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 13:04:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, mingo@...hat.com,
	hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	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



On Mon, 15 Jun 2009, Ingo Molnar wrote:
> 
> Well i guess it depends. For server apps it is true - syscalls are a 
> lot more dominant, MMs are long-running so any startup cost gets 
> amortized and pagefaults are avoided.
> 
> For something like a kernel build we have 7 times as many pagefaults 
> as syscalls:

Ingo - calm down.

This is not about page faults.

This is purely about taps FROM KERNEL SPACE.

Yes, for the kernel build we have 7 times as many page faults as system 
calls, BUT I BET 99.9% of them are from user mode!

The whole "open-code iret" only works for exceptions that happened in 
kernel mode. That's a _vanishingly_ small number (outside of device 
interrupts that happen during idle).

		Linus
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