linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Date:	Mon, 15 Jun 2009 17:54:20 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, penberg@...helsinki.fi,
	vegard.nossum@...il.com, efault@....de, jeremy@...p.org,
	npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods

* H. Peter Anvin (hpa@...or.com) wrote:
> Mathieu Desnoyers wrote:
> > 
> > As the maintainer of the out-of-tree LTTng tracer, which hooks in the
> > page fault handler with tracepoints, and which can build almost entirely
> > as modules, I am very tempted to argue that having the nmi-code entirely
> > robust wrt in-kernel page faults would be a very-nice-to-have feature.
> > 
> 
> I doubt that is ever going to be reliable, due to reentrancy issues.
> 
> 	-hpa

Do you mean the page fault handler code is no ever going to be reliable
or the tracer code ?

I spent a great deal of effort making LTTng lockless and reentrant wrt
NMIs. It would be great if the low-level kernel exception handlers would
do the same, therefore I would not have to isolate the tracer from the
kernel as I currently do. Well, I would still continue to isolate the
tracer from the kernel, but at least I would not have to spend as much
effort controlling what exceptions and faults paths the tracer is
executing.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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