Re: [PATCH 1/2] tracing: fgraph: Raise FTRACE_RETSTACK_ALLOC_SIZE to 1024

[Steven Rostedt <rostedt@goodmis.org>]

On Thu, 24 Sep 2026 12:00:40 +0200
Peter Zijlstra <peterz@infradead.org> wrote:

> 
> 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++];
> +			}

Yeah this is fine. This iteration code goes back to when function graph
was first added back in 2008. Back then, looping all tasks required
taking the task_list lock and I remember we didn't want to call
kmalloc() while holding that. This code could definitely use some
reassessment.

-- Steve


>  
> -		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]);