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

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Message-ID: <20190524093600.11ca3cb2@gandalf.local.home>
Date:   Fri, 24 May 2019 09:36:00 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Marc Zyngier <marc.zyngier@....com>,
        Mark Rutland <mark.rutland@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] clocksource/arm_arch_timer: Don't trace count reader
 functions

On Fri, 24 May 2019 10:10:25 +0100
Julien Thierry <julien.thierry@....com> 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@....com>
> Cc: Marc Zyngier <marc.zyngier@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> ---
>  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();
>  }