linux-kernel - Re: [PATCH v9 00/13] unwind_user: x86: Deferred unwinding infrastructure

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Message-ID: <20250519113339.027c2a68@batman.local.home>
Date: Mon, 19 May 2025 11:33:39 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Jiri Olsa
 <jolsa@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin"
 <hpa@...or.com>, Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH v9 00/13] unwind_user: x86: Deferred unwinding
 infrastructure

On Fri, 16 May 2025 16:39:56 -0700
Namhyung Kim <namhyung@...nel.org> wrote:

> Hi Steve,
> 
> On Wed, May 14, 2025 at 01:27:20PM -0400, Steven Rostedt wrote:
> > On Tue, 13 May 2025 18:34:35 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> >   
> > > This has modifications in x86 and I would like it to go through the x86
> > > tree. Preferably it can go into this merge window so we can focus on getting
> > > perf and ftrace to work on top of this.  
> > 
> > I think it may be best for me to remove the two x86 specific patches, and
> > rebuild the ftrace work on top of it. For testing, I'll just keep those two
> > patches in my tree locally, but then I can get this moving for this merge
> > window.  
> 
> Maybe I asked this before but I don't remember if I got the answer. :)
> How does it handle task exits as it won't go to userspace?  I guess it'll
> lose user callstacks for exit syscalls and other termination paths.
> 
> Similarly, it will miss user callstacks in the samples at the end of
> profiling if the target tasks remain in the kernel (or they sleep).
> It looks like a fundamental limitation of the deferred callchains.
> 

Ah, I think I forgot about that. I believe the exit path can also be a
faultable path. All it needs is a hook to do the exit. Is there any
"task work" clean up on exit? I need to take a look.

-- Steve