7) values that do not account for the time
spent performing this last P-state or C-state change. This error is normally
temporary in nature, in that it may be resolved after the next P-state or
C-state change.


Potential Effect on System
--------------------------

System software or software with multiple threads may observe that one
thread or processor core provides TSC values that are behind all of the
other threads or processor cores.

While a single thread operating on a single core can not observe
successively stored TSC values that incorrectly decrement, it is
possible that a single thread may be dispatched on one core, where the
software observes a TSC, and is then dispatched by the operating system
on another core that has encountered the conditions of the erratum. In
this sequence of events, the thread may observe a TSC that appears to
decrement.

In addition, software may calculate a higher effective frequency (APERF,
MSR0000_00E8, divided by MPERF).


Suggested Workaround
--------------------

Contact your AMD representative for information on a BIOS update.


Fix Planned
-----------

Yes



According to https://bugs.launchpad.net/ubuntu/+source/linux-firmware/+bug/1200533, the latest amd-ucode has a fix to this erratum; however, the bug was seen on systems with that revision of ucode, too.

[I didn't mean the bug to be private; I'd appreciate if it could be made public]

(In reply to Roman Kagan from comment #4)
> [I didn't mean the bug to be private; I'd appreciate if it could be made
> public]

np.

P.

Created attachment 941540 [details]
RHEL PATCH 1/6

Created attachment 941541 [details]
RHEL PATCH 2/6

Created attachment 941542 [details]
RHEL PATCH 3/6

Created attachment 941543 [details]
RHEL PATCH 4/6

Created attachment 941544 [details]
RHEL PATCH 5/6

Created attachment 941545 [details]
RHEL PATCH 6/6

Sorry everyone, I made the changes for RHEL7 first and accidentally used this BZ.  I'm going to clone this to RHEL7 and POST for RHEL7 from there.

P.

Created attachment 943020 [details]
RHEL PATCH 1/2

Created attachment 943021 [details]
RHEL PATCH 2/2

Created attachment 943022 [details]
RHEL PATCH 3/2

Created attachment 943023 [details]
RHEL PATCH 4/2

Created attachment 943024 [details]
RHEL PATCH 5/2

Created attachment 943025 [details]
RHEL PATCH 6/2

Sorry everyone, I mucked up this BZ pretty badly and am cleaning it up.  I'll push 6.7 patches shortly.

P.

This request was evaluated by Red Hat Product Management for
inclusion in a Red Hat Enterprise Linux release.  Product
Management has requested further review of this request by
Red Hat Engineering, for potential inclusion in a Red Hat
Enterprise Linux release for currently deployed products.
This request is not yet committed for inclusion in a release.

Created attachment 975178 [details]
RHEL PATCH 1/2

Created attachment 975179 [details]
RHEL PATCH 2/2

Patch(es) available on kernel-2.6.32-532.el6

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2015-1272.html