On Wed, Jan 16, 2019 at 12:50:42PM +0000, Valentin Schneider wrote:
> Hi,
>
> I've been wandering around preempt_schedule_irq() in sched/core.c, and
> got curious regarding how the arch code calls it.
>
> The main part of preempt_schedule_irq() is:
>
> do {
> preempt_disable();
> local_irq_enable();
> __schedule(true);
> local_irq_disable();
> sched_preempt_enable_no_resched();
> } while (need_resched());
>
> Yet all the arch entry.S I looked at (I stopped after arm64, arm, x86_32,
> MIPS, powerpc) wrap the call to preempt_schedule_irq() in another
>
> do { ... } while (need_resched())
>
> For instance, this is what's done in arm64:
>
> 1: bl preempt_schedule_irq // irq en/disable is
> done inside
> ldr x0, [tsk, #TSK_TI_FLAGS] // get new tasks TI_FLAGS
> tbnz x0, #TIF_NEED_RESCHED, 1b // needs rescheduling?
>
>
> I naively thought this could be attributed to something like
> preempt_schedule_irq() historically not having an inner loop, but it seems
> to have been there since the beginning of time (or at least up to the point
> where the git history stops).
>
> I don't see why we need to have these nested loops - AFAICT the one in
> preempt_schedule_irq() would suffice. What am I missing?
I think you're quite right; but I wasn't doing kernel work back when rml
added the preemptible bits. Ingo, do you have any recollections that far
back?
