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

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Date: Tue, 04 Jun 2024 20:13:47 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: mhklinux@...look.com, kys@...rosoft.com, haiyangz@...rosoft.com,
 wei.liu@...nel.org, decui@...rosoft.com, mingo@...hat.com, bp@...en8.de,
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 James.Bottomley@...senPartnership.com, martin.petersen@...cle.com,
 arnd@...db.de, linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-pci@...r.kernel.org, linux-scsi@...r.kernel.org,
 linux-arch@...r.kernel.org
Cc: maz@...nel.org, den@...inux.co.jp, jgowans@...zon.com,
 dawei.li@...ngroup.cn
Subject: Re: [RFC 06/12] genirq: Add per-cpu flow handler with conditional
 IRQ stats

Michael!

On Mon, Jun 03 2024 at 22:09, mhkelley58@...il.com wrote:
> Hyper-V VMBus devices generate interrupts that are multiplexed
> onto a single per-CPU architectural interrupt. The top-level VMBus
> driver ISR demultiplexes these interrupts and invokes per-device
> handlers. Currently, these per-device handlers are not modeled as
> Linux IRQs, so /proc/interrupts shows all VMBus interrupts as accounted
> to the top level architectural interrupt. Visibility into per-device
> interrupt stats requires accessing VMBus-specific entries in sysfs.
> The top-level VMBus driver ISR also handles management-related
> interrupts that are not attributable to a particular VMBus device.
>
> As part of changing VMBus to model VMBus per-device handlers as
> normal Linux IRQs, the top-level VMBus driver needs to conditionally
> account for interrupts. If it passes the interrupt off to a
> device-specific IRQ, the interrupt stats are done by that IRQ
> handler, and accounting for the interrupt at the top level
> is duplicative. But if it handles a management-related interrupt
> itself, then it should account for the interrupt itself.
>
> Introduce a new flow handler that provides this functionality.
> The new handler parallels handle_percpu_irq(), but does stats
> only if the ISR returns other than IRQ_NONE. The existing
> handle_untracked_irq() can't be used because it doesn't work for
> per-cpu IRQs, and it doesn't provide conditional stats.

There is a two other options to solve this:

   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.

   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.

Thanks,

        tglx