[PATCH v4 29/39] unwind_user/deferred: Add unwind cache

Josh Poimboeuf
2025-01-22T02:32:22Z

Cache the results of the unwind to ensure the unwind is only performed
once, even when called by multiple tracers.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
include/linux/unwind_deferred_types.h | 8 +++++++-
kernel/unwind/deferred.c | 26 ++++++++++++++++++++------
2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/include/linux/unwind_deferred_types.h b/include/linux/unwind_deferred_types.h
index 9749824aea09..6f71a06329fb 100644
--- a/include/linux/unwind_deferred_types.h
+++ b/include/linux/unwind_deferred_types.h
@@ -2,8 +2,14 @@
#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;
+ u64 cookie;
+};
+
+struct unwind_task_info {
+ struct unwind_cache cache;
u64 cookie;
};

diff --git a/kernel/unwind/deferred.c b/kernel/unwind/deferred.c
index f0dbe4069247..2f38055cce48 100644
--- a/kernel/unwind/deferred.c
+++ b/kernel/unwind/deferred.c
@@ -56,6 +56,7 @@ static void unwind_deferred_task_work(struct callback_head *head)
{
struct unwind_work *work = container_of(head, struct unwind_work, work);
struct unwind_task_info *info = &current->unwind_info;
+ struct unwind_cache *cache = &info->cache;
struct unwind_stacktrace trace;
u64 cookie;

@@ -73,17 +74,30 @@ static void unwind_deferred_task_work(struct callback_head *head)
if (!current->mm)
goto do_callback;

- if (!info->entries) {
- info->entries = kmalloc(UNWIND_MAX_ENTRIES * sizeof(long),
- GFP_KERNEL);
- if (!info->entries)
+ if (!cache->entries) {
+ cache->entries = kmalloc(UNWIND_MAX_ENTRIES * sizeof(long),
+ GFP_KERNEL);
+ if (!cache->entries)
goto do_callback;
}

- trace.entries = info->entries;
+ trace.entries = cache->entries;
+
+ if (cookie == cache->cookie) {
+ /*
+ * The user stack has already been previously unwound in this
+ * entry context. Skip the unwind and use the cache.
+ */
+ trace.nr = cache->nr_entries;
+ goto do_callback;
+ }
+
trace.nr = 0;
unwind_user(&trace, UNWIND_MAX_ENTRIES);

+ cache->cookie = cookie;
+ cache->nr_entries = trace.nr;
+
do_callback:
work->func(work, &trace, cookie);
work->pending = 0;
@@ -174,5 +188,5 @@ void unwind_task_free(struct task_struct *task)
{
struct unwind_task_info *info = &task->unwind_info;

- kfree(info->entries);
+ kfree(info->cache.entries);
}
--
2.48.1