Re: [RFC PATCH 4/4] Documentation/ftrace: Document function comm filters

Randy wrote:

Hi,

Hi Randy,

On 8/30/26 4:12 AM, hu.shengming@zte.com.cn wrote:

quoted

From: Shengming Hu <redacted>

Document set_ftrace_comm and set_ftrace_notrace_comm, including their
write and O_TRUNC behavior, accepted comm names, interaction with PID
filters, and when a task is rechecked after its comm changes.

Signed-off-by: Shengming Hu <redacted>
---
 Documentation/trace/ftrace.rst | 31 +++++++++++++++++++++++++++++++
 1 file changed, 31 insertions(+)

diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
index 7261f25f8b4b..a96fc436ec16 100644
--- a/Documentation/trace/ftrace.rst
+++ b/Documentation/trace/ftrace.rst

@@ -313,6 +313,37 @@ of ftrace. Here is a list of some of the key files:
         If a PID is in both this file and "set_ftrace_pid", then this
         file takes precedence, and the thread will not be traced.

+  set_ftrace_comm:
+
+    Have the function and function_graph tracers trace only threads whose
+    task comm is listed in this file. Names are matched exactly and must
+    contain between 1 and TASK_COMM_LEN - 1 bytes.
+
+    Each write adds one name. A single trailing newline is ignored, while
+    embedded newlines are rejected. A zero-length write or a write containing
+    only a newline adds no entry. Duplicate entries have no effect.
+
+    Opening the file with O_TRUNC clears the filter. Otherwise, writes add

Is there a way to clear the filter from (in) a shell script?

Yes. It can be cleared with:

  # echo > set_ftrace_comm

The shell opens the file with O_TRUNC, which clears the existing filter.

I described the O_TRUNC behavior but overlooked the corresponding
user-facing shell command. Thanks for pointing this out.

Based on Masami's feedback, I am now considering reworking the series
around a generic event trigger that updates set_ftrace_pid, so the
comm-specific files may not remain in the next version.

--
With Best Regards,
Shengming