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    SubjectRe: [patch 8/8] Add abilty to enable/disable nmi watchdog from sysfs
    On Wed, May 10, 2006 at 02:10:26AM -0700, Stephane Eranian wrote:
    > Don,
    >
    > Congratulations on the patch. I am glad to see that some of the SMP
    > issues I reported a long time ago are now fixed.

    Thanks and glad I could help.

    > > Adds a new /proc/sys/kernel/nmi call that will enable/disable the nmi
    > > watchdog.
    > >
    >
    > This means you can at runtime enable/disbale nmi_watchdog, i.e., reserve
    > some performance counters on the fly. This gets complicated because now
    > the perfmon subsystem (and probably oprofile) cannot check register
    > availability when they are first initialized. Basically each time,
    > the /sys entry is modified, they would have to scan the list of available
    > performance counters. I don't know exactly when Oprofile does this checking.
    > For perfmon, this is done only once, when the PMU description table is loaded.
    How often did you plan on enabling/disabling the nmi_watchdog? My
    understanding was you disable nmi_watchdog, run oprofile/perfmon,
    re-enable nmi_watchdog. I guess I don't understand what type of funky
    scenarios you are dealing with.

    >
    > Also something that I did not see in this code is the error detection in
    > case enable_lapic_nmi_watchdog() fails. Oprofile runs on all CPUs or none.
    > Perfmon lets you monitor on subsets on CPUs. In case NMI was disabled and
    > a monitoring session was active on some CPUs. The enable_lapic_nmi_watchdog()
    > will fail on some CPUs. How is that handled?

    It's not. In fact I wouldn't know what to do in such situations. Is it
    really wrong to only have a subset of cpus being monitored by the
    nmi_watchdog? This seems to be wandering into the area where the user is
    looking to do something complicated (profiling a subset of cpus) and as
    such might be expected to make sure the nmi_watchdog is properly enabled
    on all cpus when they are done.

    Cheers,
    Don
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