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    SubjectRe: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf counters
    On Thu, Sep 10, 2009 at 11:18:01PM +0200, Peter Zijlstra wrote:
    > On Thu, 2009-09-10 at 20:53 +0200, Frederic Weisbecker wrote:
    > > On Thu, Sep 10, 2009 at 07:55:40PM +0530, K.Prasad wrote:
    > > > On Thu, Sep 10, 2009 at 10:29:25AM +0200, Frederic Weisbecker wrote:
    > > > > This patch rebase the implementation of the breakpoints API on top of
    > > > > perf counters instances.
    > > > >
    > > > > The core breakpoint API has changed a bit:
    > > > >
    > > > > - register_kernel_hw_breakpoint() now takes a cpu as a parameter. For
    > > > > now it doesn't support all cpu wide breakpoints but this may be
    > > > > implemented soon.
    > > >
    > > > Is there a reason why perf doesn't support counters effective on all
    > > > CPUs (and all processes)?
    > > > Atleast, it is vital for debugging aspects of hw-breakpoints...say to
    > > > answer "Who all did a 'write' on the kernel variable that turned corrupt", etc.
    > > >
    > > > The implementation to iteratively register a breakpoint on all CPUs would
    > > > (as in trace_ksym.c) result in unclean semantics for the end user, when, a
    > > > register_kernel_<> request fails on a given CPU and all previously
    > > > registered breakpoints have to be reverted (but the user might have
    > > > received a few breakpoint triggers by then as a result of the successful
    > > > ones...i.e. register request fails, but still received 'some' output).
    > >
    > >
    > > (Please shrink the end of the message if you don't answer in further parts.
    > > I'm especially a bad example of what not to do :-)


    Oh I was meaning "a good example"...



    > >
    > > Yeah it would be very convenient to have that. Is it possible considering
    > > the current internal design of perf?
    >
    > Create the counters disabled? Maybe even group them to allow 'atomic'
    > enable/disable.


    I don't see why we need that.
    The problem is that we need "all-cpu" counters.

    May be we could pass a per cpu ptr to a
    register_hardware_breakpoint_wide() that could do the trick by itself?

    But that sounds too much workarounds while we would like only one
    handler.
    May be could we multiplex several per cpu counter into a single one?



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