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

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Message-Id: <20250521082605.b4bd632ef1312778ea51dd71@kernel.org>
Date: Wed, 21 May 2025 08:26:05 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Steven Rostedt <rostedt@...dmis.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>, 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.

Can we use a hybrid approach for this case?
It might be more balanced (from the performance point of view) to save
the full stack in a classic way only in this case, rather than faulting
on process exit or doing file access just to load the sframe.

Thanks,

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

-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>