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    SubjectRe: [PATCH 0/4] perf: Precise task / softirq / hardirq filtered stats/profiles

    * Peter Zijlstra <> wrote:

    > On Fri, 2010-05-21 at 17:12 +0200, Ingo Molnar wrote:
    > > 'exclusion' is the ABI detail. The feature your patches
    > > implement are to allow 'softirq limited' or 'task-context
    > > limited' or 'hardirq profiling' - which is way cool.
    > >
    > > One thing i'd like to see in this feature is for it to
    > > work on pure event counting - i.e. 'perf stat' as well.
    > Its not really exclusion, all it does is discard samples
    > when in the wrong context (which happens to work
    > reasonably well for all the swevents, except for the
    > timer ones).
    > If you really want to do exclusion you have to
    > disable/enable on *IRQ entry/exit, but I guess that gets
    > to be prohibitive on costs.

    Yeah, i know - this is what i tried to allude to in my
    other part of my reply:

    > > If we extended your feature to perf stat, we might be
    > > able to get a lot more precise measurements in terms
    > > of kernel optimizations (and kernel bloat).

    Right, so there's two ways to do it, one is the
    disable/enable what you mention, the other would be to
    save the count and then read again and subtract the delta.

    ( the RDPMC based delta method can be made to work for
    sampling as well, even if the NMI hits in the middle of
    the softirq or hardirq. )

    Two reads might be cheaper than a disable+enable.
    Especially if it's done using RDPMC.

    We should do it like that, not by discarding samples, and
    overhead should be OK as long as we dont do the
    disable/enable (or delta read) if the feature is off.

    If a simple enable/disable or read/read costs too much
    then we need to prod hw makers about it. But it should be
    OK i think.


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