Date:	Mon, 14 Dec 2009 20:32:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	David Miller <davem@...emloft.net>
Cc:	mingo@...e.hu, tglx@...utronix.de, linux-kernel@...r.kernel.org
Subject: Re: cpu_clock() in NMIs

On Mon, 2009-12-14 at 11:09 -0800, David Miller wrote:
> From: Peter Zijlstra <peterz@...radead.org>
> Date: Mon, 14 Dec 2009 10:02:39 +0100
> 
> > I'm not sure, traditionally sched_clock() was always called with IRQs
> > disabled, and on eg. x86 that is needed because we read the TSC and then
> > scale that value depending on CPUfreq state, which can be changed by a
> > CPUfreq interrupt. Allowing NMIs in between isn't really a problem,
> > allowing IRQs in is.
> 
> Ok, that looks fine then.
> 
> But, speaking more generally, any local_irq_{disable,save}() done in
> an NMI handler has a high probability of being a bug.  And it is a
> bug if the IRQ disabling is there to prevent re-entry of the code
> on the local cpu.
> 
> > Now, the SPARC implementation might be good without IRQs disabled, but
> > we should at least look at all other arches before we do what you
> > propose below. As it removes the IRQ disable from the callsites whereas
> > it previously always had that.
> 
> Here's a quick audit:
> 
> 1) Generic kernel/sched_clock.c implementation does a subtraction
>    of jiffies with a constant, then does some constant math on it.
>    Should be OK.
> 
> 2) sparc64 is fine, just reads a register and multiplies with a
>    boot time calculated value, then shifts down by a constant.
>    Should be OK.
> 
> 3) ARM mach-map, same situation as sparc64
> 
> 4) ARM mach-pxa, same situation as sparc64
> 
> 5) ARM mach-realview, multiplies counter a constant then divides by
>    one, should be OK.
> 
> 6) ARM mach-sa1100, same as mach-realview
> 
> 7) ARM mach-u300, uses boot time computed multiplier and shift,
>    should be OK.
> 
> 8) ARM mach-versatile, same as mach-realview
> 
> 9) ARM plat-IOP, same as mach-u300
> 
> 10) ARM plat-omap, same as mach-u300
> 
> 11) ARM plat-orion, same as sparc64
> 
> 12) Blackfin, TS version is same as sparc64
> 
>     non-TS version is unnecessary duplication of generic weak
>     function version in kernel/sched_clock.c and could be deleted
> 
> 13) CRIS, another dup of kernel/sched_clock.c weak function
> 
> 14) FRV, another dup of kernel/sched_clock.c weak function
> 
> 15) IA64, same as sparc64 for native version.
>     Paravirt version uses preemption disable, but also relies on
>     IA64 always setting HAVE_UNSTABLE_SCHED_CLOCK which it does,
>     and therefore IA64 would still disable interrupts with my change
> 
> 16) m68knommu coldfire, multiplies by a constant then shifts down by
>     one, should be OK
> 
> 17) mn10300, same as sparc64
> 
> 18) powerpc,  non-__USE_RTC() case is same as sparc64
> 
>     __USE_RTC() case also looks fine
> 
> 19) s390, depends upon preemption being disabled so that
>     stop_machine does not interrupt the sched_clock() call
> 
>     should be OK
> 
> 20) x86 sets HAVE_UNSTABLE_SCHED_CLOCK
> 
> It should be safe.

OK, you convinced me plenty ;-)

I guess we need some debug code to ensure we don't grow any
local_irq_save/restore/disable code in NMI paths, and maybe document
this some place.

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
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