linux-kernel - Re: [PATCHv2 3/5] kernel/irq: make irq_{enter,exit}() in handle_domain

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Date:   Wed, 29 Sep 2021 11:15:09 +0800
From:   Pingfan Liu <piliu@...hat.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Pingfan Liu <kernelfans@...il.com>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Joey Gouly <joey.gouly@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Julien Thierry <julien.thierry@....com>,
        Yuichi Ito <ito-yuichi@...itsu.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2 3/5] kernel/irq: make irq_{enter,exit}() in
 handle_domain_irq() arch optional

On Tue, Sep 28, 2021 at 09:55:00AM +0100, Mark Rutland wrote:
> On Fri, Sep 24, 2021 at 09:28:35PM +0800, Pingfan Liu wrote:
> > handle_domain_irq() contains two major parts:
> >   -1. irq_enter()/irq_exit(), which serves as hooks for rcu and trace etc.
> >   -2. irq mapping and dispatching
> > 
> > After the introduction of irqentry_enter()/irqentry_exit() and arch
> > specific counterpart (e.g. arm64), roughly speaking, rcu_irq_enter() has
> > already been called.  Hence here comes requirement to move
> > irq_enter/irq_exit out of handle_domain_irq(). And arches should handle
> > about irq_enter_rcu()/irq_exit_rcu() by themself.
> > 
> > Since there is still arches, which does not adopt irqentry_enter(), let
> > irq_enter()/irq_exit() arch optional in handle_domain_irq().
> 
> The patch below looks good to me, but the commit message is a little
> hard to follow. How about:
> 
>   When an IRQ is taken, some accounting needs to be performed to enter
>   and exit IRQ context around the IRQ handler. Historically arch code
>   would leave this to the irqchip or core IRQ code, but these days we
>   want this to happen in exception entry code, and architectures such as
>   arm64 do this.
> 
>   Currently handle_domain_irq() performs this entry/exit accounting, and
>   if used on an architecture where the entry code also does this, the
>   entry/exit accounting will be performed twice per IRQ. This is
>   problematic as core RCU code such as rcu_is_cpu_rrupt_from_idle()
>   depends on this happening once per IRQ, and will not detect quescent
>   periods correctly, leading to stall warnings.
> 
>   As irqchip drivers which use handle_domain_irq() need to work on
>   architectures with or without their own entry/exit accounting, this
>   patch makes handle_domain_irq() conditionally perform the entry
>   accounting depending on a new HAVE_ARCH_IRQENTRY Kconfig symbol that
>   architectures can select if they perform this entry accounting
>   themselves.
> 
>   For architectures which do not select the symbol. there should be no
>   functional change as a result of this patch.
> 
Thanks for your precious time to improve the log. It looks great, and I
will use it in V2. I will keep learning to improve my level of log.

> With that commit message:
> 
>   Reviewed-by: Mark Rutland <mark.rutland@....com>
> 
Thanks,

	Pingfan