Messages in this thread | | | Date | Tue, 16 Jun 2015 11:07:13 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 4/8] ARCv2: perf: Support sampling events using overflow interrupts |
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On Tue, Jun 16, 2015 at 05:37:40AM +0000, Vineet Gupta wrote: > On Monday 15 June 2015 09:55 PM, Peter Zijlstra wrote: > > On Tue, Jun 09, 2015 at 05:49:28PM +0530, Vineet Gupta wrote: > >> + if (arc_pmu->has_interrupts) { > >> + int irq = platform_get_irq(pdev, 0); > > Hmm, so you're requesting a regular interrupt. > > > > I see your architecture has IRQ priorities, could you play games and > > create NMIs using those? > > > > For example, never mask L1 (assuming that's the highest priority) and > > treat that as an NMI. > > I've had this idea before, however, while ARCv2 provides hardware interrupt > priorities, we really can't implement true NMI, because CLRI / SETI used at > backend of loal_irq_save() / restore() impact all priorities (statsu32 register > has a global enable interrupt bit which these wiggle). So e.g. a > spin_lock_irqsave() will lock out even the perf interrupt.
Hmm, bugger. I (of course) only looked at the kernel source, since that is all I have, and the current arch/arc/ frobs with those two En bits in status32.
So arcv2 changed all that, shame.
> OTOH, we can improve the perf isr path a bit - by not routing it thru regular > interrupt return path (song and dance of CONFIG_PREEMPT_IRQ and possible > preemption). Plus there's a bit more we can do in the isr itself - not looping > thru 32 counters etc using ffs() etc - but I'd rather do that as separate series, > once we have the core support in.
Yeah, borderline useful though, the reason the NMI thing is so useful is that you can profile _inside_ IRQ-disabled regions. Now your local_irq_enable() et al will be the hottest functions ever :-)
Regular interrupts are really only useful for userspace profiling, which if plenty useful on its own of course. But kernel profiling is very handy too :-)
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