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    SubjectRe: [PATCH] perf/x86: check ucode before disabling PEBS on SandyBridge
    On 12.06.12 19:13:23, Peter Zijlstra wrote:
    > On Tue, 2012-06-12 at 19:09 +0200, Stephane Eranian wrote:
    > > > Instead of registering a microcode notifier, why not checking the
    > > > availability of pebs dynamically with each syscall in
    > > > intel_pmu_hw_config()? It looks like intel_snb_verify_ucode() is not
    > > > that much expensive. We can perform the check only if the event
    > > could
    > > > be for pebs and if pebs is broken. The check could be repeated when
    > > > setting up a new event after ucode could potentially has been
    > > updated
    > > > (e.g. after bringing a cpu online or so).
    > Because you then end up with a for_each_online_cpu() loop in there,
    > that's not pretty and quite horrible on large systems when you need to
    > create nr_cpus events.

    But usually the check fails on the current cpu, no need to touch other
    cpus in this case. for_each_online_cpu() would run only for the case
    that there just was a ucode update and pebs is going to be enabled.
    And for this rare case we could use locking.

    > Furthermore, ucode update is the rare thing, creating events happens
    > much more frequently.
    > > That's what I had in my original version.
    > Right, but you really need to check all cpus, not just the one you
    > happen to run on or the boot cpu.

    Once pebs is enabled no further checks are needed anymore, I think.


    Advanced Micro Devices, Inc.
    Operating System Research Center

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