linux-kernel - Re: [RFC PATCH v2 12/31] function_graph: Have the instances use their own ftrace

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Date:   Fri, 10 Nov 2023 10:51:54 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Florent Revest <revest@...omium.org>,
        linux-trace-kernel@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        bpf <bpf@...r.kernel.org>, Sven Schnelle <svens@...ux.ibm.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alan Maguire <alan.maguire@...cle.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Guo Ren <guoren@...nel.org>
Subject: Re: [RFC PATCH v2 12/31] function_graph: Have the instances use
 their own ftrace_ops for filtering

Hi,

I found strange behavior that if we set two or more probes on the same function,
its callback called twice or more. Also, the stacktrace failed.
And I found the root cause is here;

On Wed,  8 Nov 2023 23:26:42 +0900
"Masami Hiramatsu (Google)" <mhiramat@...nel.org> wrote:

> @@ -910,9 +917,10 @@ int register_ftrace_graph(struct fgraph_ops *gops)
>  		 */
>  		ftrace_graph_return = return_run;
>  		ftrace_graph_entry = entry_run;
> -
> -		ret = ftrace_startup(&graph_ops, FTRACE_START_FUNC_RET);
> +		command = FTRACE_START_FUNC_RET;
>  	}
> +
> +	ret = ftrace_startup(&gops->ops, command);

So this patch registers ftrace_ops for each fgraph_ops to ftrace.
This means that the ftrace_graph_func() will be called twice or more
on the same function.
Thus should I call ftrace_startup() once when the first fgraph_ops
is registered? 
No, it's not enough. Actually each fgraph_ops can have different filters.
We need to define a shared filter and combine new filters to one and
use it. We also need to do it when a fgraph is unregistered.

Is there any function which makes a new filter from two (or more) filters?

Or, maybe we can make the common callback to find the previous ret entry on
the ret_stack and reuse it. (In this case we don't need loop on each
fgraph_array entry)

Thank you,

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>