linux-kernel - Re: [PATCH] fgraph: Move trace_clock_local() for return time to function

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Message-ID: <20250114100552.21b28c99@gandalf.local.home>
Date: Tue, 14 Jan 2025 10:05:52 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: "Masami Hiramatsu (Google)" <mhiramat@...nel.org>
Cc: Jiri Olsa <olsajiri@...il.com>, Alexei Starovoitov
 <alexei.starovoitov@...il.com>, 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>, Alexei
 Starovoitov <ast@...nel.org>, Alan Maguire <alan.maguire@...cle.com>, Mark
 Rutland <mark.rutland@....com>
Subject: Re: [PATCH] fgraph: Move trace_clock_local() for return time to
 function_graph tracer

On Tue, 14 Jan 2025 10:18:06 +0900
Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:

> > I'd rather just nuke the calltime and rettime from struct ftrace_graph_ret.  
> 
> Yeah, this one is good to me. But this will make a slightly different time
> for the same function when we enable multiple function graph tracer on
> different instances (mine records it once and reuse it but this will record
> on each instance). We may need to notice it on the document. :)

And I believe it's separate for each instance now anyway, so I was going to
send this to Linus as well:

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 272464bb7c60..2b74f96d09d5 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst
@@ -810,6 +810,12 @@ Here is the list of current tracers that may be configured.
 	to draw a graph of function calls similar to C code
 	source.
 
+	Note that the function graph calculates the timings of when the
+	function starts and returns internally and for each instance. If
+	there are two instances that run function graph tracer and traces
+	the same functions, the length of the timings may be slightly off as
+	each read the timestamp separately and not at the same time.
+
   "blk"
 
 	The block tracer. The tracer used by the blktrace user


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

Thanks,

-- Steve