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

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Date:	Sat, 7 Nov 2009 21:03:00 +1100
From:	Paul Mackerras <paulus@...ba.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Prasad <prasad@...ux.vnet.ibm.com>,
	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>
Subject: Re: [PATCH 4/6] hw-breakpoints: Rewrite the hw-breakpoints layer
	on top of perf events

Frederic Weisbecker writes:

> On Thu, Nov 05, 2009 at 10:59:44AM +1100, Paul Mackerras wrote:
> > 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.
> 
> 
> 
> That won't happen because of the set of constraints we have.
> We never overcommit the debug register resources, except in
> the case of non-pinned counter, but that's in their nature :)

Suppose you have 4 breakpoint registers per cpu and there are two
pinned per-cpu breakpoint events, three non-pinned per-cpu breakpoint
events, and one pinned per-task breakpoint event.  I believe your
constraints will allow that situation.

What will happen is that the two pinned per-cpu breakpoint events will
use two of the hardware registers, and the three non-pinned per-cpu
breakpoint events will get round-robined onto the other two hardware
registers.  The per-task breakpoint will never get to use a hardware
register, because the code in perf_event.c schedules per-cpu events
before it schedules per-task events (see for example
perf_event_task_tick()).

We will have to make the event scheduling in kernel/perf_event.c a bit
more sophisticated before we can guarantee that a pinned breakpoint
event will always get to use a hardware register.

Paul.
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