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Date:   Wed, 29 Sep 2021 10:23:58 +0100
From:   Mark Rutland <mark.rutland@arm.com>
To:     Pingfan Liu <piliu@redhat.com>
Cc:     Marc Zyngier <maz@kernel.org>, Pingfan Liu <kernelfans@gmail.com>,
        linux-arm-kernel@lists.infradead.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>, Joey Gouly <joey.gouly@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Yuichi Ito <ito-yuichi@fujitsu.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCHv2 4/5] irqchip/GICv3: let gic_handle_irq() utilize
 irqentry on arm64
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On Wed, Sep 29, 2021 at 04:27:11PM +0800, Pingfan Liu wrote:
> On Wed, Sep 29, 2021 at 08:20:35AM +0100, Marc Zyngier wrote:
> > On Wed, 29 Sep 2021 04:10:11 +0100,
> > Pingfan Liu <piliu@redhat.com> wrote:
> > > 
> > > On Tue, Sep 28, 2021 at 10:10:53AM +0100, Mark Rutland wrote:
> > > > On Fri, Sep 24, 2021 at 09:28:36PM +0800, Pingfan Liu wrote:
> > > > > The call to rcu_irq_enter() originated from gic_handle_irq() is
> > > > > redundant now, since arm64 has enter_from_kernel_mode() akin to
> > > > > irqenter_entry(), which has already called rcu_irq_enter().
> > > > 
> > > > Here I think you're referring to the call in handle_domain_irq(), but
> > > > that isn't clear from the commit message.
> > > > 
> > > Yes, and I will make it clear in V2.
> > > 
> > > > > Based on code analysis, the redundant can raise some mistake, e.g.
> > > > > rcu_data->dynticks_nmi_nesting inc 2, which causes
> > > > > rcu_is_cpu_rrupt_from_idle() unexpected.
> > > > > 
> > > > > So eliminate the call to irq_enter() in handle_domain_irq(). And
> > > > > accordingly supplementing irq_enter_rcu().
> > > > 
> > > > We support many more irqchips on arm64, and GICv3 can be used on regular
> > > > 32-bit arm, so this isn't right. Moving the irq_enter_rcu() call
> > > > into the GICv3 driver specifically breaks other drivers on arm64 by
> > > > removing the call, and breaks the GICv3 driver on arm by adding a
> > > > duplicate call.
> > > > 
> > > Oops. I forgot to protect the code in GICv3 with CONFIG_HAVE_ARCH_IRQENTRY
> > > 
> > > > It looks like this should live in do_interrupt_handler() in
> > > > arch/arm64/kernel/entry-common.c, e.g.
> > > > 
> > > > | static void do_interrupt_handler(struct pt_regs *regs,
> > > > | 				 void (*handler)(struct pt_regs *)) 
> > > > | {
> > > > | 	irq_enter_rcu();
> > > > | 	if (on_thread_stack())
> > > > | 		call_on_irq_stack(regs, handler);
> > > > | 	else
> > > > | 		handler(regs);
> > > > | 	irq_exit_rcu();
> > > > | }
> > > > 
> > > > ... unless there's some problem with that?
> > > > 
> > > Yeah, do_interrupt_handler() is a more suitable place. But to resolve
> > > the performance regression of rescheduling IPI [1], it is badly demanded to
> > > distinguish irqnr before calling irq_enter_rcu() (please see 5/5 and [2]
> > > for the context). So it is a compromise to host the code in GICv3.
> > > 
> > > Any good idea?
> > 
> > There is no way we are going to single out a particular interrupt
> > controller. As for the "regression", we'll have to look at the numbers
> > once we have fixed the whole infrastructure.
> > 
> But I just realize that at present, gic_handle_nmi() sits behind
> gic_handle_irq().  So it will make an mistaken for accounting of normal
> interrupt if calling irq_enter_rcu() in do_interrupt_handler().

We can restructure entry-common.c to avoid that if necessary.

TBH, the more I see problems in this area the more I want to rip out the
pNMI bits...

> And going through drivers/irqchip/irq-chip-gic*, I think there are only
> two files should be handled: irq-gic.c and irq-gic-v3.c.

That are irqchips other than GICv2 and GICv3 that are used as the root
irqchip on arm64. For example, Raspberry Pi 3 uses
drivers/irqchip/irq-bcm2836.c as its root irqchip.

Thanks,
Mark.