From 858fa8a3b083e862114bb6483b9fb50b3e2bc4c3 Mon Sep 17 00:00:00 2001 From: Steven Rostedt Date: Tue, 29 Jul 2025 14:23:12 -0400 Subject: [PATCH] unwind: Add USED bit to only have one conditional on way back to user space On the way back to user space, the function unwind_reset_info() is called unconditionally (but always inlined). It currently has two conditionals. One that checks the unwind_mask which is set whenever a deferred trace is called and is used to know that the mask needs to be cleared. The other checks if the cache has been allocated, and if so, it resets the nr_entries so that the unwinder knows it needs to do the work to get a new user space stack trace again (it only does it once per entering the kernel). Use one of the bits in the unwind mask as a "USED" bit that gets set whenever a trace is created. This will make it possible to only check the unwind_mask in the unwind_reset_info() to know if it needs to do work or not and eliminates a conditional that happens every time the task goes back to user space. Cc: Masami Hiramatsu Cc: Mathieu Desnoyers Cc: Josh Poimboeuf Cc: Peter Zijlstra Cc: Ingo Molnar Cc: Jiri Olsa Cc: Arnaldo Carvalho de Melo Cc: Namhyung Kim Cc: Thomas Gleixner Cc: Andrii Nakryiko Cc: Indu Bhagat Cc: "Jose E. Marchesi" Cc: Beau Belgrave Cc: Jens Remus Cc: Linus Torvalds Cc: Andrew Morton Cc: Jens Axboe Cc: Florian Weimer Cc: Sam James Link: https://lore.kernel.org/20250729182406.155422551@kernel.org Signed-off-by: Steven Rostedt (Google) --- include/linux/unwind_deferred.h | 18 +++++++++--------- kernel/unwind/deferred.c | 5 ++++- 2 files changed, 13 insertions(+), 10 deletions(-) diff --git a/include/linux/unwind_deferred.h b/include/linux/unwind_deferred.h index b9ec4c8515c74..2efbda01e959b 100644 --- a/include/linux/unwind_deferred.h +++ b/include/linux/unwind_deferred.h @@ -20,10 +20,14 @@ struct unwind_work { enum { UNWIND_PENDING_BIT = 0, + UNWIND_USED_BIT, }; enum { UNWIND_PENDING = BIT(UNWIND_PENDING_BIT), + + /* Set if the unwinding was used (directly or deferred) */ + UNWIND_USED = BIT(UNWIND_USED_BIT) }; void unwind_task_init(struct task_struct *task); @@ -49,15 +53,11 @@ static __always_inline void unwind_reset_info(void) return; } while (!try_cmpxchg(&info->unwind_mask, &bits, 0UL)); current->unwind_info.id.id = 0; - } - /* - * As unwind_user_faultable() can be called directly and - * depends on nr_entries being cleared on exit to user, - * this needs to be a separate conditional. - */ - if (unlikely(info->cache)) { - info->cache->nr_entries = 0; - info->cache->unwind_completed = 0; + + if (unlikely(info->cache)) { + info->cache->nr_entries = 0; + info->cache->unwind_completed = 0; + } } } diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c index a3d26014a2e66..2311b725d6914 100644 --- a/kernel/unwind/deferred.c +++ b/kernel/unwind/deferred.c @@ -45,7 +45,7 @@ static inline bool try_assign_cnt(struct unwind_task_info *info, u32 cnt) static DEFINE_MUTEX(callback_mutex); static LIST_HEAD(callbacks); -#define RESERVED_BITS (UNWIND_PENDING) +#define RESERVED_BITS (UNWIND_PENDING | UNWIND_USED) /* Zero'd bits are available for assigning callback users */ static unsigned long unwind_mask = RESERVED_BITS; @@ -140,6 +140,9 @@ int unwind_user_faultable(struct unwind_stacktrace *trace) cache->nr_entries = trace->nr; + /* Clear nr_entries on way back to user space */ + set_bit(UNWIND_USED_BIT, &info->unwind_mask); + return 0; } -- 2.47.2