Re: [RFC PATCH 1/2] ftrace: Introduce cmdline argument ftrace_disabled

[Tiezhu Yang]

On 06/19/2021 11:22 PM, Steven Rostedt wrote:
> On Sat, 19 Jun 2021 14:29:19 +0800
> Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:
>
> > If set CONFIG_FUNCTION_TRACER=y, we can use the following command to
> > disable ftrace after boot up:
> > echo 0 > /proc/sys/kernel/ftrace_enabled
> I'd love to just remove that. It's original purpose was to stop function
> tracing when the latency tracers were active. That's now done via a tracefs
> option. The "ftrace_enabled" really has no use anymore.
>
> > ftrace_disabled is much stronger than ftrace_enabled, introduce a new
> > cmdline argument ftrace_disabled for user to control whether to disable
> > ftrace when boot up.
> "ftrace_disabled" is triggered when an anomaly is detected, and for the
> safety of the system, ftrace shuts down. It was never meant to be a user
> triggered event.

Thanks for your explanation.

> You have no rationale for this change. What's the purpose of this?

The "System Benchmarks Index Score" of UnixBench under FUNCTION_TRACER
is lower than !FUNCTION_TRACER, I want to use this new cmdline argument
ftrace_disabled to test it, this is the original intention.

I see the following help info of "config FUNCTION_TRACER":

[If it's runtime disabled (the bootup default), then the overhead of the
instructions is very small and not measurable even in micro-benchmarks.]

I am not quite understand the above description, could you tell me how to
avoid the runtime performance overhead under FUNCTION_TRACER?

Thanks,
Tiezhu

> -- Steve
>
>
> > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > ---
> >   kernel/trace/ftrace.c | 8 ++++++++
> >   1 file changed, 8 insertions(+)
> >
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 72ef4dc..a015699 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -5517,6 +5517,14 @@ static char ftrace_filter_buf[FTRACE_FILTER_SIZE] __initdata;
> >   /* Used by function selftest to not test if filter is set */
> >   bool ftrace_filter_param __initdata;
> >   
> > +static int __init set_ftrace_disabled(char *str)
> > +{
> > +	pr_info("Set ftrace_disabled to disable ftrace\n");
> > +	ftrace_disabled = 1;
> > +	return 1;
> > +}
> > +__setup("ftrace_disabled", set_ftrace_disabled);
> > +
> >   static int __init set_ftrace_notrace(char *str)
> >   {
> >   	ftrace_filter_param = true;