linux-kernel - Re: [PATCH v16 0/4] perf: Support the deferred unwinding infrastructure

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Message-ID: <20251023124057.2a6e793a@gandalf.local.home>
Date: Thu, 23 Oct 2025 12:40:57 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...nel.org>, 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>, Arnaldo
 Carvalho de Melo <acme@...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>, Florian Weimer
 <fweimer@...hat.com>, Sam James <sam@...too.org>, Kees Cook
 <kees@...nel.org>, Carlos O'Donell <codonell@...hat.com>
Subject: Re: [PATCH v16 0/4] perf: Support the deferred unwinding
 infrastructure

On Thu, 23 Oct 2025 17:00:02 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> +/* Deferred unwinding callback for task specific events */
> +static void perf_unwind_deferred_callback(struct unwind_work *work,
> +					 struct unwind_stacktrace *trace, u64 cookie)
> +{
> +	struct perf_callchain_deferred_event deferred_event = {
> +		.trace = trace,
> +		.event = {
> +			.header = {
> +				.type = PERF_RECORD_CALLCHAIN_DEFERRED,
> +				.misc = PERF_RECORD_MISC_USER,
> +				.size = sizeof(deferred_event.event) +
> +					(trace->nr * sizeof(u64)),
> +			},
> +			.cookie = cookie,
> +			.nr = trace->nr,
> +		},
> +	};
> +
> +	perf_iterate_sb(perf_callchain_deferred_output, &deferred_event, NULL);
> +}
> +

So "perf_iterate_sb()" was the key point I was missing. I'm guessing it's
basically a demultiplexer that distributes events to all the requestors?

If I had know this, I would have done it completely different.

-- Steve