On Wed, 16 Jan 2019, Peter Zijlstra wrote:
> 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?
I just went back in the history tree and had to figure out that it's my
fault :)
preempt_schedule_irq() was introduced to plug a stupid race, but I did not
notice (and obviously nobody else) that this made the extra loop in the
entry code pointless. So it's just histerical raisins without any value.
Thanks,
tglx
