On Wed, May 27, 2026 at 09:33:53AM -0700, Boqun Feng wrote:
> On Wed, May 27, 2026 at 06:18:44PM +0200, Peter Zijlstra wrote:
> > On Tue, May 26, 2026 at 08:21:36AM -0700, Boqun Feng wrote:
> > > Hi Peter,
> > >
> > > This is a follow-up for Lyude's work [1]. After learning the current
> > > preempt_count() usage and how ARM64 handle this, I came up with this
> > > series that could resolve your feedback [2]. The basic idea is based on:
> > >
> > > 1) preempt_count() previously already masks our NEED_RESCHED bit, so the
> > > effective bits is 31bits
> > > 2) with a 64bit preempt count implementation (as in your PREEMPT_LONG
> > > proposal), the effective bits that record "whether we CAN preempt or
> > > not" still fit in 32bit (i.e. an int)
> > >
> > > as a result, I don't think we need to change the existing
> > > preempt_count() API, but rather keep "32bit vs 64bit" as an
> > > implementation detail. This saves us the need to change the printk code
> > > for preempt_count().
> >
> > >
> > > v1:
> > > https://lore.kernel.org/rust-for-linux/[email protected]/
> > >
> > > Changes since v1:
> > >
> > > * Rename PREEMPT_COUNT_64BIT to HAS_SEPARATE_PREEMPT_RESCHED_BITS per
> > > Mark Rutland.
> >
> > Blergh, so I really don't like that new name. It isn't that
> > PREEMPT_RESCHED is separate, it really is a 64bit preempt count.
> >
> > Shashiko has a few fits, but its mostly being stupid. Although I think
> > it might be useful to perhaps put a WARN_ON_ONCE(in_nmi()) in
> > local_interrupt_disable().
>
> Yes, that's also what I'm thinking of, and also we need to check
> local_softirq_pending() in local_interrupt_enable() I think, because now
> HARDIRQ_DISABLE bits being non-zero means deferring softirq. (Shashiko
> also reported this as well).
>
After some thoughts on this, I will postpone the idea of checking of
local_softirq_pending() in local_interrupt_enable(), because that
essientially makes every interrupt enabling a point to trigger softirq,
and that's different from the current local_irq_{enable,restore}().
Regards,
Boqun
> >
> > Anyway, I'll re-read things again tomorrow, but I suppose this will do.
> >
>
> Thank you!
>
> Regards,
> Boqun
