linux-kernel - Re: [PATCH 5/6] hw-breakpoints: Arbitrate access to pmu following registers constraints

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Date:	Thu, 5 Nov 2009 12:24:01 +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 5/6] hw-breakpoints: Arbitrate access to pmu following
	registers constraints

On Thu, Nov 05, 2009 at 09:58:30PM +1100, Paul Mackerras wrote:
> Frederic Weisbecker writes:
> 
> > Allow or refuse to build a counter using the breakpoints pmu following
> > given constraints.
> 
> As far as I can see, you assume each CPU has HBP_NUM breakpoint
> registers which are all interchangeable and can all be used either for
> data breakpoints or instruction breakpoints.  Is that accurate?



Yes, they are interchangeable at runtime while calling
enable/disable callbacks of the pmu.

I'm not sure instruction breakpoints are supported though.

 
> If so, we'll need to extend it a bit for Power since we have some CPUs
> that have one data breakpoint register and one instruction breakpoint
> register.  In general on powerpc the instruction and data breakpoint
> facilities are separate, i.e. we have no registers that can be used
> for either.



Sure. I would be glad to help in that area. That said I won't be
able to test anything as I don't have a PowerPc box.



> > +static void toggle_bp_slot(struct perf_event *bp, bool enable)
> > +{
> > +	int cpu = bp->cpu;
> > +	unsigned int *nr;
> > +	struct task_struct *tsk = bp->ctx->task;
> > +
> > +	/* Flexible */
> > +	if (!bp->attr.pinned) {
> > +		if (cpu >= 0) {
> > +			nr = &per_cpu(nr_bp_flexible, cpu);
> > +			goto toggle;
> > +		}
> > +
> > +		for_each_online_cpu(cpu) {
> > +			nr = &per_cpu(nr_bp_flexible, cpu);
> > +			goto toggle;
> 
> ...
> 
> > +toggle:
> > +	*nr = enable ? *nr + 1 : *nr - 1;
> > +}
> 
> This won't do what I think you want.  In the case where
> !bp->attr.pinned and cpu == -1, it will only update the count for the
> first online cpu, not all of them.
> 
> Paul.


Oh right! That's really idiotic. Will fix.

Thanks!



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