Re: [PATCH 11/11] arm64: sched/preempt: Enable PREEMPT_COUNT_64BIT

[Mark Rutland <mark.rutland@arm.com>]

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 <boqun@kernel.org>
> ---
>  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)
> 
>