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    SubjectRe: [PATCH 10/17] 2.6.17.1 perfmon2 patch for review: PMU context switch
    Andi,

    On Fri, Jun 30, 2006 at 03:41:22PM +0200, Andi Kleen wrote:
    >
    > > So why do we need care about context switch in cpu-wide mode?
    > > It is because we support a mode where the idle thread is excluded
    > > from cpu-wide monitoring. This is very useful to distinguish
    > > 'useful kernel work' from 'idle'.
    >

    The exclude-idle feature is an option you select when you create
    your cpu-wide session. By default, it is off.

    > I don't quite see the point because on x86 the PMU doesn't run
    > during C states anyways. So you get idle excluded automatically.
    >
    Yes, but that may not necessarily be troe of all architectures.
    At least with the option, the interfaces provides some guarantee.

    > And on the other hand a lot of people especially want idle
    > accounting too and boot with idle=poll. Your explicit
    > code would likely defeat that.
    >
    > > As you realize, that means
    > > that we need to turn off when the idle thread is context switched
    > > in and turn it back on when it is switched off.
    >
    > Also x86-64 has idle notifiers for this if you really wanted
    > to do it properly.
    >
    That looks like a useful feature I could leverage but why is it just
    on x86-64 at the moment?

    --
    -Stephane
    -
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