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: "H. Peter Anvin" <hpa@xxxxxxxxx>
- Date: Tue, 16 Jun 2009 15:37:11 -0700
- Cc: Ingo Molnar <mingo@xxxxxxx>, Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>, mingo@xxxxxxxxxx, paulus@xxxxxxxxx, acme@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, penberg@xxxxxxxxxxxxxx, vegard.nossum@xxxxxxxxx, efault@xxxxxx, jeremy@xxxxxxxx, npiggin@xxxxxxx, tglx@xxxxxxxxxxxxx, linux-tip-commits@xxxxxxxxxxxxxxx
- In-reply-to: <20090616211357.GB21305@Krystal>
- References: <20090615223038.GA15903@Krystal> <4A36CCFC.8070908@xxxxxxxxx> <20090615224908.GA16661@Krystal> <4A36F520.6020604@xxxxxxxxx> <20090616030522.GA22162@Krystal> <20090616083343.GD16229@xxxxxxx> <20090616141956.GB6541@Krystal> <4A37B8CF.8090804@xxxxxxxxx> <20090616190634.GA16430@Krystal> <4A380000.5090206@xxxxxxxxx> <20090616211357.GB21305@Krystal>
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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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