Subject: [PATCH] arm64: percpu: Implement this_cpu operations

From: [email-masked] (Will Deacon)
Date: Thu, 6 Nov 2014 12:27:53 +0000

Hi Steve,

Thanks for looking at this!

On Thu, Nov 06, 2014 at 11:12:57AM +0000, Steve Capper wrote:
> The generic this_cpu operations disable interrupts to ensure that the
> requested operation is protected from pre-emption. For arm64, this is
> overkill and can hurt throughput and latency.
> 
> This patch provides arm64 specific implementations for the this_cpu
> operations. Rather than disable interrupts, we use the exclusive
> monitor or atomic operations as appropriate.
> 
> The following operations are implemented: add, add_return, and, or,
> read, write, xchg. We also wire up a cmpxchg implementation from
> cmpxchg.h.
> 
> Testing was performed using the percpu_test module and hackbench on a
> Juno board running 3.18-rc3.

[...]

> diff --git a/arch/arm64/include/asm/cmpxchg.h b/arch/arm64/include/asm/cmpxchg.h
> index 3e02245..3e51f49 100644
> --- a/arch/arm64/include/asm/cmpxchg.h
> +++ b/arch/arm64/include/asm/cmpxchg.h
> @@ -237,8 +237,10 @@ static inline unsigned long __cmpxchg_mb(volatile void *ptr, unsigned long old,
>  	__ret; \
>  })
>  
> -#define this_cpu_cmpxchg_8(ptr, o, n) \
> -	cmpxchg(raw_cpu_ptr(&(ptr)), o, n);
> +#define this_cpu_cmpxchg_1(ptr, o, n) cmpxchg(raw_cpu_ptr(&(ptr)), o, n)
> +#define this_cpu_cmpxchg_2(ptr, o, n) cmpxchg(raw_cpu_ptr(&(ptr)), o, n)
> +#define this_cpu_cmpxchg_4(ptr, o, n) cmpxchg(raw_cpu_ptr(&(ptr)), o, n)
> +#define this_cpu_cmpxchg_8(ptr, o, n) cmpxchg(raw_cpu_ptr(&(ptr)), o, n)

You can use cmpxchg_local here, as we don't require barrier semantics.

>  #define this_cpu_cmpxchg_double_8(ptr1, ptr2, o1, o2, n1, n2) \
>  	cmpxchg_double(raw_cpu_ptr(&(ptr1)), raw_cpu_ptr(&(ptr2)), \
> diff --git a/arch/arm64/include/asm/percpu.h b/arch/arm64/include/asm/percpu.h
> index 5279e57..e751681 100644
> --- a/arch/arm64/include/asm/percpu.h
> +++ b/arch/arm64/include/asm/percpu.h
> @@ -44,6 +44,237 @@ static inline unsigned long __my_cpu_offset(void)
>  
>  #endif /* CONFIG_SMP */
>  
> +#define PERCPU_OP(op, asm_op)						\
> +static inline unsigned long __percpu_##op(void *ptr,			\
> +			unsigned long val, int size)			\
> +{									\
> +	unsigned long loop, ret;					\
> +									\
> +	switch (size) {							\
> +	case 1:								\
> +		do {							\
> +			asm ("//__per_cpu_" #op "_1\n"			\
> +			"ldxrb	  %w[ret], %[ptr]\n"			\
> +			#asm_op " %w[ret], %w[ret], %w[val]\n"		\
> +			"stxrb	  %w[loop], %w[ret], %[ptr]\n"		\
> +			: [loop] "=&r" (loop), [ret] "=&r" (ret),	\
> +			  [ptr] "+Q"(*(u8 *)ptr)			\
> +			: [val] "Ir" (val));				\
> +		} while (loop);						\
> +		break;							\

Curious, but do you see any difference in code generation over an explicit
cbnz, like we use in the ATOMIC_OP macro?

> +	case 2:								\
> +		do {							\
> +			asm ("//__per_cpu_" #op "_2\n"			\
> +			"ldxrh	  %w[ret], %[ptr]\n"			\
> +			#asm_op " %w[ret], %w[ret], %w[val]\n"		\
> +			"stxrh	  %w[loop], %w[ret], %[ptr]\n"		\
> +			: [loop] "=&r" (loop), [ret] "=&r" (ret),	\
> +			  [ptr]  "+Q"(*(u16 *)ptr)			\
> +			: [val] "Ir" (val));				\
> +		} while (loop);						\
> +		break;							\
> +	case 4:								\
> +		do {							\
> +			asm ("//__per_cpu_" #op "_4\n"			\
> +			"ldxr	  %w[ret], %[ptr]\n"			\
> +			#asm_op " %w[ret], %w[ret], %w[val]\n"		\
> +			"stxr	  %w[loop], %w[ret], %[ptr]\n"		\
> +			: [loop] "=&r" (loop), [ret] "=&r" (ret),	\
> +			  [ptr] "+Q"(*(u32 *)ptr)			\
> +			: [val] "Ir" (val));				\
> +		} while (loop);						\
> +		break;							\
> +	case 8:								\
> +		do {							\
> +			asm ("//__per_cpu_" #op "_8\n"			\
> +			"ldxr	  %[ret], %[ptr]\n"			\
> +			#asm_op " %[ret], %[ret], %[val]\n"		\
> +			"stxr	  %w[loop], %[ret], %[ptr]\n"		\
> +			: [loop] "=&r" (loop), [ret] "=&r" (ret),	\
> +			  [ptr] "+Q"(*(u64 *)ptr)			\
> +			: [val] "Ir" (val));				\
> +		} while (loop);						\
> +		break;							\
> +	default:							\
> +		BUILD_BUG();						\
> +	}								\
> +									\
> +	return ret;							\
> +}
> +
> +PERCPU_OP(add, add)
> +PERCPU_OP(and, and)
> +PERCPU_OP(or, orr)

Can you use these to generate local_t versions too?

Will