On Wed, Feb 27, 2019 at 11:24:45AM +0100, Peter Zijlstra wrote:
>
> And because it's one of _those_ days, I forgot to include one patch...
>
> ---
> Subject: x86/percpu: Optimize raw_cpu_xchg()
> From: Peter Zijlstra <pet...@infradead.org>
> Date: Wed Feb 27 11:09:56 CET 2019
>
> Since raw_cpu_xchg() doesn't need to be IRQ-safe, like
> this_cpu_xchg(), we can use a simple load-store instead of the cmpxchg
> loop.
>
> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
> ---
> arch/x86/include/asm/percpu.h | 18 +++++++++++++++---
> 1 file changed, 15 insertions(+), 3 deletions(-)
>
> --- a/arch/x86/include/asm/percpu.h
> +++ b/arch/x86/include/asm/percpu.h
> @@ -407,9 +407,21 @@ do {
> \
> #define raw_cpu_or_1(pcp, val) percpu_to_op(, "or", (pcp), val)
> #define raw_cpu_or_2(pcp, val) percpu_to_op(, "or", (pcp), val)
> #define raw_cpu_or_4(pcp, val) percpu_to_op(, "or", (pcp), val)
> -#define raw_cpu_xchg_1(pcp, val) percpu_xchg_op(, pcp, val)
> -#define raw_cpu_xchg_2(pcp, val) percpu_xchg_op(, pcp, val)
> -#define raw_cpu_xchg_4(pcp, val) percpu_xchg_op(, pcp, val)
> +
> +/*
> + * raw_cpu_xchg() can use a load-store since it is not required to be
> + * IRQ-safe.
> + */
> +#define raw_percpu_xchg_op(var, nval)
> \
> +({ \
> + typeof(var) pxo_ret__ = raw_cpu_read(var); \
> + raw_cpu_write(var, (nval)); \
> + pxo_ret__; \
> +})
> +
> +#define raw_cpu_xchg_1(pcp, val) raw_percpu_xchg_op(pcp, val)
> +#define raw_cpu_xchg_2(pcp, val) raw_percpu_xchg_op(pcp, val)
> +#define raw_cpu_xchg_4(pcp, val) raw_percpu_xchg_op(pcp, val)
>
> #define this_cpu_read_1(pcp) percpu_from_op(volatile, "mov", pcp)
> #define this_cpu_read_2(pcp) percpu_from_op(volatile, "mov", pcp)
And yes, I just added raw_cpu_xchg_8... *sigh*
