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Date: Tue, 10 Dec 2024 11:31:28 -0500
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Walleij <linus.walleij@linaro.org>
Cc: LKML <linux-kernel@vger.kernel.org>, Linux trace kernel
 <linux-trace-kernel@vger.kernel.org>, Masami Hiramatsu
 <mhiramat@kernel.org>, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
 Mark Rutland <mark.rutland@arm.com>, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] fgraph: Use CPU hotplug mechanism to initialize idle
 shadow stacks
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On Tue, 10 Dec 2024 16:11:16 +0100
Linus Walleij <linus.walleij@linaro.org> wrote:

> This patch regressed boot-time tracing for me.
> 
> How to reproduce:
> - Enable CONFIG_FTRACE, CONFIG_FUNCTION_TRACER,
>   CONFIG_BOOTTIME_TRACING
> - Pass command line
>   ftrace=function_graph ftrace_graph_filter=do_idle
>   to make ftrace trace this function all through the boot process.
> 
> Before this patch:
> 
> cd /sys/kernel/debug/tracing
> cat trace
> 
> gives a nice trace of all invocations of do_idle() during boot.
> 
> After this patch:
> 
> cd /sys/kernel/debug/tracing
> cat trace
> 
> Gives an empty trace :(
> 
> And:
> 
> cat current_tracer
> function_graph
> cat set_graph_function
> do_idle
> cat tracing_on
> 1
> 
> So all *is* set up, just not performing
> 
> I tried to figure out why this happens but I'm not good with tracing
> internals. Any ideas?

Thanks for the report. I'm currently at the ELISA workshop this week,
but will try to reproduce it.

-- Steve