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

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Date:	Tue, 16 Jun 2009 15:37:11 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
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

Mathieu Desnoyers wrote:
> 
> So, in summary :
> 
> - near-zero measurable runtime cost.
> - NMI-reentrancy constraint on a very small and well-defined trap
>   handler code path.
> - simplifies life of tracer and profilers. (meaning : makes a lot of
>   _other_ kernel code much easier to write and understand)
> - removes ad-hoc corner cases management from those users.
> - provides early error detection because the nmi-reentrant code path is
>   shared by all users.
> 
> So I'll use your own argument : making this trap handler code path
> nmi-reentrant will simplify an already existing bunch of in-kernel users
> (oprofile, perf counter tool, ftrace..). Moving the burden from
> subsystems spread across the kernel tree to a single, well defined spot
> looks like a constraint that will _diminish_ overall kernel development
> cost.
> 

No, this is utter bullshit.

YOU ARE ADDING A CONSTRAINT TO ONE OF THE HOTTEST PATHS IN THE KERNEL.

Constraining future optimizations.

To support tools.

That is what I'm objecting to.

	-hpa



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