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    SubjectRe: [PATCH 4/4] [x86] perf: fix accidentally ack'ing a second event on intel perf counter
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    Robert,

    Do you have the test program you used to test this?
    I believe the NHM hack does not solve the problem, it
    just makes it harder to appear.

    I suspect the real issue is that the GLOBAL_STATUS
    bitmask cannot be trusted. I'd like to verify this.

    Has the problem appear only on Nehalem or also on
    Westmere?


    On Wed, Sep 1, 2010 at 4:57 PM, Robert Richter <robert.richter@amd.com> wrote:
    > On 01.09.10 09:04:45, Stephane Eranian wrote:
    >> Don,
    >>
    >> Found your patch on LKML (I am not on it).
    >>
    >> In your changelog you said:
    >>
    >> > During testing of a patch to stop having the perf subsytem swallow nmis,
    >> > it was uncovered that Nehalem boxes were randomly getting unknown nmis
    >> > when using the perf tool.
    >> >
    >> > Moving the ack'ing of the PMI closer to when we get the status allows
    >> > the hardware to properly re-set the PMU bit signaling another PMI was
    >> > triggered during the processing of the first PMI.  This allows the new
    >> > logic for dealing with the shortcomings of multiple PMIs to handle the
    >> > extra NMI by 'eat'ing it later.
    >>
    >> > Now one can wonder why are we getting a second PMI when we disable all
    >> > the PMUs in the beginning of the NMI handler to prevent such a case, for
    >> > that I do not know.  But I know the fix below helps deal with this quirk.
    >> >
    >>
    >> I am assuming you're talking about back-to-back NMIs here, not nested NMIs.
    >> I don't quite understand the scenario here. Is it the case that you handled 1
    >> overflow, and then right as you return from the interrupt, you get a second
    >> PMI with a ovfl_status=0 ?
    >>
    >> What events did you measure? Which counters did you use?
    >> Did you have HT turned on?
    >
    > It is related to this thread:
    >
    >  http://lkml.org/lkml/2010/8/25/124
    >
    > Not acking the status immediately triggered an nmi, but the status was
    > 0. Acking after reading and before processing the counters results in
    > a non-zero status and thus, no empty nmi.
    >
    > -Robert
    >
    >>
    >> > Tested on multiple Nehalems where the problem was occuring.  With the
    >> > patch, the code now loops a second time to handle the second PMI (whereas
    >> > before it was not).
    >>
    >
    > --
    > Advanced Micro Devices, Inc.
    > Operating System Research Center
    >
    >
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