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

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Message-ID: <20090729145718.GB3040@ghostprotocols.net>
Date:	Wed, 29 Jul 2009 11:57:18 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	prasad@...ux.vnet.ibm.com,
	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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 5/5] perfcounter: Add support for kernel hardware
	breakpoints

Em Wed, Jul 29, 2009 at 11:22:52AM +0200, Peter Zijlstra escreveu:
> On Wed, 2009-07-29 at 12:07 +0530, K.Prasad wrote:
> 
> > > That still doesn't provide per-cpu breakpoints.
> > > 
> > 
> > Yes, it doesn't provide a per-cpu only implementation. One can obtain
> > the per-cpu data from the system-wide breakpoints by filtering it for a
> > given CPU (agreed, it will associated overhead).
> > 
> > A true per-cpu breakpoint implementation that co-exists with
> > system-wide and per-task breakpoints will be difficult. It might require
> > the re-introduction of some old features and a few new ones (like switching
> > between kernel and user-space breakpoints at syscall time) that were
> > rejected earlier by the community.
> 
> I'm not clear on why you'd need to switch breakpoints on syscall entry.
> You can simply leave the kernel address breakpoint around in userspace,
> they're not able to poke at that address space anyway.
> 
> > Also, the reason for a per-cpu only breakpoint (user and kernel-space)
> > isn't very obvious. While kernel variables can be read/written
> > throughout the system and user-space variables are per-task, the need
> > for obtaining per-cpu information isn't clear.
> 
> Well, suppose you're monitoring a per-cpu variable, or interested in the
> effects of a workload confined to 1 cpu or node, there is no reason to
> have this breakpoint on all cpus.

I was going to talk about exactly that, CPU isolation for one specific
workload, I don't want that hits on tcp_v4_rcv on another CPU get
counted, just the ones on that specific CPU.

The other CPUs can be given a break (pun intended :-P) by not having to
process traps we're uninterested in.

- Arnaldo
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