linux-kernel - Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <20250702163609.GR1613200@noisy.programming.kicks-ass.net>
Date: Wed, 2 Jul 2025 18:36:09 +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>,
	Jens Axboe <axboe@...nel.dk>, Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH v12 06/14] unwind_user/deferred: Add deferred unwinding
 interface

On Mon, Jun 30, 2025 at 08:53:27PM -0400, Steven Rostedt wrote:

> +/*
> + * Read the task context timestamp, if this is the first caller then
> + * it will set the timestamp.
> + *
> + * For this to work properly, the timestamp (local_clock()) must
> + * have a resolution that will guarantee a different timestamp
> + * everytime a task makes a system call. That is, two short
> + * system calls back to back must have a different timestamp.
> + */
> +static u64 get_timestamp(struct unwind_task_info *info)
> +{
> +	lockdep_assert_irqs_disabled();
> +
> +	if (!info->timestamp)
> +		info->timestamp = local_clock();
> +
> +	return info->timestamp;
> +}

I'm very hesitant about this. Modern hardware can do this, but older
hardware (think Intel Core and AMD Bulldozer etc hardware) might
struggle with this. They don't have stable TSC and as such will use the
magic in kernel/sched/clock.c; which can get stuck on a window edge for
a little bit and re-use timestamps.