linux-kernel - RE: [RFC 06/12] genirq: Add per-cpu flow handler with conditional IRQ stats

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Date: Wed, 05 Jun 2024 15:20:17 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Michael Kelley <mhklinux@...look.com>, "kys@...rosoft.com"
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Subject: RE: [RFC 06/12] genirq: Add per-cpu flow handler with conditional
 IRQ stats

On Tue, Jun 04 2024 at 23:03, Michael Kelley wrote:
> From: Thomas Gleixner <tglx@...utronix.de> Sent: Tuesday, June 4, 2024 11:14 AM
>>    1) Move the inner workings of handle_percpu_irq() out into
>>       a static function which returns the 'handled' value and
>>       share it between the two handler functions.
>
> The "inner workings" aren't quite the same in the two cases.
> handle_percpu_irq() uses handle_irq_event_percpu() while
> handle_percpu_demux_irq() uses __handle_irq_event_percpu().
> The latter doesn't do add_interrupt_randomness() because the
> demultiplexed IRQ handler will do it.  Doing add_interrupt_randomness()
> twice doesn't break anything, but it's more overhead in the hard irq
> path, which I'm trying to avoid.  The extra functionality in the
> non-double-underscore version could be hoisted up to
> handle_percpu_irq(), but that offsets gains from sharing the
> inner workings.

That's not rocket science to solve:

static irqreturn_t helper(desc, func)
{
	boiler_plate..
        ret = func(desc)
	boiler_plate..
        return ret;
}

No?

TBH, I still hate that conditional accounting :)

>>    2) Allocate a proper interrupt for the management mode and invoke it
>>       via generic_handle_irq() just as any other demultiplex interrupt.
>>       That spares all the special casing in the core code and just
>>       works.
>
> Yes, this would work on x86, as the top-level interrupt isn't a Linux IRQ,
> and the interrupt counting is done in Hyper-V specific code that could be
> removed.  The demux'ed interrupt does the counting.
>
> But on arm64 the top-level interrupt *is* a Linux IRQ, so each
> interrupt will get double-counted, which is a problem.

What is the problem?

You have: toplevel, mgmt, device[], right?

They are all accounted for seperately and each toplevel interrupt might
result in demultiplexing one or more interrupts (mgmt, device[]), no?

IMO accounting the toplevel interrupt seperately is informative because
it allows you to figure out whether demultiplexing is clustered or not,
but I lost that argument long ago. That's why most demultiplex muck
installs a chained handler, which is a design fail on it's own.

Thanks,

        tglx