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Date:   Wed, 29 Sep 2021 08:20:35 +0100
Message-ID: <87mtnvu9to.wl-maz@kernel.org>
From:   Marc Zyngier <maz@kernel.org>
To:     Pingfan Liu <piliu@redhat.com>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        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
In-Reply-To: <YVPZEwfi8OFkzcd1@piliu.users.ipa.redhat.com>
References: <20210924132837.45994-1-kernelfans@gmail.com>
        <20210924132837.45994-5-kernelfans@gmail.com>
        <20210928091053.GD1924@C02TD0UTHF1T.local>
        <YVPZEwfi8OFkzcd1@piliu.users.ipa.redhat.com>
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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.

	M.

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