On Tue, Sep 22, 2026 at 03:55:25PM -0700, Vineet Gupta wrote:
> When ftrace graphing is turned on, all tasks in the system missing
> return stack page are assigned one. This is done in a simplistic
> multi-sweep loop of FTRACE_RETSTACK_ALLOC_SIZE (currently 32) tasks
> at a time as follows:
>
> start_graph_tracing()
> do {
> alloc_retstack_tasklist
> } while (-EAGAIN);
>
> alloc_retstack_tasklist()
> alloc x32 # GFP_KERNEL, may sleep
> rcu_read_lock() # preempt off
> for_each_process_thread walk N_total, no cond_resched
> t->ret_stack = new_page
> rcu_read_unlock() # preempt enable but no explicit yield
>
> Each successive iteration of loop invokes for_each_process_thread()
> which doesn't support cursor based resume and always restarts from the
> init_task. Thus each successive loop needs to skip the tasks assigned
> ret_stack in prior sweeps and thus take longer and longer to find the
> candidate 32 tasks.
Does this work?
diff --git a/kernel/trace/fgraph.c b/kernel/trace/fgraph.c
index ed455b53513b..155dafad474d 100644
--- a/kernel/trace/fgraph.c
+++ b/kernel/trace/fgraph.c
@@ -1036,10 +1036,9 @@ trace_func_graph_ent_t ftrace_graph_entry =
ftrace_graph_entry_stub;
/* Try to assign a return stack array on FTRACE_RETSTACK_ALLOC_SIZE tasks. */
static int alloc_retstack_tasklist(unsigned long **ret_stack_list)
{
- int i;
- int ret = 0;
int start = 0, end = FTRACE_RETSTACK_ALLOC_SIZE;
struct task_struct *g, *t;
+ int i, ret = 0;
if (WARN_ON_ONCE(!fgraph_stack_cachep))
return -ENOMEM;
@@ -1054,26 +1053,29 @@ static int alloc_retstack_tasklist(unsigned long
**ret_stack_list)
}
}
- rcu_read_lock();
- for_each_process_thread(g, t) {
- if (start == end) {
- ret = -EAGAIN;
- goto unlock;
- }
+ scoped_guard (rcu) {
+ for_each_process_thread(g, t) {
+ unsigned long *rs;
+
+ if (t->ret_stack)
+ continue;
+
+ rs = kmem_cache_alloc(fgraph_stack_cachep, GFP_NOWAIT);
+ if (!rs) {
+ if (start == end)
+ return -EAGAIN;
+ rs = ret_stack_list[start++];
+ }
- if (t->ret_stack == NULL) {
atomic_set(&t->trace_overrun, 0);
- ret_stack_init_task_vars(ret_stack_list[start]);
+ ret_stack_init_task_vars(rs);
t->curr_ret_stack = 0;
t->curr_ret_depth = -1;
/* Make sure the tasks see the 0 first: */
- smp_wmb();
- t->ret_stack = ret_stack_list[start++];
+ smp_store_release(&t->ret_stack, rs);
}
}
-unlock:
- rcu_read_unlock();
free:
for (i = start; i < end; i++)
kmem_cache_free(fgraph_stack_cachep, ret_stack_list[i]);
