Re: [PATCH 7/8] xtensa: implement counting and sampling perf events

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 06, 2015 at 04:56:09PM +0300, Max Filippov wrote:
> On Mon, Jul 6, 2015 at 4:47 PM, Peter Zijlstra <peterz@infradead.org> wrote:
> > On Mon, Jul 06, 2015 at 04:32:48PM +0300, Max Filippov wrote:
> >> +static int __init xtensa_pmu_init(void)
> >> +{
> >> +     int ret;
> >> +     int irq = irq_create_mapping(NULL, XCHAL_PROFILING_INTERRUPT);
> >
> > Does this platform have interrupt priorities which you can partially
> > mask in order to create NMI like behaviour?
> 
> Not sure what you mean by "NMI like".

There's a number of archs where we implement NMIs by having
local_irq_disable() only disable part of the interrupt priority range
and making sure all 'normal' IRQs are mapped in that priority range.

We then map our NMI handlers to a priority above the 'normal' range,
such that these interrupts can indeed happen when interrupts are
'disabled.

See for example:

b4f4372f96e0 ("sparc64: Make %pil level 15 a pseudo-NMI.")
0c25e9e6cbe7 ("sparc64: Adjust __raw_local_irq_save() to cooperate in NMIs.")
c011f80ba091 ("sparc64: Add some more commentary to __raw_local_irq_save()")

> Interrupt priorities are fixed in the current xtensa architecture, and
> we can in theory mask certain level and below, but practically we
> always mask all low- and medium- level interrupts.
> 
> Also we currently can't have handlers for high priority interrupts written in C.

Why not? Surely this can be cured with an assembly stub?

The advantage of having NMIs is that profiling information for the
kernel becomes much more useful. Without this local_irq_enable() will be
a very 'hot' function.