Subject: Re: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer
 on top of perf counters
From: Peter Zijlstra <peterz@infradead.org>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: "K.Prasad" <prasad@linux.vnet.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       LKML <linux-kernel@vger.kernel.org>,
       Alan Stern <stern@rowland.harvard.edu>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Steven Rostedt <rostedt@goodmis.org>,
       Jan Kiszka <jan.kiszka@siemens.com>, Jiri Slaby <jirislaby@gmail.com>,
       Li Zefan <lizf@cn.fujitsu.com>, Avi Kivity <avi@redhat.com>,
       Paul Mackerras <paulus@samba.org>, Mike Galbraith <efault@gmx.de>,
       Masami Hiramatsu <mhiramat@redhat.com>
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Date: Thu, 10 Sep 2009 23:18:01 +0200
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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 :-)
> 
> 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.

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