Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

[David Miller <davem@davemloft.net>]

From: David Miller <davem@davemloft.net>
Date: Fri, 30 Jan 2009 13:54:09 -0800 (PST)

> From: Ingo Molnar <mingo@elte.hu>
> Date: Fri, 30 Jan 2009 16:01:25 +0100
> 
> > 
> > * David Miller <davem@davemloft.net> wrote:
> > 
> > Reducing it to 1 HZ was kind of a performance hack: running NMIs at HZ 
> > needlessly interrupts the CPU HZ times a second. It's more than enough to 
> > have 1 nmi-watchdog tick per second to notice deadlocks that take longer 
> > than 5 seconds.
> 
> For the NMI watchdog's purposes I understand the intent, and this
> is perfectly fine.
> 
> The problem is that it stays at '1' when oprofile starts using the NMI
> watchdog, and we certainly want more than one oprofile tick per second
> :-)

Just making sure you understand the problem, here is the
sequence of events:

1) At bootup, the NMI watchdog is tested.

   It is tested with nmi_hz=HZ

2) If the test passes, nmi_hz is reduced down to '1'

As I stated, everything up to this point is fine.  Next:

3) oprofile initializes and if we choose to use the NMI
   timer for oprofile profiling it is implemented using
   a simple DIE_NMI notifier.

   However, nmi_hz is still just '1' which means that oprofile
   will only receive one sample per-second.  And this is definitely
   not what we want.

Somehow the code in arch/x86/oprofile/nmi_timer_int.c needs to
have an interface into the NMI watchdog core so that it can
increase nmi_hz back up to "HZ" when the NMI timer profiling is
enabled and back down to "1" when such profiling stops.