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

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Message-ID: <20250618142000.GS1613376@noisy.programming.kicks-ass.net>
Date: Wed, 18 Jun 2025 16:20:00 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.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 Tue, Jun 10, 2025 at 08:54:27PM -0400, Steven Rostedt wrote:
> From: Josh Poimboeuf <jpoimboe@...nel.org>
> 
> Add an interface for scheduling task work to unwind the user space stack
> before returning to user space. This solves several problems for its
> callers:
> 
>   - Ensure the unwind happens in task context even if the caller may be
>     running in interrupt context.
> 
>   - Avoid duplicate unwinds, whether called multiple times by the same
>     caller or by different callers.
> 
>   - Take a timestamp when the first request comes in since the task
>     entered the kernel. This will be returned to the calling function
>     along with the stack trace when the task leaves the kernel. This
>     timestamp can be used to correlate kernel unwinds/traces with the user
>     unwind.
> 
> The timestamp is created to detect when the stacktrace is the same. It is
> generated the first time a user space stacktrace is requested after the
> task enters the kernel.
> 
> 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?