Re: [PATCH] clocksource/arm_arch_timer: Don't trace count reader functions

[Steven Rostedt]

On Fri, 24 May 2019 10:10:25 +0100
Julien Thierry <julien.thierry@xxxxxxx> wrote:

> With v5.2-rc1, The ftrace functions_graph tracer locks up whenever it is
> enabled on arm64.
> 
> Since commit 0ea415390cd3 ("clocksource/arm_arch_timer: Use
> arch_timer_read_counter to access stable counters") a function pointer
> is consistently used to read the counter instead of potentially
> referencing an inlinable function.
> 
> The graph tacers relies on accessing the timer counters to compute the
> time spent in functions which causes the lockup when attempting to trace
> these code paths.
> 
> Annontate the arm arch timer counter accessors as notrace.
> 
> Fixes: 0ea415390cd3 ("clocksource/arm_arch_timer: Use
>        arch_timer_read_counter to access stable counters")
> Signed-off-by: Julien Thierry <julien.thierry@xxxxxxx>
> Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> ---
>  drivers/clocksource/arm_arch_timer.c | 8 ++++----

Is there any reason to function trace any of the functions in this
file? If not, then I would suggest removing all the "notrace"
annotations from this file, and add in the Makefile for this directory:

CFLAGS_REMOVE_arm_arch_timer.o = $(CC_FLAGS_FTRACE)

Hmm, I need to go through all the Makefiles in the kernel and remove
the "-pg" and replace it with "$(CC_FLAGS_FTRACE)" as that's the safer
way of doing it now.

-- Steve


>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/clocksource/arm_arch_timer.c b/drivers/clocksource/arm_arch_timer.c
> index b2a951a..5c69c9a 100644
> --- a/drivers/clocksource/arm_arch_timer.c
> +++ b/drivers/clocksource/arm_arch_timer.c
> @@ -149,22 +149,22 @@ u32 arch_timer_reg_read(int access, enum arch_timer_reg reg,
>  	return val;
>  }
>  
> -static u64 arch_counter_get_cntpct_stable(void)
> +static notrace u64 arch_counter_get_cntpct_stable(void)
>  {
>  	return __arch_counter_get_cntpct_stable();
>  }
>  
> -static u64 arch_counter_get_cntpct(void)
> +static notrace u64 arch_counter_get_cntpct(void)
>  {
>  	return __arch_counter_get_cntpct();
>  }
>  
> -static u64 arch_counter_get_cntvct_stable(void)
> +static notrace u64 arch_counter_get_cntvct_stable(void)
>  {
>  	return __arch_counter_get_cntvct_stable();
>  }
>  
> -static u64 arch_counter_get_cntvct(void)
> +static notrace u64 arch_counter_get_cntvct(void)
>  {
>  	return __arch_counter_get_cntvct();
>  }