Re: [RFC 1/3] Unified NMI delayed call mechanism

[Huang Ying <ying.huang@intel.com>]

On Sat, 2010-06-12 at 18:25 +0800, Ingo Molnar wrote:
> * Huang Ying <ying.huang@intel.com> wrote:
> 
> > NMI can be triggered even when IRQ is masked. So it is not safe for NMI 
> > handler to call some functions. One solution is to delay the call via self 
> > interrupt, so that the delayed call can be done once the interrupt is 
> > enabled again. This has been implemented in MCE and perf event. This patch 
> > provides a unified version and make it easier for other NMI semantic handler 
> > to take use of the delayed call.
> > 
> > Signed-off-by: Huang Ying <ying.huang@intel.com>
> > ---
> >  arch/x86/include/asm/entry_arch.h  |    1 
> >  arch/x86/include/asm/hw_irq.h      |    1 
> >  arch/x86/include/asm/irq_vectors.h |    5 +
> >  arch/x86/include/asm/nmi.h         |    7 ++
> >  arch/x86/kernel/entry_64.S         |    3 +
> >  arch/x86/kernel/irqinit.c          |    3 +
> >  arch/x86/kernel/traps.c            |  104 +++++++++++++++++++++++++++++++++++++
> >  7 files changed, 124 insertions(+)
> 
> Instead of introducing this extra intermediate facility please use the same 
> approach the unified NMI watchdog is using (see latest -tip): a perf event 
> callback gives all the extra functionality needed.

Sorry, if my understanding is correct, the perf event overflow callback
should be run in NMI context instead of a delayed context (such as IRQ,
soft_irq, process context). That is, the backtrace of
watchdog_overflow_callback should be something as follow:

x86_pmu_handle_irq
  perf_event_overflow
    __perf_event_overflow
      watchdog_overflow_callback

I do not find the delayed mechanism here.

> The MCE code needs to be updated to use that - and then it will be integrated
> into the events framework.

MCE is NMI-like, and there are other NMI users too. I think some of them
will need some kind of delayed call mechanism. In fact, perf itself uses
self-made NMI delayed call mechanism too, I just want to generalize it
for other users too.

Best Regards,
Huang Ying