Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
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- Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Date: Mon, 15 Jun 2009 13:04:02 -0700 (PDT)
- Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, mingo@xxxxxxxxxx, hpa@xxxxxxxxx, paulus@xxxxxxxxx, acme@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, a.p.zijlstra@xxxxxxxxx, penberg@xxxxxxxxxxxxxx, vegard.nossum@xxxxxxxxx, efault@xxxxxx, jeremy@xxxxxxxx, npiggin@xxxxxxx, tglx@xxxxxxxxxxxxx, linux-tip-commits@xxxxxxxxxxxxxxx
- In-reply-to: <20090615194344.GA12554@xxxxxxx>
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- User-agent: Alpine 2.01 (LFD 1184 2008-12-16)
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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