Re: [POC 1/6] irq & spin_lock: Add counted interrupt disabling/enabling

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Oct 24, 2024 at 10:17:33AM +0200, Thomas Gleixner wrote:
> On Wed, Oct 23 2024 at 22:05, Boqun Feng wrote:
> > On Wed, Oct 23, 2024 at 09:34:27PM +0200, Thomas Gleixner wrote:
> >> local_interrupt_enable()
> >> {
> >>         if ((preempt_count() & LOCALIRQ_MASK) == LOCALIRQ_OFFSET) {
> >>         	local_irq_restore(this_cpu_read(...flags);
> >>                 preempt_count_sub_test_resched(LOCALIRQ_OFFSET);
> >>         } else {
> >>                 // Does not need a resched test because it's not going
> >>                 // to 0
> >>                 preempt_count_sub(LOCALIRQ_OFFSET);
> >>         }
> >> }
> >> 
> >
> > Yes, this looks nice, one tiny problem is that it requires
> > PREEMPT_COUNT=y ;-) Maybe we can do: if PREEMPT_COUNT=y, we use preempt
> > count, otherwise use a percpu?
> >
> > Hmm... but this will essentially be: we have a irq_disable_count() which
> > is always built-in, and we also uses it as preempt count if
> > PREEMPT_COUNT=y. This doesn't look too bad to me.
> 
> The preempt counter is always there even when PREEMPT_COUNT=n. It's
> required for tracking hard/soft interrupt and NMI context.
> 
> The only difference is that preempt_disable()/enable() are NOOPs. So in
> that case preempt_count_sub_test_resched() becomes a plain preempt_count_sub().
> 

Ah, good point!

> >> and then the lock thing becomes
> >> 
> >> spin_lock_irq_disable()
> >> {
> >>         local_interrupt_disable();
> >>         lock();
> >> }
> >> 
> >> spin_unlock_irq_enable()
> >> {
> >>         unlock();
> >>         local_interrupt_enable();
> >> }
> >> 
> >> instead having to do:
> >> 
> >> spin_unlock_irq_enable()
> >> {
> >>         unlock();
> >>         local_interrupt_enable();
> >>         preempt_enable();
> >> }
> >> 
> >> Which needs two distinct checks, one for the interrupt and one for the
> >
> > No? Because now since we fold the interrupt disable count into preempt
> > count, so we don't need to care about preempt count any more if we we
> > local_interrupt_{disable,enable}(). For example, in the above
> > local_interrupt_enable(), interrupts are checked at local_irq_restore()
> > and preemption is checked at preempt_count_sub_test_resched(). Right?
> 
> Correct. That's what I pointed out. By folding it into preempt count
> this becomes one operation, while in your POC it's two distinct checks
> and operations.
> 

Yes, I seemed to mis-read what you meant previously, much clear now, let
me put this into implementation for a POC v2.

Regards,
Boqun

> Thanks,
> 
>         tglx