linux-kernel - [PATCH 6/6] Documentation: tracing: Add the startup timing of boot-time tracing

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Date:   Thu, 10 Sep 2020 21:39:27 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>,
        Jonathan Corbet <corbet@....net>
Cc:     mhiramat@...nel.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: [PATCH 6/6] Documentation: tracing: Add the startup timing of boot-time tracing

Add the note about when to start the boot-time tracing.
This will be needed for the people who wants to trace
earlier boot sequence.

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
---
 Documentation/trace/boottime-trace.rst |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/Documentation/trace/boottime-trace.rst b/Documentation/trace/boottime-trace.rst
index c216f5695ae2..3931b43f902c 100644
--- a/Documentation/trace/boottime-trace.rst
+++ b/Documentation/trace/boottime-trace.rst
@@ -120,6 +120,20 @@ instance node, but those are also visible from other instances. So please
 take care for event name conflict.
 
 
+When to Start
+=============
+
+All boot-time tracing options starting with ``ftrace`` will be enabled at the
+end of core_initcall. This means you can trace the events from postcore_initcall.
+Most of the subsystems and architecture dependent drivers will be initialized
+after that (arch_initcall or subsys_initcall). Thus, you can trace those with
+boot-time tracing.
+If you want to trace events before core_initcall, you can use the options
+starting with ``kernel``. Some of them will be enabled eariler than the initcall
+processing (for example,. ``kernel.ftrace=function`` and ``kernel.trace_event``
+will start before the initcall.)
+
+
 Examples
 ========