linux-kernel - Re: [RESEND][PATCH v15 2/4] perf: Support deferred user callchains

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Message-ID: <20250923100137.GC3419281@noisy.programming.kicks-ass.net>
Date: Tue, 23 Sep 2025 12:01:37 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...nel.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>,
	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: [RESEND][PATCH v15 2/4] perf: Support deferred user callchains

On Mon, Sep 08, 2025 at 01:14:14PM -0400, Steven Rostedt wrote:
> +	/*
> +	 * All accesses related to the event are within the same RCU section in
> +	 * perf_event_callchain_deferred(). The RCU grace period before the
> +	 * event is freed will make sure all those accesses are complete by then.
> +	 */
> +	rcuwait_wait_event(&event->pending_unwind_wait, !event->pending_unwind_callback, TASK_UNINTERRUPTIBLE);

You need a narrower terminal, this is again excessive. I mostly code
with my screen split in 4 columns (3 if I can't find my glasses), and
that gets me around 90 character columns.

> +	if (event->attr.defer_callchain)
> +		init_task_work(&event->pending_unwind_work,
> +			       perf_event_callchain_deferred);

And let me hand you a bucket of {}, I've heard they're getting expensive
over in the US due to this tariff nonsense :-)