Re: [PATCH] fgraph: Use CPU hotplug mechanism to initialize idle shadow stacks

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 10 Dec 2024 16:11:16 +0100
Linus Walleij <linus.walleij@linaro.org> wrote:

> Hi Stephen,
> 
> On Sat, Oct 19, 2024 at 3:43 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > The function graph infrastructure allocates a shadow stack for every task
> > when enabled. This includes the idle tasks. The first time the function
> > graph is invoked, the shadow stacks are created and never freed until the
> > task exits. This includes the idle tasks.
> (...)
> > Cc: stable@vger.kernel.org
> > Fixes: 868baf07b1a25 ("ftrace: Fix memory leak with function graph and cpu hotplug")
> > Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> 
> 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?

Interesting. Does this happen only on boot-time tracing or after boot too?
If it does not work only for boot-time, cpuhp_setup_state() may not work
before starting boot-time function graph tracing.

Thank you,

> 
> Yours,
> Linus Walleij


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>