[tip: locking/core] preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS
"tip-bot2 for Boqun Feng" <[email protected]>
| Newsgroups | gmane.linux.kernel |
|---|---|
| Message-ID | <178622210430.442315.12906041115066212059.tip-bot2@tip-bot2> |
The following commit has been merged into the locking/core branch of tip: Commit-ID: a1ad9bd8e22a26a15f57100e9feef92386808ebe Gitweb: https://git.kernel.org/tip/a1ad9bd8e22a26a15f57100e9feef92386808ebe Author: Boqun Feng <[email protected]> AuthorDate: Tue, 04 Aug 2026 09:14:32 -07:00 Committer: Peter Zijlstra <[email protected]> CommitterDate: Sat, 08 Aug 2026 22:44:08 +02:00 preempt: Introduce HAS_SEPARATE_PREEMPT_RESCHED_BITS 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. Therefore HAS_SEPARATE_PREEMPT_RESCHED_BITS is introduced to allow architectures to select this. Note that under this Kconfig, preempt count is maintained in a 64-bit word however preempt_count() still remains as an int because all the effective bits still fit in (previously we mask out NEED_RESCHED bit in preempt_count()). This should make no functional changes for existing preempt_count() users. Enable this for x86_64 along with the introduction of the Kconfig. [boqun: Undo the __preempt_count_{add,sub}() optimization in 32-bit preempt count since it may introduce {over,under}flow] Originally-by: Peter Zijlstra <[email protected]> Signed-off-by: Boqun Feng <[email protected]> Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Link: https://patch.msgid.link/[email protected] --- arch/x86/Kconfig | 1 +- arch/x86/include/asm/preempt.h | 55 ++++++++++++++++++++++----------- arch/x86/kernel/cpu/common.c | 2 +- include/linux/hardirq.h | 47 ++++++++++++++++++++-------- include/linux/preempt.h | 23 ++++++++++++-- kernel/Kconfig.preempt | 4 ++- kernel/sched/core.c | 12 +++++-- kernel/softirq.c | 6 ++++- lib/locking-selftest.c | 2 +- 9 files changed, 115 insertions(+), 37 deletions(-) diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig index bdad90f..6a7067d 100644 --- a/arch/x86/Kconfig +++ b/arch/x86/Kconfig @@ -326,6 +326,7 @@ config X86 select USER_STACKTRACE_SUPPORT select HAVE_ARCH_KCSAN if X86_64 select PROC_PID_ARCH_STATUS if PROC_FS + select HAS_SEPARATE_PREEMPT_RESCHED_BITS if X86_64 && PREEMPT_COUNT select HAVE_ARCH_NODE_DEV_GROUP if X86_SGX select FUNCTION_ALIGNMENT_16B if X86_64 || X86_ALIGNMENT_16 select FUNCTION_ALIGNMENT_4B diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h index 1220656..fafb6f8 100644 --- a/arch/x86/include/asm/preempt.h +++ b/arch/x86/include/asm/preempt.h @@ -7,10 +7,20 @@ #include <linux/static_call_types.h> -DECLARE_PER_CPU_CACHE_HOT(int, __preempt_count); +DECLARE_PER_CPU_CACHE_HOT(unsigned long, __preempt_count); -/* We use the MSB mostly because its available */ -#define PREEMPT_NEED_RESCHED 0x80000000 +/* + * We use the MSB for PREEMPT_NEED_RESCHED mostly because it is available. + */ +#define PREEMPT_NEED_RESCHED (~(((unsigned long)-1L) >> 1)) + +#ifdef CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS +#define __pc_dec "decq" +#define __pc_op(op, ...) raw_cpu_##op##_8(__VA_ARGS__) +#else +#define __pc_dec "decl" +#define __pc_op(op, ...) raw_cpu_##op##_4(__VA_ARGS__) +#endif /* * We use the PREEMPT_NEED_RESCHED bit as an inverted NEED_RESCHED such @@ -24,18 +34,26 @@ DECLARE_PER_CPU_CACHE_HOT(int, __preempt_count); */ static __always_inline int preempt_count(void) { - return raw_cpu_read_4(__preempt_count) & ~PREEMPT_NEED_RESCHED; + return __pc_op(read, __preempt_count) & ~PREEMPT_NEED_RESCHED; } -static __always_inline void preempt_count_set(int pc) +/* + * unsigned long preempt count parameter works for both 32bit and 64bit cases: + * + * - For 32bit, "int" (the return of preempt_count()) and "unsigned long" have + * the same size. + * - For 64bit, the effective bits of a preempt count sit in 32bit, and we + * preserve the NEED_RESCHED bit from the old count. + */ +static __always_inline void preempt_count_set(unsigned long pc) { - int old, new; + unsigned long old, new; - old = raw_cpu_read_4(__preempt_count); + old = __pc_op(read, __preempt_count); do { new = (old & PREEMPT_NEED_RESCHED) | (pc & ~PREEMPT_NEED_RESCHED); - } while (!raw_cpu_try_cmpxchg_4(__preempt_count, &old, new)); + } while (!__pc_op(try_cmpxchg, __preempt_count, &old, new)); } /* @@ -58,17 +76,17 @@ static __always_inline void preempt_count_set(int pc) static __always_inline void set_preempt_need_resched(void) { - raw_cpu_and_4(__preempt_count, ~PREEMPT_NEED_RESCHED); + __pc_op(and, __preempt_count, ~PREEMPT_NEED_RESCHED); } static __always_inline void clear_preempt_need_resched(void) { - raw_cpu_or_4(__preempt_count, PREEMPT_NEED_RESCHED); + __pc_op(or, __preempt_count, PREEMPT_NEED_RESCHED); } static __always_inline bool test_preempt_need_resched(void) { - return !(raw_cpu_read_4(__preempt_count) & PREEMPT_NEED_RESCHED); + return !(__pc_op(read, __preempt_count) & PREEMPT_NEED_RESCHED); } /* @@ -77,22 +95,22 @@ static __always_inline bool test_preempt_need_resched(void) static __always_inline void __preempt_count_add(int val) { - raw_cpu_add_4(__preempt_count, val); + __pc_op(add, __preempt_count, val); } static __always_inline void __preempt_count_sub(int val) { - raw_cpu_add_4(__preempt_count, -val); + __pc_op(add, __preempt_count, -val); } static __always_inline int __preempt_count_add_return(int val) { - return raw_cpu_add_return_4(__preempt_count, val); + return __pc_op(add_return, __preempt_count, val); } static __always_inline int __preempt_count_sub_return(int val) { - return raw_cpu_add_return_4(__preempt_count, -val); + return __pc_op(add_return, __preempt_count, -val); } /* @@ -102,7 +120,7 @@ static __always_inline int __preempt_count_sub_return(int val) */ static __always_inline bool __preempt_count_dec_and_test(void) { - return GEN_UNARY_RMWcc("decl", __my_cpu_var(__preempt_count), e, + return GEN_UNARY_RMWcc(__pc_dec, __my_cpu_var(__preempt_count), e, __percpu_arg([var])); } @@ -111,7 +129,7 @@ static __always_inline bool __preempt_count_dec_and_test(void) */ static __always_inline bool should_resched(int preempt_offset) { - return unlikely(raw_cpu_read_4(__preempt_count) == preempt_offset); + return unlikely(__pc_op(read, __preempt_count) == preempt_offset); } #ifdef CONFIG_PREEMPTION @@ -158,4 +176,7 @@ do { \ #endif /* PREEMPTION */ +#undef __pc_op +#undef __pc_dec + #endif /* __ASM_PREEMPT_H */ diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c index a3df21d..73a6d9f 100644 --- a/arch/x86/kernel/cpu/common.c +++ b/arch/x86/kernel/cpu/common.c @@ -2236,7 +2236,7 @@ DEFINE_PER_CPU_CACHE_HOT(struct task_struct *, current_task) = &init_task; EXPORT_PER_CPU_SYMBOL(current_task); EXPORT_PER_CPU_SYMBOL(const_current_task); -DEFINE_PER_CPU_CACHE_HOT(int, __preempt_count) = INIT_PREEMPT_COUNT; +DEFINE_PER_CPU_CACHE_HOT(unsigned long, __preempt_count) = INIT_PREEMPT_COUNT; EXPORT_PER_CPU_SYMBOL(__preempt_count); DEFINE_PER_CPU_CACHE_HOT(unsigned long, cpu_current_top_of_stack) = TOP_OF_INIT_STACK; diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h index 8d48955..73b48dd 100644 --- 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); \ + /* 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); \ + } 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) diff --git a/include/linux/preempt.h b/include/linux/preempt.h index 33fc4c8..8299657 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -34,14 +34,31 @@ * SOFTIRQ_MASK: 0x0000ff00 * HARDIRQ_DISABLE_MASK: 0x00ff0000 * HARDIRQ_MASK: 0x0f000000 + * + * When HAS_SEPARATE_PREEMPT_RESCHED_BITS=y, PREEMPT_NEED_RESCHED is put in a + * separate word and that allows 64bit load-store architectures to 'set' + * PREEMPT_NEED_RESCHED without messing up the otherwise symmetric + * modifications used on preempt_count and still load the whole thing + * (single-copy) atomically, without having to resort to full atomic + * operations. + * + * Because of the above, NMI_MASK bits are different depending on + * HAS_SEPARATE_PREEMPT_RESCHED_BITS: + * + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=n: + * * NMI_MASK: 0x10000000 * PREEMPT_NEED_RESCHED: 0x80000000 + * + * - HAS_SEPARATE_PREEMPT_RESCHED_BITS=y: + * NMI_MASK: 0xf0000000 + * (PREEMPT_NEED_RESCHED is in a different word) */ #define PREEMPT_BITS 8 #define SOFTIRQ_BITS 8 #define HARDIRQ_DISABLE_BITS 8 #define HARDIRQ_BITS 4 -#define NMI_BITS 1 +#define NMI_BITS (1 + 3*IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS)) #define PREEMPT_SHIFT 0 #define SOFTIRQ_SHIFT (PREEMPT_SHIFT + PREEMPT_BITS) @@ -116,8 +133,8 @@ static __always_inline unsigned char interrupt_context_level(void) * preempt_count() is commonly implemented with READ_ONCE(). */ -#define nmi_count() (preempt_count() & NMI_MASK) -#define hardirq_count() (preempt_count() & HARDIRQ_MASK) +#define nmi_count() (preempt_count() & NMI_MASK) +#define hardirq_count() (preempt_count() & HARDIRQ_MASK) #ifdef CONFIG_PREEMPT_RT # define softirq_count() (current->softirq_disable_cnt & SOFTIRQ_MASK) # define irq_count() ((preempt_count() & (NMI_MASK | HARDIRQ_MASK)) | softirq_count()) diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt index 88c594c..35f546a 100644 --- a/kernel/Kconfig.preempt +++ b/kernel/Kconfig.preempt @@ -122,6 +122,10 @@ config PREEMPT_RT_NEEDS_BH_LOCK config PREEMPT_COUNT bool +config HAS_SEPARATE_PREEMPT_RESCHED_BITS + bool + depends on PREEMPT_COUNT && 64BIT + config PREEMPTION bool select PREEMPT_COUNT diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 9b3f176..6d88343 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -5973,8 +5973,13 @@ void preempt_count_add(int val) #ifdef CONFIG_DEBUG_PREEMPT /* * Underflow? + * + * Cannot detect underflow based on the current preempt_count() value + * if using HAS_SEPARATE_PREEMPT_RESCHED_BITS because preempt count takes all 32 + * bits. */ - if (DEBUG_LOCKS_WARN_ON((preempt_count() < 0))) + if (!IS_ENABLED(CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS) && + DEBUG_LOCKS_WARN_ON((preempt_count() < 0))) return; #endif __preempt_count_add(val); @@ -6006,7 +6011,10 @@ void preempt_count_sub(int val) /* * Underflow? */ - if (DEBUG_LOCKS_WARN_ON(val > preempt_count())) + unsigned int uval = val; + unsigned int pc = preempt_count(); + + if (DEBUG_LOCKS_WARN_ON(pc - uval > pc)) return; /* * Is the spinlock portion underflowing? diff --git a/kernel/softirq.c b/kernel/softirq.c index 0c9b226..7980a4a 100644 --- a/kernel/softirq.c +++ b/kernel/softirq.c @@ -103,7 +103,13 @@ void _local_interrupt_enable(void) } EXPORT_SYMBOL(_local_interrupt_enable); +#ifndef CONFIG_HAS_SEPARATE_PREEMPT_RESCHED_BITS +/* + * Any 32bit architecture that still cares about performance should + * probably ensure this is near preempt_count. + */ DEFINE_PER_CPU(unsigned int, nmi_nesting); +#endif /* * SOFTIRQ_OFFSET usage: diff --git a/lib/locking-selftest.c b/lib/locking-selftest.c index bfafe12..c3d976c 100644 --- a/lib/locking-selftest.c +++ b/lib/locking-selftest.c @@ -1429,7 +1429,7 @@ static int unexpected_testcase_failures; static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask) { - int saved_preempt_count = preempt_count(); + long saved_preempt_count = preempt_count(); #ifdef CONFIG_PREEMPT_RT int saved_mgd_count = current->migration_disabled; int saved_rcu_count = current->rcu_read_lock_nesting;