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

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Message-ID: <aCfMzJ-zN0JKKTjO@google.com>
Date: Fri, 16 May 2025 16:39:56 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Steven Rostedt <rostedt@...dmis.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

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.

Thanks,
Namhyung

> 
> Next merge window, we can spend more time on getting the perf API working
> properly.
> 
> -- Steve