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

Valentin Schneider
2020-12-05T19:25:21Z

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