linux-kernel - Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer on top of perf events

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Message-ID: <20091105060008.GA3261@in.ibm.com>
Date:	Thu, 5 Nov 2009 11:30:08 +0530
From:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
To:	Paul Mackerras <paulus@...ba.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Peter Zijlstra <peterz@...radead.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jan Kiszka <jan.kiszka@....de>,
	Jiri Slaby <jirislaby@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Avi Kivity <avi@...hat.com>,
	Mike Galbraith <efault@....de>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Paul Mundt <lethal@...ux-sh.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
	on top of perf events

On Thu, Nov 05, 2009 at 10:59:44AM +1100, Paul Mackerras wrote:
> Frederic Weisbecker writes:
> 
> > This patch rebase the implementation of the breakpoints API on top of
> > perf events instances.
> > 
> > Each breakpoints are now perf events that handle the
> > register scheduling, thread/cpu attachment, etc..
> 
> What I haven't managed to understand yet is how you provide reliable
> breakpoints for debugging purposes.  If I'm debugging a program and I
> have set a breakpoint, I'll be very unhappy if the breakpoint should
> trigger but doesn't because the perf_event infrastructure has decided
> it can't schedule that breakpoint in.  If the breakpoint isn't going
> to work then I want to know that at the time that I set it.
>

The hw-breakpoint layer avoids such issues by not over-committing debug
registers i.e. 'n' number of debug registers are reserved for user-space
where n = max(no. of breakpoints requested by any given process/thread).
And book-keeping certainly helps here too.

Thanks,
K.Prasad
 
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