Re: Enhancing Conditional Filtering for Function Graph Tracer

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 14 Feb 2026 18:36:24 +0800
Donglin Peng <dolinux.peng@gmail.com> wrote:

> To simplify implementation, I propose extending a new trigger type
> (e.g., "funcgraph").
> In ftrace_graph_ignore_func, we could look up the corresponding trace_fprobe and
> trace_event_file based on trace->func, then decide whether to trace
> the function using
> a helper like the following:
> 
> static bool ftrace_graph_filter(struct trace_fprobe *tf, struct
> ftrace_regs *fregs,
>                                struct trace_event_file *trace_file)
> {
>     struct fentry_trace_entry_head *entry;
>     struct trace_event_buffer fbuffer;
>     struct event_trigger_data *data;
>     int dsize;
> 
>     dsize = __get_data_size(&tf->tp, fregs, NULL);
>     entry = trace_event_buffer_reserve(&fbuffer, trace_file,
>                                        sizeof(*entry) + tf->tp.size + dsize);
>     if (!entry)
>         return false;
> 
>     entry = ring_buffer_event_data(fbuffer.event);
>     store_trace_args(&entry[1], &tf->tp, fregs, NULL, sizeof(*entry), dsize);
> 
>     list_for_each_entry_rcu(data, &trace_file->triggers, list) {
>         if (data->cmd_ops->trigger_type == TRIGGER_TYPE_FUNCGRAPH) {
>             struct event_filter *filter = rcu_dereference_sched(data->filter);
>             if (filter && filter_match_preds(filter, entry))
>                 return true; // Allow tracing
>         }
>     }
>     return false; // Skip tracing
> }
> 
> Does this approach make sense? Any suggestions or concerns?

My biggest concern is with performance. You want to run this against all
functions being traced?

How is this different than just using fprobes?

-- Steve