[PATCH v6 08/18] unwind_user/deferred: Add unwind cache

Steven Rostedt
2025-04-25T14:56:15Z

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) <rostedt@goodmis.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v5: https://lore.kernel.org/20250424162632.532889212@goodmis.org

- Instead of clearing cache->nr_entries when entering from user, clear it
when exiting to user mode.

include/linux/entry-common.h | 2 ++
include/linux/unwind_deferred.h | 7 +++++++
include/linux/unwind_deferred_types.h | 7 ++++++-
kernel/unwind/deferred.c | 27 ++++++++++++++++++++-------
4 files changed, 35 insertions(+), 8 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index fc61d0205c97..725ec0e87cdd 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>

@@ -361,6 +362,7 @@ static __always_inline void exit_to_user_mode(void)
lockdep_hardirqs_on_prepare();
instrumentation_end();

+ unwind_exit_to_user_mode();
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 5064ebe38c4f..c2d760e5e257 100644
--- a/include/linux/unwind_deferred.h
+++ b/include/linux/unwind_deferred.h
@@ -12,6 +12,11 @@ void unwind_task_free(struct task_struct *task);

int unwind_deferred_trace(struct unwind_stacktrace *trace);

+static __always_inline void unwind_exit_to_user_mode(void)
+{
+ current->unwind_info.cache.nr_entries = 0;
+}
+
#else /* !CONFIG_UNWIND_USER */

static inline void unwind_task_init(struct task_struct *task) {}
@@ -19,6 +24,8 @@ static inline void unwind_task_free(struct task_struct *task) {}

static inline int unwind_deferred_trace(struct unwind_stacktrace *trace) { return -ENOSYS; }

+static inline void unwind_exit_to_user_mode(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..b3b7389ee6eb 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_task_info {
+struct unwind_cache {
unsigned long *entries;
+ unsigned int nr_entries;
+};
+
+struct unwind_task_info {
+ struct unwind_cache cache;
};

#endif /* _LINUX_UNWIND_USER_DEFERRED_TYPES_H */
diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index 6a4cec4877c5..99d4d9e049cd 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -12,6 +12,7 @@
int unwind_deferred_trace(struct unwind_stacktrace *trace)
{
struct unwind_task_info *info = &current->unwind_info;
+ struct unwind_cache *cache = &info->cache;

/* Should always be called from faultable context */
might_fault();
@@ -20,17 +21,29 @@ int unwind_deferred_trace(struct unwind_stacktrace *trace)
if (!current->mm)
return -EINVAL;

- if (!info->entries) {
- info->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
- GFP_KERNEL);
- if (!info->entries)
- return -ENOMEM;
+ if (!cache->entries) {
+ cache->entries = kmalloc_array(UNWIND_MAX_ENTRIES, sizeof(long),
+ GFP_KERNEL);
+ if (!cache->entries)
+ return -ENOMEM;
+ }
+
+ 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;
}

@@ -45,5 +58,5 @@ void unwind_task_free(struct task_struct *task)
{
struct unwind_task_info *info = &task->unwind_info;

- kfree(info->entries);
+ kfree(info->cache.entries);
}
--
2.47.2