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

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Date:	Sat, 25 Jul 2009 18:27:09 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	"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

On Sat, 2009-07-25 at 11:51 -0400, Mathieu Desnoyers wrote:
> I agree with your idea to split the hardware counter management from
> virtual per-process counters. IMO, this limited resource needs to be
> managed centrally at one location and because system-wide level
> performance counters do not need to flip the performance counters
> depending on the current task.

System wide counters never care about the task state.

Its task counters we sometimes don't re-program counters for on context
switch when both tasks have the same configuration, saving greatly on
context switch costs.

>  We can easily think of an embedded system
> where providing system-wide performance counters would be important (for
> tracing for instance), but which would compile-out the per-task
> performance counters to save space.

That doesn't make sense, the per task/global parts of perf counters are
tightly interwoven and don't differ much.

> Note that you will have to deal with some policy here, because you can
> have performance counter reservation asked from both the kernel
> (for either kernel and per-task watchpoints) and from userspace (for
> per-task watchpoints).

cpu counters can be both kernel and user
task counter can be both kernel and user.

Your above use of 'and' doesn't make sense.

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