Re: [PATCH 2/2] tracing: Keep pid and comm[] in the same structure

[David Laight]

On Wed, 1 Jul 2026 06:38:22 -0400
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> On Wed, 1 Jul 2026 11:04:07 +0100
> David Laight <david.laight.linux@xxxxxxxxx> wrote:
> 
> > 
> > I thought it was just used to do a pid->string lookup when you run 'cat trace'.
> > But then I found the code that lets userspace read the table....
> > I guess the latter is used by the userspace code that reads the raw trace buffer.  
> 
> Yes, trace-cmd uses it.
...
> > The userspace code could probably be given comm[] for all the running
> > processes and those that exited while tracing_on() set.
> > (I didn't see anything that would clear the table when the trace buffer
> > was cleared.)  
> 
> Well, that would break trace-cmd. As reading the raw buffers clears the
> trace, and trace-cmd reads the saved_cmdlines file *after* it reads the
> trace, as during the trace it gets populated.

So you'd need to clear it when tracing is enabled after the buffer is cleared.
Just a matter of getting the timing right.

If trace-cmd is currently given all 6000 entries (if you've run tracing for
long enough), then giving it all the active processes isn't going to be any
more of a problem.
So you could just save comm[] in the process exit path when the trace buffer
is non-empty, or better those started before tracing was last stopped.
You'd need to give trace-cmd the active ones first and delete the entry
from the cache because the pid might have been reused.

All just needs some coding, testing, and fixing of corner cases.

-- David 

> 
> -- Steve