From: Thomas Gleixner Date: Mon, 27 Oct 2025 08:44:52 +0000 (+0100) Subject: rseq: Expose lightweight statistics in debugfs X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=5412910487d0839111e4f2f3a6f33f6c9af9b007;p=thirdparty%2Fkernel%2Flinux.git rseq: Expose lightweight statistics in debugfs Analyzing the call frequency without actually using tracing is helpful for analysis of this infrastructure. The overhead is minimal as it just increments a per CPU counter associated to each operation. The debugfs readout provides a racy sum of all counters. Signed-off-by: Thomas Gleixner Signed-off-by: Peter Zijlstra (Intel) Signed-off-by: Ingo Molnar Reviewed-by: Mathieu Desnoyers Link: https://patch.msgid.link/20251027084307.027916598@linutronix.de --- diff --git a/include/linux/rseq.h b/include/linux/rseq.h index a200836a6fe38..7f347c3a4af8b 100644 --- a/include/linux/rseq.h +++ b/include/linux/rseq.h @@ -29,21 +29,6 @@ static inline void rseq_sched_switch_event(struct task_struct *t) } } -static __always_inline void rseq_exit_to_user_mode(void) -{ - struct rseq_event *ev = ¤t->rseq.event; - - if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) - WARN_ON_ONCE(ev->sched_switch); - - /* - * Ensure that event (especially user_irq) is cleared when the - * interrupt did not result in a schedule and therefore the - * rseq processing did not clear it. - */ - ev->events = 0; -} - /* * KVM/HYPERV invoke resume_user_mode_work() before entering guest mode, * which clears TIF_NOTIFY_RESUME. To avoid updating user space RSEQ in @@ -92,7 +77,6 @@ static inline void rseq_sched_switch_event(struct task_struct *t) { } static inline void rseq_virt_userspace_exit(void) { } static inline void rseq_fork(struct task_struct *t, u64 clone_flags) { } static inline void rseq_execve(struct task_struct *t) { } -static inline void rseq_exit_to_user_mode(void) { } #endif /* !CONFIG_RSEQ */ #ifdef CONFIG_DEBUG_RSEQ diff --git a/include/linux/rseq_entry.h b/include/linux/rseq_entry.h index 5be507a127eb4..ff9080b89be39 100644 --- a/include/linux/rseq_entry.h +++ b/include/linux/rseq_entry.h @@ -2,6 +2,37 @@ #ifndef _LINUX_RSEQ_ENTRY_H #define _LINUX_RSEQ_ENTRY_H +/* Must be outside the CONFIG_RSEQ guard to resolve the stubs */ +#ifdef CONFIG_RSEQ_STATS +#include + +struct rseq_stats { + unsigned long exit; + unsigned long signal; + unsigned long slowpath; + unsigned long ids; + unsigned long cs; + unsigned long clear; + unsigned long fixup; +}; + +DECLARE_PER_CPU(struct rseq_stats, rseq_stats); + +/* + * Slow path has interrupts and preemption enabled, but the fast path + * runs with interrupts disabled so there is no point in having the + * preemption checks implied in __this_cpu_inc() for every operation. + */ +#ifdef RSEQ_BUILD_SLOW_PATH +#define rseq_stat_inc(which) this_cpu_inc((which)) +#else +#define rseq_stat_inc(which) raw_cpu_inc((which)) +#endif + +#else /* CONFIG_RSEQ_STATS */ +#define rseq_stat_inc(x) do { } while (0) +#endif /* !CONFIG_RSEQ_STATS */ + #ifdef CONFIG_RSEQ #include @@ -39,8 +70,26 @@ static __always_inline void rseq_note_user_irq_entry(void) current->rseq.event.user_irq = true; } +static __always_inline void rseq_exit_to_user_mode(void) +{ + struct rseq_event *ev = ¤t->rseq.event; + + rseq_stat_inc(rseq_stats.exit); + + if (IS_ENABLED(CONFIG_DEBUG_RSEQ)) + WARN_ON_ONCE(ev->sched_switch); + + /* + * Ensure that event (especially user_irq) is cleared when the + * interrupt did not result in a schedule and therefore the + * rseq processing did not clear it. + */ + ev->events = 0; +} + #else /* CONFIG_RSEQ */ static inline void rseq_note_user_irq_entry(void) { } +static inline void rseq_exit_to_user_mode(void) { } #endif /* !CONFIG_RSEQ */ #endif /* _LINUX_RSEQ_ENTRY_H */ diff --git a/init/Kconfig b/init/Kconfig index cab3ad28ca49e..f39fdfb287978 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -1913,6 +1913,18 @@ config RSEQ If unsure, say Y. +config RSEQ_STATS + default n + bool "Enable lightweight statistics of restartable sequences" if EXPERT + depends on RSEQ && DEBUG_FS + help + Enable lightweight counters which expose information about the + frequency of RSEQ operations via debugfs. Mostly interesting for + kernel debugging or performance analysis. While lightweight it's + still adding code into the user/kernel mode transitions. + + If unsure, say N. + config DEBUG_RSEQ default n bool "Enable debugging of rseq() system call" if EXPERT diff --git a/kernel/rseq.c b/kernel/rseq.c index f49d3118c3e00..c0dbe2ed26a89 100644 --- a/kernel/rseq.c +++ b/kernel/rseq.c @@ -67,12 +67,16 @@ * F1. */ +/* Required to select the proper per_cpu ops for rseq_stats_inc() */ +#define RSEQ_BUILD_SLOW_PATH + +#include +#include +#include #include -#include #include -#include +#include #include -#include #include #define CREATE_TRACE_POINTS @@ -108,6 +112,56 @@ void __rseq_trace_ip_fixup(unsigned long ip, unsigned long start_ip, } #endif /* CONFIG_TRACEPOINTS */ +#ifdef CONFIG_RSEQ_STATS +DEFINE_PER_CPU(struct rseq_stats, rseq_stats); + +static int rseq_debug_show(struct seq_file *m, void *p) +{ + struct rseq_stats stats = { }; + unsigned int cpu; + + for_each_possible_cpu(cpu) { + stats.exit += data_race(per_cpu(rseq_stats.exit, cpu)); + stats.signal += data_race(per_cpu(rseq_stats.signal, cpu)); + stats.slowpath += data_race(per_cpu(rseq_stats.slowpath, cpu)); + stats.ids += data_race(per_cpu(rseq_stats.ids, cpu)); + stats.cs += data_race(per_cpu(rseq_stats.cs, cpu)); + stats.clear += data_race(per_cpu(rseq_stats.clear, cpu)); + stats.fixup += data_race(per_cpu(rseq_stats.fixup, cpu)); + } + + seq_printf(m, "exit: %16lu\n", stats.exit); + seq_printf(m, "signal: %16lu\n", stats.signal); + seq_printf(m, "slowp: %16lu\n", stats.slowpath); + seq_printf(m, "ids: %16lu\n", stats.ids); + seq_printf(m, "cs: %16lu\n", stats.cs); + seq_printf(m, "clear: %16lu\n", stats.clear); + seq_printf(m, "fixup: %16lu\n", stats.fixup); + return 0; +} + +static int rseq_debug_open(struct inode *inode, struct file *file) +{ + return single_open(file, rseq_debug_show, inode->i_private); +} + +static const struct file_operations dfs_ops = { + .open = rseq_debug_open, + .read = seq_read, + .llseek = seq_lseek, + .release = single_release, +}; + +static int __init rseq_debugfs_init(void) +{ + struct dentry *root_dir = debugfs_create_dir("rseq", NULL); + + debugfs_create_file("stats", 0444, root_dir, NULL, &dfs_ops); + return 0; +} +__initcall(rseq_debugfs_init); +#endif /* CONFIG_RSEQ_STATS */ + #ifdef CONFIG_DEBUG_RSEQ static struct rseq *rseq_kernel_fields(struct task_struct *t) { @@ -187,12 +241,13 @@ static int rseq_update_cpu_node_id(struct task_struct *t) u32 node_id = cpu_to_node(cpu_id); u32 mm_cid = task_mm_cid(t); - /* - * Validate read-only rseq fields. - */ + rseq_stat_inc(rseq_stats.ids); + + /* Validate read-only rseq fields on debug kernels */ if (rseq_validate_ro_fields(t)) goto efault; WARN_ON_ONCE((int) mm_cid < 0); + if (!user_write_access_begin(rseq, t->rseq.len)) goto efault; @@ -403,6 +458,8 @@ static int rseq_ip_fixup(struct pt_regs *regs, bool abort) struct rseq_cs rseq_cs; int ret; + rseq_stat_inc(rseq_stats.cs); + ret = rseq_get_rseq_cs(t, &rseq_cs); if (ret) return ret; @@ -412,8 +469,10 @@ static int rseq_ip_fixup(struct pt_regs *regs, bool abort) * If not nested over a rseq critical section, restart is useless. * Clear the rseq_cs pointer and return. */ - if (!in_rseq_cs(ip, &rseq_cs)) + if (!in_rseq_cs(ip, &rseq_cs)) { + rseq_stat_inc(rseq_stats.clear); return clear_rseq_cs(t->rseq.usrptr); + } ret = rseq_check_flags(t, rseq_cs.flags); if (ret < 0) return ret; @@ -422,6 +481,7 @@ static int rseq_ip_fixup(struct pt_regs *regs, bool abort) ret = clear_rseq_cs(t->rseq.usrptr); if (ret) return ret; + rseq_stat_inc(rseq_stats.fixup); trace_rseq_ip_fixup(ip, rseq_cs.start_ip, rseq_cs.post_commit_offset, rseq_cs.abort_ip); instruction_pointer_set(regs, (unsigned long)rseq_cs.abort_ip); @@ -462,6 +522,11 @@ void __rseq_handle_notify_resume(struct ksignal *ksig, struct pt_regs *regs) if (unlikely(t->flags & PF_EXITING)) return; + if (ksig) + rseq_stat_inc(rseq_stats.signal); + else + rseq_stat_inc(rseq_stats.slowpath); + /* * Read and clear the event pending bit first. If the task * was not preempted or migrated or a signal is on the way,