Re: [Bug translator/13187] Reconsider the semantics of process(number).thread.begin/end

Re: [Bug translator/13187] Reconsider the semantics of process(number).thread.begin/end

[Dave Brolley]

No reason why this discussion should not be public. The discussion 
started on IRC within Red Hat when I tried to test the 
process.thread.begin/end variants and ran into what was, for me anyway, 
unexpected semantics. The scenario is an already-running process, foo, 
with process id 1234.

stap -e 'probe process("foo").thread.begin,process("foo").thread.end {}' 
catches each child thread of foo as they begin and end, as expected.

I expected stap -e 'probe 
process(1234).thread.begin,process(1234).thread.end {}' to do the same, 
but it does not. Instead, it never fires, because the current semantics 
are to probe the child thread with task id == 1234, which doesn't exist.

My proposal for new semantics, and David Smith's response are in the PR 
13187. My subsequent reply to David is below.

Feel free to add your thoughts and opinions!

--------------------------------------------------------

On 09/14/2011 10:28 AM, dsmith at redhat dot com wrote:
>
>  There is a situation where process(PID).thread.begin probes will fire.
>
>  - Start a process that creates several threads that will run for an extended
>  period of time.  Determine the pids of those threads.
>  - RUn systemtap with a script that probes those pids.
>  - As systemtap attaches to those threads, the process(PID).thread.begin probe
>  will fire.
I think that you're getting caught up in the notion that the PID here
refers to a task because that happens to be the current implementation.
Sure we can jump through some hoops and get this probe type to fire for
a particular type of application (long running threads) in a particular
situation (want to probe one thread in particular, probe fires at some
random point in the already-running thread), but is this really the most
useful and most intuitive interpretation of this probe type from the
user's point of view?

I think that there is a lack of orthogonality in the current
implementation that is confusing. At least it is for me.

1) stap -e 'process("PATH").thread.begin {}' catches *all* child threads
of *processes* (not tasks) identified by PATH, *as they start*
2) stap -e 'process.thread.begin {}' -c CMD catches *all* child threads
of the specific *process* (not task) created by running CMD, *as they start*

whereas

3) stap -e 'process(NUMBER).thread.begin {}': catches only the thread
with task id equal to NUMBER.

The behavior of variant 3 is not intuitive at all, to me, given the
behavior of the other two variants, combined with the name of the probe
itself being process(NUMBER), and not task(NUMBER).

Furthermore, the number in

   process(number).statement(stmtnumber).absolute
   process(number).statement(stmenumber).absolute.return
   process(number).syscall
   process(number).syscall.return

refers to the process id and these probes all fire in the main thread of
the process with that id and also in its children. i.e. it simply
identifies the target process. So why should the number in not also
refer to the process id?

   process(number).thread.begin
   process(number).thread.end
   process(number).begin
   process(number).end

the only difference should be that the latter two only refer to the main
thread and the first two only refer to the child threads, as they do for
the other begin/end and thread.begin/end variants.

It seems to me that the thread.begin family of probes was intended to
provide for the probing of threading activity within a process (as
opposed to probing of a particular thread/task) and that providing a
*process* (not task) NUMBER instead of a *process* PATH or a -c option
shouldn't change this. i.e. the purpose of providing process(NUMBER)
probes is to allow the specification of an already-running *process* as
the target and not a specific thread/task id.

The point of my proposal in the BZ is that simply having variant 3 use
the same "follow all child threads" semantics as the other two would
accomplish this. As fche pointed out, this is exactly what variant 2
does for the *process* id generated by -c PATH, so why deny it for the
*process* id provided by NUMBER?

>  In my opinion, changing the semantics here is too big/messy of a change.
>  Instead, a new probe type, something like 'process(PATH/PID).thread.create',
>  could be created.
I see it the other way. I think that the current
process(NUMBER).thread.start/end should behave like its counterparts and
that if you really want to probe a specific thread/task, you can:

1) filter on the task id or
2) create something like task(TID).begin/end or
process(PATH/PID).thread(TID).begin/end.

Note also, that process(PATH).thread.create would duplicate
functionality already provided by process(PATH).thread.begin.

At the end of the day, don't we want what is most flexible, useful and
intuitive for the end user?

With my proposed change:
useful: probes which are possible with the current implementation will
still be possible after the change (via filtering)
useful and flexible: a whole range of probes on already-running
processes, which are now impossible would become possible
intuitive: the NUMBER in process(NUMBER).thread.begin would refer to a
process id as the name of the probe suggests

As for the concern about changing the semantics. The new semantics are a
superset of the current semantics and, given the current semantics, I
doubt that anyone is currently using process(NUMBER).thread.begin/end at
all.

I welcome your further thoughts on this!

Thanks,
Dave

Re: [Bug translator/13187] Reconsider the semantics of process(number).thread.begin/end

[David Smith]

On 09/14/2011 01:23 PM, Dave Brolley wrote:

Sorry for the delay in responding to this.  I'm also sorry for the
incomplete response.  But here goes with what I've got.

... stuff deleted ...

Let's start with a discussion of 'PID'.  What does that exactly mean?
You probably already know this, I just want to get our terminology straight.

From a user's point of view, there are process IDs (returned by
getpid()) and thread IDs (returned by gettid()).  The gettid() man page
states:

    In a single-threaded process, the thread ID is equal to the
    process ID (PID, as returned  by getpid(2)).  In a
    multithreaded process, all threads have the same PID,
    but each one has a unique TID.

Here's where things get confusing.  From the kernel's point of view, the
thread ID is called 'pid' in the task structure and the process ID is
called 'tgid' (thread group id) in the task structure.  (It happened
this way for historical reasons.)

To avoid confusion, I'm going to call an individual thread a 'task' and
the group of tasks with the same 'tgid' value a 'thread group'.

[Here's something I've rediscovered about 'process(PID).*'
probes.  The task_finder only looks for processes in its initial pass.
So, after stap starts up, it looks for all the PID probes, then never
looks for them again.  This was actually done on purpose, since task ids
are non-predictable.  If you ask us to probe a PID, it finishes and
later a completely different process starts and ends up with the same
PID (because PIDs will wrap after a certain number), we didn't want to
accidently process that.  The thought was that if you're probing
specific PIDs, you meant those very tasks, not random ones that might
come along later.]

While researching this, I've discovered a bug in the task_finder.  Where
we're talking about 'process(PID).*' probes, there is actually
some confusion in the task_finder code about what PID means (which
actually relates to your question here).  This command:

  stap -x PID -e 'probe process.* {}'

treats PID differently than this one:

  stap -e 'probe process(PID).* {}'

In the first one, we look for a thread group id value of PID. In the
second case, we look for a task id value of PID.

> I think that there is a lack of orthogonality in the current
> implementation that is confusing. At least it is for me.
> 
> 1) stap -e 'process("PATH").thread.begin {}' catches *all* child threads
> of *processes* (not tasks) identified by PATH, *as they start*


Your "as they start" distinction is not correct.
'process("PATH").begin' and 'process("PATH").thread.begin' probes will
fire when attaching to existing tasks.

So, if you "stap -e 'process("PATH").thread.begin {}", that
.thread.begin probe will fire for all existing threads whose execname is
PATH.  The .thread.begin probe will later fire for all new threads whose
execname is PATH also.

Also note here that the .thread.begin probes fire in the context of the
new thread, not the parent thread.  If your probe body was something like:

    process("PATH").thread.begin {
	printf("pid %d tid %d\n", pid(), tid())
    }

You'll see 2 different numbers there.

[Internally, the task_finder first loops through all existing tasks,
looking for PATH and also attaches a probe to every task in the system,
to help in monitoring new threads.]

> 2) stap -e 'process.thread.begin {}' -c CMD catches *all* child threads
> of the specific *process* (not task) created by running CMD, *as they
> start*


To avoid any misunderstandings, let's break this one down:

- 'process.thread.begin' means we're interested in *all* threads in the
system.

- The '-c CMD' (which basically devolves into a '-x PID') means we're
*only* interested in PID.  [See 'NOTE 1' for more '-c CMD' details.]

We resolve this conflict by being interested in all threads started by PID.

[Internally, the task_finder first loops through all existing tasks,
looking for PID and only attaching a probe to PID that monitors for new
threads.]

> whereas
> 
> 3) stap -e 'process(NUMBER).thread.begin {}': catches only the thread
> with task id equal to NUMBER.
> 
> The behavior of variant 3 is not intuitive at all, to me, given the
> behavior of the other two variants, combined with the name of the probe
> itself being process(NUMBER), and not task(NUMBER).
> 
> Furthermore, the number in
> 
>   process(number).statement(stmtnumber).absolute
>   process(number).statement(stmenumber).absolute.return
>   process(number).syscall
>   process(number).syscall.return
> 
> refers to the process id and these probes all fire in the main thread of
> the process with that id and also in its children. i.e. it simply
> identifies the target process. So why should the number in not also
> refer to the process id?


I haven't tested this, but I'd bet you are incorrect there about
'process(PID).syscall' probes.  I'll bet they only apply to task id PID,
not thread group id PID.

... more stuff deleted ...

I'm going to have to ignore the rest of this email for now, or I'll
never send this response.


NOTE 1: Here's what happens when you use 'stap -c CMD'

- stap passes the '-c CMD' argument down to staprun

- staprun loads the module and passes the '-c CMD' arg down to stapio

- stapio runs the command and eventually sends the pid to the module

There ends up being little difference (except a little timing) between
"stap -c CMD foo.stp" and "CMD; stap -x PID foo.stp".

-- 
David Smith
dsmith@redhat.com
Red Hat
http://www.redhat.com
256.217.0141 (direct)
256.837.0057 (fax)

[Bug translator/13187] Reconsider the semantics of process(number).thread.begin/end

[fche at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=13187

Frank Ch. Eigler <fche at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |WONTFIX

--- Comment #2 from Frank Ch. Eigler <fche at redhat dot com> ---
neat idea, but no recent need

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[Bug translator/13187] Reconsider the semantics of process(number).thread.begin/end

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=13187

--- Comment #1 from David Smith <dsmith at redhat dot com> 2011-09-14 14:28:23 UTC ---
(In reply to comment #0)
> However, for a running process, these probes will currently never fire, because
> the task ids of the child threads do not match the given process id.

There is a situation where process(PID).thread.begin probes will fire.

- Start a process that creates several threads that will run for an extended
period of time.  Determine the pids of those threads.
- RUn systemtap with a script that probes those pids.
- As systemtap attaches to those threads, the process(PID).thread.begin probe
will fire.


In my opinion, changing the semantics here is too big/messy of a change. 
Instead, a new probe type, something like 'process(PATH/PID).thread.create',
could be created.

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