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:

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On Wed, 29 Sep 2021 04:10:11 +0100,
Pingfan Liu [off-list ref] wrote:

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On Tue, Sep 28, 2021 at 10:10:53AM +0100, Mark Rutland wrote:

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On Fri, Sep 24, 2021 at 09:28:36PM +0800, Pingfan Liu wrote:

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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.

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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

quoted

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.

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