Re: [RFC PATCH 3/3] sched/Documentation: Point out use of preempt_schedule_irq()

[Julien Thierry <julien.thierry@arm.com>]

Hi Valentin,

On 31/01/2019 18:23, Valentin Schneider wrote:
> Since there are a few archs out there that call preempt_schedule_irq()
> within a need_resched() loop, point out that it's not needed.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Julien Thierry <julien.thierry@arm.com>
> Cc: linux-doc@vger.kernel.org
> ---
>  Documentation/scheduler/sched-arch.txt | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/Documentation/scheduler/sched-arch.txt b/Documentation/scheduler/sched-arch.txt
> index a2f27bbf2cba..ae41a94da700 100644
> --- a/Documentation/scheduler/sched-arch.txt
> +++ b/Documentation/scheduler/sched-arch.txt
> @@ -59,6 +59,16 @@ Your cpu_idle routines need to obey the following rules:
>  arch/x86/kernel/process.c has examples of both polling and
>  sleeping idle functions.
>  
> +Kernel preemption
> +=================
> +When returning from interrupt context, you should call either of
> +preempt_schedule() or preempt_schedule_irq() if preemption is enabled
> +and need_resched() is true.
> +

I don't think preempt_schedule() is really an option for a return from
interrupt. First thing preempt_schedule() does is:

        if (likely(!preemptible()))
		return;

And preemptible() is:

preempt_count() == 0 && !irqs_disabled()

Generally on return from interrupt context interrupts are disabled, so
we would never be preemptible() and preempt_schedule() would just do
nothing.

Unless I'm missing something.

Cheers,

-- 
Julien Thierry