linux-kernel - Re: [PATCH v1 1/3] irqchip/gic: enable irq target all

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Date:   Fri, 27 Nov 2020 18:56:10 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Hanks Chen <hanks.chen@...iatek.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Russell King <linux@...linux.org.uk>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mediatek@...ts.infradead.org,
        CC Hwang <cc.hwang@...iatek.com>,
        Kuohong Wang <kuohong.wang@...iatek.com>,
        Loda Chou <loda.chou@...iatek.com>
Subject: Re: [PATCH v1 1/3] irqchip/gic: enable irq target all

On Fri, Nov 27, 2020 at 06:11:01PM +0000, Marc Zyngier wrote:
> On 2020-11-27 14:15, Hanks Chen wrote:
> > Support for interrupt distribution design for SMP system solutions.
> 
> As far as I know, we have been supporting interrupt distribution on
> ARM SMP systems pretty well for the past... what... 15 years?
> I'm sure Russell can dig out an ARM926 SMP system that even predates
> that.
> 
> > With this feature enabled ,the SPI interrupts would be routed to
> > all the cores rather than boot core to achieve better
> > load balance of interrupt handling.
> 
> Please quantify this compared to the current distribution method.
> 
> > That is, interrupts might be serviced simultaneously on different CPUs.
> 
> They already can. What is new here? Or do you mean the *same* interrupt
> being serviced by different CPUs *at the same time*? That'd be fun.

IIRC (it's been many years since I looked at the GIC), more than one CPU
is woken and if they all read the INTACK, only one of them gets the
actual IRQ number, the others see a spurious interrupt. I thought we
decided that's not an efficient way to handle interrupts, so a software
irqbalancer is better.

Has anything changed since then?

I'm also concerned that in a big.LITTLE system, you may see the big CPUs
taking the interrupts all the time, which is nice for energy efficiency.

-- 
Catalin