linux-kernel - Re: [PATCH v10 04/14] unwind_user/deferred: Add unwind_deferred

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Message-ID: <20250618112939.76f4bb87@gandalf.local.home>
Date: Wed, 18 Jun 2025 11:29:39 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.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>, Ingo Molnar <mingo@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Namhyung Kim <namhyung@...nel.org>, Thomas
 Gleixner <tglx@...utronix.de>, Andrii Nakryiko <andrii@...nel.org>, Indu
 Bhagat <indu.bhagat@...cle.com>, "Jose E. Marchesi" <jemarch@....org>, Beau
 Belgrave <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, Andrew Morton
 <akpm@...ux-foundation.org>
Subject: Re: [PATCH v10 04/14] unwind_user/deferred: Add
 unwind_deferred_trace()

On Wed, 18 Jun 2025 16:02:47 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Jun 10, 2025 at 08:54:25PM -0400, Steven Rostedt wrote:
> > +/**
> > + * unwind_deferred_trace - Produce a user stacktrace in faultable context
> > + * @trace: The descriptor that will store the user stacktrace
> > + *
> > + * This must be called in a known faultable context (usually when entering
> > + * or exiting user space). Depending on the available implementations
> > + * the @trace will be loaded with the addresses of the user space stacktrace
> > + * if it can be found.  
> 
> I am confused -- why would we ever want to call this on exiting
> user-space, or rather kernel entry?
> 
> I thought the whole point was to request a user trace while in-kernel,
> and defer that to return-to-user.

This code was broken out of the unwind deferred trace to be more stand
alone. Actually, it should be renamed to unwind_faultable_trace() or
something to denote that it must be called from a faultable context.

When Josh made perf use the task_work directly, it used this function to do
the trace as it handled the deferring.

Note, a request from the gcc folks is to add a system call that gives the
user space application a backtrace from its current location. This can be
handy for debugging as it would be similar to how we use dump_stack().

This function would be used for that.

-- Steve