linux-kernel - Re: [PATCH 3/5] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf counters

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Message-ID: <20090914211822.GJ6045@nowhere>
Date:	Mon, 14 Sep 2009 23:18:24 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>
Subject: Re: [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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