From: Josh Poimboeuf <jpoim...@kernel.org> Cache the results of the unwind to ensure the unwind is only performed once, even when called by multiple tracers.
The cache nr_entries gets cleared every time the task exits the kernel. When a stacktrace is requested, nr_entries gets set to the number of entries in the stacktrace. If another stacktrace is requested, if nr_entries is not zero, then it contains the same stacktrace that would be retrieved so it is not processed again and the entries is given to the caller. Co-developed-by: Steven Rostedt (Google) <rost...@goodmis.org> Signed-off-by: Josh Poimboeuf <jpoim...@kernel.org> Signed-off-by: Steven Rostedt (Google) <rost...@goodmis.org> --- Changes since v10: https://lore.kernel.org/20250611010428.603778...@goodmis.org - Rename unwind_exit_to_user_mode() to unwind_reset_info() as it is called in a noinstr section and the current name looks like it does instrumentation (Peter Zijlstra) - Make the allocated cache fit inside a 4K page. (Peter Zijlstra) include/linux/entry-common.h | 2 ++ include/linux/unwind_deferred.h | 8 +++++++ include/linux/unwind_deferred_types.h | 7 +++++- kernel/unwind/deferred.c | 31 +++++++++++++++++++++------ 4 files changed, 40 insertions(+), 8 deletions(-) diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h index f94f3fdf15fc..8908b8eeb99b 100644 --- a/include/linux/entry-common.h +++ b/include/linux/entry-common.h @@ -12,6 +12,7 @@ #include <linux/resume_user_mode.h> #include <linux/tick.h> #include <linux/kmsan.h> +#include <linux/unwind_deferred.h> #include <asm/entry-common.h> #include <asm/syscall.h> @@ -362,6 +363,7 @@ static __always_inline void exit_to_user_mode(void) lockdep_hardirqs_on_prepare(); instrumentation_end(); + unwind_reset_info(); user_enter_irqoff(); arch_exit_to_user_mode(); lockdep_hardirqs_on(CALLER_ADDR0); diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h index a5f6e8f8a1a2..baacf4a1eb4c 100644 --- a/include/linux/unwind_deferred.h +++ b/include/linux/unwind_deferred.h @@ -12,6 +12,12 @@ void unwind_task_free(struct task_struct *task); int unwind_user_faultable(struct unwind_stacktrace *trace); +static __always_inline void unwind_reset_info(void) +{ + if (unlikely(current->unwind_info.cache)) + current->unwind_info.cache->nr_entries = 0; +} + #else /* !CONFIG_UNWIND_USER */ static inline void unwind_task_init(struct task_struct *task) {} @@ -19,6 +25,8 @@ static inline void unwind_task_free(struct task_struct *task) {} static inline int unwind_user_faultable(struct unwind_stacktrace *trace) { return -ENOSYS; } +static inline void unwind_reset_info(void) {} + #endif /* !CONFIG_UNWIND_USER */ #endif /* _LINUX_UNWIND_USER_DEFERRED_H */ diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h index aa32db574e43..db5b54b18828 100644 --- a/include/linux/unwind_deferred_types.h +++ b/include/linux/unwind_deferred_types.h @@ -2,8 +2,13 @@ #ifndef _LINUX_UNWIND_USER_DEFERRED_TYPES_H #define _LINUX_UNWIND_USER_DEFERRED_TYPES_H +struct unwind_cache { + unsigned int nr_entries; + unsigned long entries[]; +}; + struct unwind_task_info { - unsigned long *entries; + struct unwind_cache *cache; }; #endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */ diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c index a0badbeb3cc1..96368a5aa522 100644 --- a/kernel/unwind/deferred.c +++ b/kernel/unwind/deferred.c @@ -4,10 +4,13 @@ */ #include <linux/kernel.h> #include <linux/sched.h> +#include <linux/sizes.h> #include <linux/slab.h> #include <linux/unwind_deferred.h> -#define UNWIND_MAX_ENTRIES 512 +/* Make the cache fit in a 4K page */ +#define UNWIND_MAX_ENTRIES \ + ((SZ_4K - sizeof(struct unwind_cache)) / sizeof(long)) /** * unwind_user_faultable - Produce a user stacktrace in faultable context @@ -24,6 +27,7 @@ int unwind_user_faultable(struct unwind_stacktrace *trace) { struct unwind_task_info *info = ¤t->unwind_info; + struct unwind_cache *cache; /* Should always be called from faultable context */ might_fault(); @@ -31,17 +35,30 @@ int unwind_user_faultable(struct unwind_stacktrace *trace) if (current->flags & PF_EXITING) return -EINVAL; - if (!info->entries) { - info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long), - GFP_KERNEL); - if (!info->entries) + if (!info->cache) { + info->cache = kzalloc(struct_size(cache, entries, UNWIND_MAX_ENTRIES), + GFP_KERNEL); + if (!info->cache) return -ENOMEM; } + cache = info->cache; + trace->entries = cache->entries; + + if (cache->nr_entries) { + /* + * The user stack has already been previously unwound in this + * entry context. Skip the unwind and use the cache. + */ + trace->nr = cache->nr_entries; + return 0; + } + trace->nr = 0; - trace->entries = info->entries; unwind_user(trace, UNWIND_MAX_ENTRIES); + cache->nr_entries = trace->nr; + return 0; } @@ -56,5 +73,5 @@ void unwind_task_free(struct task_struct *task) { struct unwind_task_info *info = &task->unwind_info; - kfree(info->entries); + kfree(info->cache); } -- 2.47.2
