Re: PMU and timer interrupts

[Jack Steiner <steiner@sgi.com>]

On Thu, Feb 09, 2006 at 10:39:49AM -0800, Stephane Eranian wrote:
> Tony, Jack,
> 
> On Thu, Feb 09, 2006 at 10:20:18AM -0800, Luck, Tony wrote:
> > >It was recently pointed out to me that the PMU interrupt
> > >vector is set to 0xee just below the timer interrupt at 0xef.
> > >As such, the timer interrupt has higher priority than the PMU
> > >interrupt. This would make sense except that a side effect is
> > >that it is not possible to collect samples from within the timer
> > >interrupt, i.e. we have a blind spot. Another side effect is
> > >that events happening during the timer handler, may be falsely
> > >attributed to the point where we write the EOI register.
> > >
> > >I looked at the timer_interrupt() path, and I did not see anything special
> > >that would prevent us from swapping the interrupt vectors thereby removing
> > >the blind spot we have.
> > >
> > >Does anybody see a problem with this?
> > 
> > How much would this help?  Both 0xEF and 0xEE are part of the
> > same priority class (see table 5-8 on p. 2-112 of SDM).
> > 
> Correct me if I am wrong, but the class masking happens only with TPR
> masking. So if we had timer=0xee, PMU=0xef and if the kernel does not
> raise the TPR class masking to the priority class of 0xef/0xee, then
> we could still get a PMU interrupt while executing the timer handler.
> 

What you say is true. However, the kernel does use the TPR to enforce 
priority classes:

	void
	ia64_handle_irq (ia64_vector vector, struct pt_regs *regs)
	{
		...
		saved_tpr = ia64_getreg(_IA64_REG_CR_TPR);
		while (vector != IA64_SPURIOUS_INT_VECTOR) {
			if (!IS_RESCHEDULE(vector)) {
				ia64_setreg(_IA64_REG_CR_TPR, vector);
				__do_IRQ(local_vector_to_irq(vector), regs);
		....
	}

Use of priority classes prevents excessive recursion. Without it, as a worse case,
	- a priority 16 interrupt could be interrupted by a priority 17
	- which is interrupted by an 18
	- which is interrupted by an 19
	- which is interrupted by an 20
	- etc. and the kernel stack goes BOOM   :-)

Priority classes limits the recursion to 15 levels at most.