From 3785c7dbae0f733f13f8857beaaada5d7dc63e02 Mon Sep 17 00:00:00 2001 From: Yujun Dong Date: Mon, 30 Dec 2024 22:16:24 +0800 Subject: [PATCH] cpuidle, sched: Use smp_mb__after_atomic() in current_clr_polling() In architectures that use the polling bit, current_clr_polling() employs smp_mb() to ensure that the clearing of the polling bit is visible to other cores before checking TIF_NEED_RESCHED. However, smp_mb() can be costly. Given that clear_bit() is an atomic operation, replacing smp_mb() with smp_mb__after_atomic() is appropriate. Many architectures implement smp_mb__after_atomic() as a lighter-weight barrier compared to smp_mb(), leading to performance improvements. For instance, on x86, smp_mb__after_atomic() is a no-op. This change eliminates a smp_mb() instruction in the cpuidle wake-up path, saving several CPU cycles and thereby reducing wake-up latency. Architectures that do not use the polling bit will retain the original smp_mb() behavior to ensure that existing dependencies remain unaffected. Signed-off-by: Yujun Dong Signed-off-by: Ingo Molnar Link: https://lore.kernel.org/r/20241230141624.155356-1-yujundong@pascal-lab.net --- include/linux/sched/idle.h | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) diff --git a/include/linux/sched/idle.h b/include/linux/sched/idle.h index e670ac282333e..439f6029d3b94 100644 --- a/include/linux/sched/idle.h +++ b/include/linux/sched/idle.h @@ -79,6 +79,21 @@ static __always_inline bool __must_check current_clr_polling_and_test(void) return unlikely(tif_need_resched()); } +static __always_inline void current_clr_polling(void) +{ + __current_clr_polling(); + + /* + * Ensure we check TIF_NEED_RESCHED after we clear the polling bit. + * Once the bit is cleared, we'll get IPIs with every new + * TIF_NEED_RESCHED and the IPI handler, scheduler_ipi(), will also + * fold. + */ + smp_mb__after_atomic(); /* paired with resched_curr() */ + + preempt_fold_need_resched(); +} + #else static inline void __current_set_polling(void) { } static inline void __current_clr_polling(void) { } @@ -91,21 +106,15 @@ static inline bool __must_check current_clr_polling_and_test(void) { return unlikely(tif_need_resched()); } -#endif static __always_inline void current_clr_polling(void) { __current_clr_polling(); - /* - * Ensure we check TIF_NEED_RESCHED after we clear the polling bit. - * Once the bit is cleared, we'll get IPIs with every new - * TIF_NEED_RESCHED and the IPI handler, scheduler_ipi(), will also - * fold. - */ smp_mb(); /* paired with resched_curr() */ preempt_fold_need_resched(); } +#endif #endif /* _LINUX_SCHED_IDLE_H */