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

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Message-ID: <aCxM_BizulyIVcdb@gmail.com>
Date: Tue, 20 May 2025 11:35:56 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Namhyung Kim <namhyung@...nel.org>, 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>, 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


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 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.

Could you please not rush this facility into v6.16? It barely had any 
design review so far, and I'm still not entirely sure about the 
approach.

Thanks,

	Ingo