For now, migrate_enable and migrate_disable are global, which makes them become hotspots in some case. Take BPF for example, the function calling to migrate_enable and migrate_disable in BPF trampoline can introduce significant overhead, and following is the 'perf top' of FENTRY's benchmark (./tools/testing/selftests/bpf/bench trig-fentry): 54.63% bpf_prog_2dcccf652aac1793_bench_trigger_fentry [k] bpf_prog_2dcccf652aac1793_bench_trigger_fentry 10.43% [kernel] [k] migrate_enable 10.07% bpf_trampoline_6442517037 [k] bpf_trampoline_6442517037 8.06% [kernel] [k] __bpf_prog_exit_recur 4.11% libc.so.6 [.] syscall 2.15% [kernel] [k] entry_SYSCALL_64 1.48% [kernel] [k] memchr_inv 1.32% [kernel] [k] fput 1.16% [kernel] [k] _copy_to_user 0.73% [kernel] [k] bpf_prog_test_run_raw_tp So in this commit, we make migrate_enable/migrate_disable inline to obtain better performance. The struct rq is defined internally in kernel/sched/sched.h, and the field "nr_pinned" is accessed in migrate_enable/migrate_disable, which makes it hard to make them inline. Alexei Starovoitov suggests to generate the offset of "nr_pinned" in [1], so we can define the migrate_enable/migrate_disable in include/linux/sched.h and access "this_rq()->nr_pinned" with "(void *)this_rq() + RQ_nr_pinned". The offset of "nr_pinned" is generated in include/generated/rq-offsets.h by kernel/sched/rq-offsets.c. Generally speaking, we move the definition of migrate_enable and migrate_disable to include/linux/sched.h from kernel/sched/core.c. The calling to __set_cpus_allowed_ptr() is leaved in ___migrate_enable(). The "struct rq" is not available in include/linux/sched.h, so we can't access the "runqueues" with this_cpu_ptr(), as the compilation will fail in this_cpu_ptr() -> raw_cpu_ptr() -> __verify_pcpu_ptr(): typeof((ptr) + 0) So we introduce the this_rq_raw() and access the runqueues with arch_raw_cpu_ptr/PERCPU_PTR directly. The variable "runqueues" is not visible in the kernel modules, and export it is not a good idea. As Peter Zijlstra advised in [2], we define and export migrate_enable/migrate_disable in kernel/sched/core.c too, and use them for the modules. Before this patch, the performance of BPF FENTRY is: fentry : 113.030 ± 0.149M/s fentry : 112.501 ± 0.187M/s fentry : 112.828 ± 0.267M/s fentry : 115.287 ± 0.241M/s After this patch, the performance of BPF FENTRY increases to: fentry : 143.644 ± 0.670M/s fentry : 149.764 ± 0.362M/s fentry : 149.642 ± 0.156M/s fentry : 145.263 ± 0.221M/s Link: https://lore.kernel.org/bpf/CAADnVQ+5sEDKHdsJY5ZsfGDO_1SEhhQWHrt2SMBG5SYyQ+jt7w@mail.gmail.com/ [1] Link: https://lore.kernel.org/all/20250819123214.GH4067720@noisy.programming.kicks-ass.net/ [2] Signed-off-by: Menglong Dong --- v3: - don't export runqueues, define migrate_enable and migrate_disable in kernel/sched/core.c and use them for kernel modules instead - define the macro this_rq_pinned() - add some comment for this_rq_raw() v2: - use PERCPU_PTR() for this_rq_raw() if !CONFIG_SMP --- Kbuild | 13 ++++- include/linux/preempt.h | 3 -- include/linux/sched.h | 106 ++++++++++++++++++++++++++++++++++++++ kernel/bpf/verifier.c | 1 + kernel/sched/core.c | 63 +++++----------------- kernel/sched/rq-offsets.c | 12 +++++ 6 files changed, 145 insertions(+), 53 deletions(-) create mode 100644 kernel/sched/rq-offsets.c diff --git a/Kbuild b/Kbuild index f327ca86990c..13324b4bbe23 100644 --- a/Kbuild +++ b/Kbuild @@ -34,13 +34,24 @@ arch/$(SRCARCH)/kernel/asm-offsets.s: $(timeconst-file) $(bounds-file) $(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s FORCE $(call filechk,offsets,__ASM_OFFSETS_H__) +# Generate rq-offsets.h + +rq-offsets-file := include/generated/rq-offsets.h + +targets += kernel/sched/rq-offsets.s + +kernel/sched/rq-offsets.s: $(offsets-file) + +$(rq-offsets-file): kernel/sched/rq-offsets.s FORCE + $(call filechk,offsets,__RQ_OFFSETS_H__) + # Check for missing system calls quiet_cmd_syscalls = CALL $< cmd_syscalls = $(CONFIG_SHELL) $< $(CC) $(c_flags) $(missing_syscalls_flags) PHONY += missing-syscalls -missing-syscalls: scripts/checksyscalls.sh $(offsets-file) +missing-syscalls: scripts/checksyscalls.sh $(rq-offsets-file) $(call cmd,syscalls) # Check the manual modification of atomic headers diff --git a/include/linux/preempt.h b/include/linux/preempt.h index 1fad1c8a4c76..92237c319035 100644 --- a/include/linux/preempt.h +++ b/include/linux/preempt.h @@ -424,8 +424,6 @@ static inline void preempt_notifier_init(struct preempt_notifier *notifier, * work-conserving schedulers. * */ -extern void migrate_disable(void); -extern void migrate_enable(void); /** * preempt_disable_nested - Disable preemption inside a normally preempt disabled section @@ -471,7 +469,6 @@ static __always_inline void preempt_enable_nested(void) DEFINE_LOCK_GUARD_0(preempt, preempt_disable(), preempt_enable()) DEFINE_LOCK_GUARD_0(preempt_notrace, preempt_disable_notrace(), preempt_enable_notrace()) -DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable()) #ifdef CONFIG_PREEMPT_DYNAMIC diff --git a/include/linux/sched.h b/include/linux/sched.h index f8188b833350..ae73fb70cc05 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -49,6 +49,9 @@ #include #include #include +#ifndef COMPILE_OFFSETS +#include +#endif /* task_struct member predeclarations (sorted alphabetically): */ struct audit_context; @@ -2312,4 +2315,107 @@ static __always_inline void alloc_tag_restore(struct alloc_tag *tag, struct allo #define alloc_tag_restore(_tag, _old) do {} while (0) #endif +#ifndef MODULE +#ifndef COMPILE_OFFSETS + +extern void ___migrate_enable(void); + +struct rq; +DECLARE_PER_CPU_SHARED_ALIGNED(struct rq, runqueues); + +/* The "struct rq" is not available here, so we can't access the + * "runqueues" with this_cpu_ptr(), as the compilation will fail in + * this_cpu_ptr() -> raw_cpu_ptr() -> __verify_pcpu_ptr(): + * typeof((ptr) + 0) + * + * So use arch_raw_cpu_ptr()/PERCPU_PTR() directly here. + */ +#ifdef CONFIG_SMP +#define this_rq_raw() arch_raw_cpu_ptr(&runqueues) +#else +#define this_rq_raw() PERCPU_PTR(&runqueues) +#endif +#define this_rq_pinned() (*(unsigned int *)((void *)this_rq_raw() + RQ_nr_pinned)) + +static inline void __migrate_enable(void) +{ + struct task_struct *p = current; + +#ifdef CONFIG_DEBUG_PREEMPT + /* + * Check both overflow from migrate_disable() and superfluous + * migrate_enable(). + */ + if (WARN_ON_ONCE((s16)p->migration_disabled <= 0)) + return; +#endif + + if (p->migration_disabled > 1) { + p->migration_disabled--; + return; + } + + /* + * Ensure stop_task runs either before or after this, and that + * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule(). + */ + guard(preempt)(); + if (unlikely(p->cpus_ptr != &p->cpus_mask)) + ___migrate_enable(); + /* + * Mustn't clear migration_disabled() until cpus_ptr points back at the + * regular cpus_mask, otherwise things that race (eg. + * select_fallback_rq) get confused. + */ + barrier(); + p->migration_disabled = 0; + this_rq_pinned()--; +} + +static inline void __migrate_disable(void) +{ + struct task_struct *p = current; + + if (p->migration_disabled) { +#ifdef CONFIG_DEBUG_PREEMPT + /* + *Warn about overflow half-way through the range. + */ + WARN_ON_ONCE((s16)p->migration_disabled < 0); +#endif + p->migration_disabled++; + return; + } + + guard(preempt)(); + this_rq_pinned()++; + p->migration_disabled = 1; +} +#else /* !COMPILE_OFFSETS */ +static inline void __migrate_disable(void) { } +static inline void __migrate_enable(void) { } +#endif /* !COMPILE_OFFSETS */ + +#ifndef CREATE_MIGRATE_DISABLE +static inline void migrate_disable(void) +{ + __migrate_disable(); +} + +static inline void migrate_enable(void) +{ + __migrate_enable(); +} +#else /* CREATE_MIGRATE_DISABLE */ +extern void migrate_disable(void); +extern void migrate_enable(void); +#endif /* CREATE_MIGRATE_DISABLE */ + +#else /* MODULE */ +extern void migrate_disable(void); +extern void migrate_enable(void); +#endif /* MODULE */ + +DEFINE_LOCK_GUARD_0(migrate, migrate_disable(), migrate_enable()) + #endif diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index c4f69a9e9af6..de9078a9df3a 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -23855,6 +23855,7 @@ int bpf_check_attach_target(struct bpf_verifier_log *log, BTF_SET_START(btf_id_deny) BTF_ID_UNUSED #ifdef CONFIG_SMP +BTF_ID(func, ___migrate_enable) BTF_ID(func, migrate_disable) BTF_ID(func, migrate_enable) #endif diff --git a/kernel/sched/core.c b/kernel/sched/core.c index be00629f0ba4..58164a69449d 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7,6 +7,8 @@ * Copyright (C) 1991-2002 Linus Torvalds * Copyright (C) 1998-2024 Ingo Molnar, Red Hat */ +#define CREATE_MIGRATE_DISABLE +#include #include #include #include @@ -2381,28 +2383,7 @@ static void migrate_disable_switch(struct rq *rq, struct task_struct *p) __do_set_cpus_allowed(p, &ac); } -void migrate_disable(void) -{ - struct task_struct *p = current; - - if (p->migration_disabled) { -#ifdef CONFIG_DEBUG_PREEMPT - /* - *Warn about overflow half-way through the range. - */ - WARN_ON_ONCE((s16)p->migration_disabled < 0); -#endif - p->migration_disabled++; - return; - } - - guard(preempt)(); - this_rq()->nr_pinned++; - p->migration_disabled = 1; -} -EXPORT_SYMBOL_GPL(migrate_disable); - -void migrate_enable(void) +void ___migrate_enable(void) { struct task_struct *p = current; struct affinity_context ac = { @@ -2410,35 +2391,19 @@ void migrate_enable(void) .flags = SCA_MIGRATE_ENABLE, }; -#ifdef CONFIG_DEBUG_PREEMPT - /* - * Check both overflow from migrate_disable() and superfluous - * migrate_enable(). - */ - if (WARN_ON_ONCE((s16)p->migration_disabled <= 0)) - return; -#endif + __set_cpus_allowed_ptr(p, &ac); +} +EXPORT_SYMBOL_GPL(___migrate_enable); - if (p->migration_disabled > 1) { - p->migration_disabled--; - return; - } +void migrate_disable(void) +{ + __migrate_disable(); +} +EXPORT_SYMBOL_GPL(migrate_disable); - /* - * Ensure stop_task runs either before or after this, and that - * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule(). - */ - guard(preempt)(); - if (p->cpus_ptr != &p->cpus_mask) - __set_cpus_allowed_ptr(p, &ac); - /* - * Mustn't clear migration_disabled() until cpus_ptr points back at the - * regular cpus_mask, otherwise things that race (eg. - * select_fallback_rq) get confused. - */ - barrier(); - p->migration_disabled = 0; - this_rq()->nr_pinned--; +void migrate_enable(void) +{ + __migrate_enable(); } EXPORT_SYMBOL_GPL(migrate_enable); diff --git a/kernel/sched/rq-offsets.c b/kernel/sched/rq-offsets.c new file mode 100644 index 000000000000..a23747bbe25b --- /dev/null +++ b/kernel/sched/rq-offsets.c @@ -0,0 +1,12 @@ +// SPDX-License-Identifier: GPL-2.0 +#define COMPILE_OFFSETS +#include +#include +#include "sched.h" + +int main(void) +{ + DEFINE(RQ_nr_pinned, offsetof(struct rq, nr_pinned)); + + return 0; +} -- 2.50.1