linux-kernel - Re: [RFC PATCH v1 2/9] genirq: Support shared per_cpu

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Date:   Mon, 25 Sep 2017 23:06:39 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paul Burton <paul.burton@...tec.com>
cc:     Ralf Baechle <ralf@...ux-mips.org>, dianders@...omium.org,
        James Hogan <james.hogan@...tec.com>,
        Brian Norris <briannorris@...omium.org>,
        Jason Cooper <jason@...edaemon.net>, jeffy.chen@...k-chips.com,
        Marc Zyngier <marc.zyngier@....com>,
        linux-kernel@...r.kernel.org, linux-mips@...ux-mips.org,
        tfiga@...omium.org
Subject: Re: [RFC PATCH v1 2/9] genirq: Support shared per_cpu_devid
 interrupts

On Thu, 7 Sep 2017, Paul Burton wrote:
> Up until now per_cpu_devid interrupts have not supported sharing. On
> MIPS we have some percpu interrupts which are shared in many systems -
> a single CPU interrupt line may be used to indicate a timer interrupt,
> performance counter interrupt or fast debug channel interrupt. We have
> up until now supported this with a series of hacks, wherein drivers call
> each other's interrupt handlers & our MIPS GIC irqchip driver includes a
> hack which configures the interrupt(s) for all CPUs. In order to allow
> this mess to be cleaned up, this patch introduces support for shared
> per_cpu_devid interrupts.
> 
> The major portion of this is supporting per_cpu_devid interrupts in
> __handle_irq_event_percpu() and then making use of this, via
> handle_irq_event_percpu(), from handler_percpu_devif_irq() to invoke the
> handler for all actions associated with the shared interrupt. This does
> have a few side effects worth noting:
> 
>  - per_cpu_devid interrupts will now add to the entropy pool via
>    add_interrupt_randomness(), where they previously did not.
> 
>  - per_cpu_devid interrupts will record timings when IRQS_TIMINGS is
>    set, via record_irq_time(), where they previously did not.
> 
>  - per_cpu_devid interrupts will handle an IRQ_WAKE_THREAD return from
>    their handlers to wake a thread, where they previously did not.

That's broken because it lacks the magic synchronization which is described
in the comment in __irq_wake_thread().

> I'm not aware of any reason the above should be bad side effects, so
> sharing __handle_irq_event_percpu() for per_cpu_devid interrupts seems
> like a positive.

Now you are :)

The other side effect of this is the extra overhead. You add an extra
conditional into the main interrupt handling function
__handle_irq_event_percpu() and the extra loop and hoops overhead for
handle_percpu_devid_irq().

Thanks,

	tglx