Messages in this thread | | | Date | Mon, 14 Sep 2009 23:18:24 +0200 | From | Frederic Weisbecker <> | Subject | Re: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf counters |
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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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