Re: [PATCH v4 10/17] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS

Hi Boqun,

On 8/4/26 9:44 PM, Boqun Feng wrote:
> With the changes that enable preempt count to track IRQ disabling
> nesting, we don't have enough bits in 32-bit preempt count
> implementation, as a result we move NMI nesting bits out of the 32-bit
> preempt count. However on the architectures that can support 64-bit
> preempt count implementation, we can keep the NMI nesting bits in the
> 32-bit preempt count and avoid maintaining NMI nesting bits outside of
> the same cache line.
> 

[...]

> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -10,8 +10,6 @@
>   #include <linux/vtime.h>
>   #include <asm/hardirq.h>
>   
> -DECLARE_PER_CPU(unsigned int, nmi_nesting);
> -
>   extern void synchronize_irq(unsigned int irq);
>   extern bool synchronize_hardirq(unsigned int irq);
>   
> @@ -94,6 +92,37 @@ void irq_exit_rcu(void);
>   #define arch_nmi_exit()		do { } while (0)
>   #endif
>   
> +#ifdef CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS
> +static __always_inline void __preempt_count_nmi_enter(void)
> +{
> +	__preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);
> +}
> +
> +static __always_inline void __preempt_count_nmi_exit(void)
> +{
> +	__preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);
> +}
> +#else
> +DECLARE_PER_CPU(unsigned int, nmi_nesting);
> +
> +#define __preempt_count_nmi_enter()				\
> +	do {							\
> +		__preempt_count_add(HARDIRQ_OFFSET);		\

nit: This limit is because to have the same behavior as other case when
NMI_BITS=4 right?
It is not easy to infer that from comment.

> +		/* Maximum NMI nesting is 15. */		\
> +		BUG_ON(__this_cpu_read(nmi_nesting) >= 15);	\
> +		__this_cpu_inc(nmi_nesting);			\
> +		preempt_count_set(preempt_count() | NMI_MASK);  \


Is there a reason preempt count updates are split rather than
folded into a single preempt_count update?

> +	} while (0)
> +
> +#define __preempt_count_nmi_exit()				\
> +	do {							\
> +		__preempt_count_sub(HARDIRQ_OFFSET);		\
> +		if (!__this_cpu_dec_return(nmi_nesting))	\
> +			preempt_count_set(preempt_count() & ~NMI_MASK); \
> +	} while (0)
> +
> +#endif
> +
>   /*
>    * NMI vs Tracing
>    * --------------
> @@ -110,18 +139,14 @@ void irq_exit_rcu(void);
>   	do {							\
>   		lockdep_off();					\
>   		arch_nmi_enter();				\
> -		/* Maximum NMI nesting is 15. */		\
> -		BUG_ON(__this_cpu_read(nmi_nesting) >= 15);	\
> -		__this_cpu_inc(nmi_nesting);			\
> -		__preempt_count_add(HARDIRQ_OFFSET);		\
> -		preempt_count_set(preempt_count() | NMI_MASK);	\
> +		__preempt_count_nmi_enter();			\
>   	} while (0)
>   
>   #define nmi_enter()						\
>   	do {							\
>   		__nmi_enter();					\
>   		lockdep_hardirq_enter();			\
> -		ct_nmi_enter();				\
> +		ct_nmi_enter();					\
>   		instrumentation_begin();			\
>   		ftrace_nmi_enter();				\
>   		instrumentation_end();				\
> @@ -129,12 +154,8 @@ void irq_exit_rcu(void);
>   
>   #define __nmi_exit()						\
>   	do {							\
> -		unsigned int nesting;				\
>   		BUG_ON(!in_nmi());				\
> -		__preempt_count_sub(HARDIRQ_OFFSET);		\
> -		nesting = __this_cpu_dec_return(nmi_nesting);	\
> -		if (!nesting)					\
> -			preempt_count_set(preempt_count() & ~NMI_MASK);	\
> +		__preempt_count_nmi_exit();			\
>   		arch_nmi_exit();				\
>   		lockdep_on();					\
>   	} while (0)