linux-kernel - Re: [PATCH v2 2/6] genirq: Allow an interrupt to be marked as 'raw'

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Date:   Thu, 03 Dec 2020 15:52:44 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marc Zyngier <maz@...nel.org>,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@....linux.org.uk>,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH v2 2/6] genirq: Allow an interrupt to be marked as 'raw'


On 03/12/20 13:03, Peter Zijlstra wrote:
> On Thu, Nov 26, 2020 at 06:18:33PM +0000, Valentin Schneider wrote:
>> If I got the RCU bits right from what Thomas mentioned in
>>
>>   https://lore.kernel.org/r/87ft5q18qs.fsf@nanos.tec.linutronix.de
>>   https://lore.kernel.org/r/87lfewnmdz.fsf@nanos.tec.linutronix.de
>>
>> then we're still missing something to notify RCU in the case the IRQ hits
>> the idle task. All I see on our entry path is
>>
>>   trace_hardirqs_off();
>>   ...
>>   irq_handler()
>>     handle_domain_irq();
>>   ...
>>   trace_hardirqs_on();
>>
>> so we do currently rely on handle_domain_irq()'s irq_enter() + irq_exit()
>> for that. rcu_irq_enter() says CONFIG_RCU_EQS_DEBUG=y can detect missing
>> bits, but I don't get any warnings with your series on my Juno.
>
> The scheduler IPI really doesn't need RCU either ;-)

Because it doesn't enter any new read-side section, right?
But as with any other interrupt, we could then go through:

  preempt_schedule_irq() ~> pick_next_task_fair() -> newidle_balance()

which does enter a read-side section, so RCU would need to be
watching. Looking at kernel/entry/common.c:irqentry_exit_cond_resched(), it
seems we do check for this via rcu_irq_exit_check_preempt().

I however cannot grok why irqentry_exit() *doesn't* call into
preempt_schedule_irq() if RCU wasn't watching on IRQ entry, so I'm starting
to question everything (again).