linux-kernel - Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain user space unwinding

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Message-ID: <2b0d0133-3156-47d8-b046-f93ea6cd40fd@linux.ibm.com>
Date: Fri, 1 Aug 2025 17:49:08 +0200
From: Jens Remus <jremus@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>, Josh Poimboeuf <jpoimboe@...nel.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
        bpf@...r.kernel.org, x86@...nel.org,
        Steven Rostedt <rostedt@...nel.org>, Vasily Gorbik <gor@...ux.ibm.com>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>,
        Sam James <sam@...too.org>
Subject: Re: [RFC PATCH v1 12/16] unwind_user/backchain: Introduce back chain
 user space unwinding

On 8/1/2025 2:36 PM, Heiko Carstens wrote:
> On Thu, Jul 17, 2025 at 10:19:54PM -0700, Josh Poimboeuf wrote:
>> On Thu, Jul 17, 2025 at 02:20:12PM +0200, Jens Remus wrote:

>> I believe stack_trace_save_user() is only used by ftrace, and that will
>> no longer be needed once ftrace starts using unwind_user.
>>
>> Maybe Heiko knows if that backchain user stacktrace code has any users?
>>
>> If distros aren't building with -mbackchain, maybe backchain support can
>> be considered obsoleted by sframe, and we can get away with not
>> implementing it.
> 
> I guess that's a valid option. I know only of some special cases where
> users compile everything on their own with -mbackchain to make this
> work on a per-case basis. It shouldn't cause to much pain for them to
> switch to sframe, as soon as that is available.

Let me have another stab at a cleaner implementation.  Shouldn't be too
much effort.  We can then decide whether to leave it out or not.

Will be away from keyboard for a few weeks.

Regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@...ibm.com

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