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

[<hu.shengming@zte.com.cn>]

Randy wrote:
> Hi,
> 

Hi Randy,

> On 8/30/26 4:12 AM, hu.shengming@zte.com.cn wrote:
> > From: Shengming Hu <hu.shengming@zte.com.cn>
> > 
> > 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 <hu.shengming@zte.com.cn>
> > ---
> >  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