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

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Date:   Sat, 05 Dec 2020 19:24:15 +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 15:52, Valentin Schneider wrote:
> On 03/12/20 13:03, Peter Zijlstra wrote:
[...]
>> The scheduler IPI really doesn't need RCU either ;-)
[...]
> 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

RCU wasn't watching on IRQ entry:
  -> we should be on the idle task
  -> no unvoluntary preemption for the idle task, scheduling always happens
     at the tail of the idle loop
  -> ignore what I've been saying, current patch is fine