On Fri, May 08, 2026 at 09:22:20AM +0100, Mark Rutland wrote:
> Hi Boqun,
>
> I have a question at the end, with some context for other reviewers
> before that.
>
Thank you for adding the context.
> On Thu, May 07, 2026 at 09:21:11PM -0700, Boqun Feng wrote:
> > ARM64 already uses 64bit preempt count and the need reschedule bit is
> > maintained in a separate 32bit than the preempt count.
>
> For the benefit of those reading the list, arm64 has a separate 32-bit
> count and a 32-bit field for need_resched, which are unioned together as
> a composite 64-bit value:
>
> union {
> u64 preempt_count; /* 0 => preemptible, <0 => bug
> */
> struct {
> u32 count;
> u32 need_resched;
> } preempt;
> };
>
> All of our "count" operations work on the 32-bit count, e.g.
>
> static inline int preempt_count(void)
> {
> return READ_ONCE(current_thread_info()->preempt.count);
> }
>
>
> static inline void __preempt_count_add(int val)
> {
> u32 pc = READ_ONCE(current_thread_info()->preempt.count);
> pc += val;
> WRITE_ONCE(current_thread_info()->preempt.count, pc);
> }
>
> static inline void __preempt_count_sub(int val)
> {
> u32 pc = READ_ONCE(current_thread_info()->preempt.count);
> pc -= val;
> WRITE_ONCE(current_thread_info()->preempt.count, pc);
> }
>
> ... but some operations use the 64-bit 'preempt_count' field from the union,
> e.g.
>
> static inline bool should_resched(int preempt_offset)
> {
> u64 pc = READ_ONCE(current_thread_info()->preempt_count);
> return pc == preempt_offset;
> }
>
> > Therefore preempt count has enough bits to represent 16 level of NMI
> > nesting, hence enable it for ARM64. This saves a per-CPU variable and
> > additional instructions in the NMI path.
>
> This might be true, but I think the name "PREEMPT_COUNT_64BIT" is
> misleading given the above. What exactly does PREEMPT_COUNT_64BIT tell
> core code it can do?
It tells the core code that all 32bit of preempt_count() can be used.
>
> If this is just telling core code that it doesn't need ot reserve space
> in preempt_count for the resched bits, can this be called something
> else, e.g. HAS_SEPARATE_PREEMPT_RESCHED_BITS?
You are right on that the resched bit detail is a major difference
between PREEMPT_COUNT_64BIT=y or n. I haven't gone that far to resolve
the #1 difficult issue in programming (i.e. naming) ;-)
HAS_SEPARATE_PREEMPT_RESCHED_BITS seems reasonable, I will use it if no
other better ideas. Thanks!
Regards,
Boqun
>
> Mark.
>
> >
> > Signed-off-by: Boqun Feng <[email protected]>
> > ---
> > arch/arm64/Kconfig | 1 +
> > 1 file changed, 1 insertion(+)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index fe60738e5943..1ed5173872fc 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -248,6 +248,7 @@ config ARM64
> > select PCI_SYSCALL if PCI
> > select POWER_RESET
> > select POWER_SUPPLY
> > + select PREEMPT_COUNT_64BIT
> > select SPARSE_IRQ
> > select SWIOTLB
> > select SYSCTL_EXCEPTION_TRACE
> > --
> > 2.50.1 (Apple Git-155)
> >
> >
