Hi Boqun,
I have a question at the end, with some context for other reviewers
before that.
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?
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?
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)
>
>
