linux-kernel - Re: [PATCH v10 06/14] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <20250618113706.2eb46544@gandalf.local.home>
Date: Wed, 18 Jun 2025 11:37:06 -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 06/14] unwind_user/deferred: Add deferred unwinding
 interface

On Wed, 18 Jun 2025 16:20:00 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> > The timestamp is passed to the caller on request, and when the stacktrace is
> > generated upon returning to user space, it call the requester's callback
> > with the timestamp as well as the stacktrace.  
> 
> This whole story hinges on there being a high resolution time-stamp
> available... Good thing we killed x86 !TSC support when we did. You sure
> there's no other architectures you're interested in that lack a high res
> time source?
> 
> What about two CPUs managing to request an unwind at exactly the same
> time?

It's mapped to a task. As long as each timestamp is unique for a task it
should be fine. As the trace can record the current->pid along with the
timestamp to map to the unique user space stack trace.

As for resolution, as long as there can't be two system calls back to back
within the same time stamp. Otherwise, yeah, we have an issue.

-- Steve