From c2daa3bed53a81171cf8c1a36db798e82b91afe8 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 14 Aug 2013 14:51:00 +0200 Subject: sched, x86: Provide a per-cpu preempt_count implementation Convert x86 to use a per-cpu preemption count. The reason for doing so is that accessing per-cpu variables is a lot cheaper than accessing thread_info variables. We still need to save/restore the actual preemption count due to PREEMPT_ACTIVE so we place the per-cpu __preempt_count variable in the same cache-line as the other hot __switch_to() variables such as current_task. NOTE: this save/restore is required even for !PREEMPT kernels as cond_resched() also relies on preempt_count's PREEMPT_ACTIVE to ignore task_struct::state. Also rename thread_info::preempt_count to ensure nobody is 'accidentally' still poking at it. Suggested-by: Linus Torvalds Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/n/tip-gzn5rfsf8trgjoqx8hyayy3q@git.kernel.org Signed-off-by: Ingo Molnar --- arch/x86/include/asm/preempt.h | 98 ++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 98 insertions(+) create mode 100644 arch/x86/include/asm/preempt.h (limited to 'arch/x86/include/asm/preempt.h') diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h new file mode 100644 index 000000000000..1309942b95e5 --- /dev/null +++ b/arch/x86/include/asm/preempt.h @@ -0,0 +1,98 @@ +#ifndef __ASM_PREEMPT_H +#define __ASM_PREEMPT_H + +#include +#include +#include + +DECLARE_PER_CPU(int, __preempt_count); + +/* + * We mask the PREEMPT_NEED_RESCHED bit so as not to confuse all current users + * that think a non-zero value indicates we cannot preempt. + */ +static __always_inline int preempt_count(void) +{ + return __this_cpu_read_4(__preempt_count) & ~PREEMPT_NEED_RESCHED; +} + +static __always_inline void preempt_count_set(int pc) +{ + __this_cpu_write_4(__preempt_count, pc); +} + +/* + * must be macros to avoid header recursion hell + */ +#define task_preempt_count(p) \ + (task_thread_info(p)->saved_preempt_count & ~PREEMPT_NEED_RESCHED) + +#define init_task_preempt_count(p) do { \ + task_thread_info(p)->saved_preempt_count = PREEMPT_DISABLED; \ +} while (0) + +#define init_idle_preempt_count(p, cpu) do { \ + task_thread_info(p)->saved_preempt_count = PREEMPT_ENABLED; \ + per_cpu(__preempt_count, (cpu)) = PREEMPT_ENABLED; \ +} while (0) + +/* + * We fold the NEED_RESCHED bit into the preempt count such that + * preempt_enable() can decrement and test for needing to reschedule with a + * single instruction. + * + * We invert the actual bit, so that when the decrement hits 0 we know we both + * need to resched (the bit is cleared) and can resched (no preempt count). + */ + +static __always_inline void set_preempt_need_resched(void) +{ + __this_cpu_and_4(__preempt_count, ~PREEMPT_NEED_RESCHED); +} + +static __always_inline void clear_preempt_need_resched(void) +{ + __this_cpu_or_4(__preempt_count, PREEMPT_NEED_RESCHED); +} + +static __always_inline bool test_preempt_need_resched(void) +{ + return !(__this_cpu_read_4(__preempt_count) & PREEMPT_NEED_RESCHED); +} + +/* + * The various preempt_count add/sub methods + */ + +static __always_inline void __preempt_count_add(int val) +{ + __this_cpu_add_4(__preempt_count, val); +} + +static __always_inline void __preempt_count_sub(int val) +{ + __this_cpu_add_4(__preempt_count, -val); +} + +static __always_inline bool __preempt_count_dec_and_test(void) +{ + GEN_UNARY_RMWcc("decl", __preempt_count, __percpu_arg(0), "e"); +} + +/* + * Returns true when we need to resched -- even if we can not. + */ +static __always_inline bool need_resched(void) +{ + return unlikely(test_preempt_need_resched()); +} + +/* + * Returns true when we need to resched and can (barring IRQ state). + */ +static __always_inline bool should_resched(void) +{ + return unlikely(!__this_cpu_read_4(__preempt_count)); +} + +#endif /* __ASM_PREEMPT_H */ -- cgit v1.2.3