Messages in this thread | | | Date | Tue, 3 Feb 2009 13:27:32 +0100 | From | Ingo Molnar <> | Subject | Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c |
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* David Miller <davem@davemloft.net> wrote:
> 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.
btw., these types of interactions will be solved in a natural way via perfcounters: in that model the NMI watchdog is a set of per-CPU counters running on each CPU [with a NMI watchdog callback in the IRQ handling routine] - and oprofile uses its own perfcounter - what is left of the PMU hardware.
I.e. each PMU using facility can just use performance counters transparently and interactions will be solved naturally by perfcounters resource management.
Ingo
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