linux-kernel - Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware breakpoints

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Date:	Fri, 24 Jul 2009 16:02:56 +0200
From:	Frédéric Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Anton Blanchard <anton@...ba.org>,
	Li Zefan <lizf@...fujitsu.com>,
	Zhaolei <zhaolei@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"K . Prasad" <prasad@...ux.vnet.ibm.com>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware 
	breakpoints

2009/7/23 Peter Zijlstra <a.p.zijlstra@...llo.nl>:
> On Mon, 2009-07-20 at 13:08 -0400, Frederic Weisbecker wrote:
>> This adds the support for kernel hardware breakpoints in perfcounter.
>> It is added as a new type of software counter and can be defined by
>> using the counter number 5 and by passsing the address of the
>> breakpoint to set through the config attribute.
>
> Is there a limit to these hardware breakpoints? If so, the software
> counter model is not sufficient, since we assume we can always schedule
> all software counters. However if you were to add more counters than you
> have hardware breakpoints you're hosed.
>
>

Hmm, indeed. But this patch handles this case:

+static const struct pmu *bp_perf_counter_init(struct perf_counter *counter)
+{
+       if (hw_breakpoint_perf_init((unsigned long)counter->attr.config))
+               return NULL;
+

IIRC, hw_breakpoint_perf_init() calls register_kernel_breakpoint() which in turn
returns -ENOSPC if we haven't any breakpoint room left.

It seems we can only set 4 breakpoints simultaneously in x86, or
something close to that.
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