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

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Date:	Thu, 5 Nov 2009 12:09:52 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Paul Mackerras <paulus@...ba.org>
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

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.



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 :)


 
> We can go some distance towards that with the pinned attribute, but
> not far enough.  The pinned attribute doesn't guarantee that the event
> will always be scheduled in, it just says that we'll do our best to
> schedule it in, and if we can't, we'll put the event into error state
> so that the user knows we didn't manage to schedule it in.  But
> there's no way to get back to gdb and tell it that a breakpoint that
> it had previously successfully created is no longer working.
> 
> Also, we don't currently fail the creation of a pinned event if it
> would conflict with another pinned event already created in the same
> context.  We would need to do something like that if we want to use
> pinned events for debugging breakpoints (as distinct from breakpoints
> for performance monitoring purposes, for which it matters less if they
> are sometimes not scheduled in).
> 
> And then there's the question of whether a per-cpu pinned breakpoint
> event conflicts with a per-task pinned breakpoint event if you only
> have one breakpoint register (as is the case on Power server CPUs).
> Plus the fact that we don't currently give per-task pinned events
> priority over per-cpu non-pinned events (perhaps that would be a good
> idea anyway).
> 
> Paul.


All that is already handled by the constraints.

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