Re: [PATCHv2 4/5] irqchip/GICv3: let gic_handle_irq() utilize irqentry on arm64

[Marc Zyngier]

On Wed, 29 Sep 2021 04:10:11 +0100,
Pingfan Liu <piliu@xxxxxxxxxx> 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.

	M.

-- 
Without deviation from the norm, progress is not possible.