linux-kernel - Re: [RFC PATCH 2/5] unwind: Export unwind

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Message-ID: <6871537ec0461_1d3d100c6@dwillia2-xfh.jf.intel.com.notmuch>
Date: Fri, 11 Jul 2025 11:10:06 -0700
From: <dan.j.williams@...el.com>
To: Steven Rostedt <rostedt@...dmis.org>, Greg KH <gregkh@...uxfoundation.org>
CC: Christoph Hellwig <hch@...radead.org>, Mathieu Desnoyers
	<mathieu.desnoyers@...icios.com>, <linux-kernel@...r.kernel.org>, "Josh
 Poimboeuf" <jpoimboe@...nel.org>, Masami Hiramatsu <mhiramat@...nel.org>,
	Peter Zijlstra <peterz@...radead.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>, <tech-board-discuss@...ts.linuxfoundation.org>
Subject: Re: [RFC PATCH 2/5] unwind: Export unwind_user symbol to GPL modules

Steven Rostedt wrote:
> On Fri, 11 Jul 2025 13:38:07 +0200
> Greg KH <gregkh@...uxfoundation.org> wrote:
> 
> > Yes, lttng is a "good citizen" of the kernel community, and yes, it's
> > not merged into the tree, but that's not our issue, and living outside
> > of the tree has it's penalities, both economic and technical.  This is
> > one of those penalities, sorry.
> 
> BTW, when it comes to tracers, being out of tree is actually a huge
> advantage. Tracers, unlike drivers, are only monitoring the kernel. Which
> means it doesn't really rely on the workings of the kernel, so it really
> doesn't get much help from changes made by other kernel developers. Like
> the BPF folks have said keeping BPF programs up to date, isn't that much of
> a burden.
> 
> The huge advantage that LTTng has over perf and ftrace for being out of
> tree is that it controls the ABI between the LTTng kernel side and the user
> space side. LTTng can experiment with new interfaces, and if something
> breaks, it can simply change it and deliver a new tool that includes the
> new module with the update.

It is odd to read this claimed benefit when viewing it from the wider
Linux kernel project. Upstream maintenance of ABI contracts is the
fundamental struggle of subsystems. The request, "can we get the kernel
out of the way and maintain our own ABI to our users?" is a consistent
refrain, and it consistently receives a qualified "no" for regression,
security, and other interface evolution concerns.