linux-kernel - Re: [RFC/PATCH] Prevent wasting time to find out get_parent

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Date:	Tue, 24 Apr 2012 14:16:37 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Minho Ban <mhban@...sung.com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Josh Triplett <josh@...htriplett.org>
Subject: Re: [RFC/PATCH] Prevent wasting time to find out get_parent_ip

On Tue, 2012-04-24 at 21:36 +0900, Minho Ban wrote:
> trace_preempt_on/off looks empty if PREEMPT_TRACER is off. But actually it is
> spending time to find out get_parent_ip(even CALLER_ADDR for some ARCH) which is
> in argument. This seems not fair for those who expect to do nothing but increase
> or decrease count.
> 

Yuck what an ugly patch. Please don't uglify the C code with #ifdefs!

> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Minho Ban <mhban@...sung.com>
> ---
>  include/linux/ftrace.h |    3 ---
>  kernel/sched/core.c    |    5 ++++-
>  kernel/softirq.c       |    3 ++-
>  3 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/ftrace.h b/include/linux/ftrace.h
> index 72a6cab..7cd11fe 100644
> --- a/include/linux/ftrace.h
> +++ b/include/linux/ftrace.h
> @@ -484,9 +484,6 @@ static inline void __ftrace_enabled_restore(int enabled)
>  #ifdef CONFIG_PREEMPT_TRACER
>    extern void trace_preempt_on(unsigned long a0, unsigned long a1);
>    extern void trace_preempt_off(unsigned long a0, unsigned long a1);
> -#else
> -  static inline void trace_preempt_on(unsigned long a0, unsigned long a1) { }
> -  static inline void trace_preempt_off(unsigned long a0, unsigned long a1) { }

If you want to hide the "CALLER_ADDR" for other archs, then simply do:

# define trace_preempt_on(a0, a1) do { } while (0)
# define trace_preempt_off(a0, a1) do { } while (0)

and be done with it.

Oh, you should add a comment before these defines to the effect of:

 /*
  * Use defines instead of static inlines because some arches will make 
  * code out of the CALLER_ADDR, when we really want these to be a real 
  * nop.
  */

That way, you will document why we use defines instead of static
inlines.

-- Steve

>  #endif
>  
>  #ifdef CONFIG_FTRACE_MCOUNT_RECORD
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4603b9d..efdd5a0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3068,8 +3068,10 @@ void __kprobes add_preempt_count(int val)
>  	DEBUG_LOCKS_WARN_ON((preempt_count() & PREEMPT_MASK) >=
>  				PREEMPT_MASK - 10);
>  #endif
> +#ifdef CONFIG_PREEMPT_TRACER
>  	if (preempt_count() == val)
>  		trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> +#endif
>  }
>  EXPORT_SYMBOL(add_preempt_count);
>  
> @@ -3088,9 +3090,10 @@ void __kprobes sub_preempt_count(int val)
>  			!(preempt_count() & PREEMPT_MASK)))
>  		return;
>  #endif
> -
> +#ifdef CONFIG_PREEMPT_TRACER
>  	if (preempt_count() == val)
>  		trace_preempt_on(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> +#endif
>  	preempt_count() -= val;
>  }
>  EXPORT_SYMBOL(sub_preempt_count);
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 671f959..e7c8336 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -112,9 +112,10 @@ static void __local_bh_disable(unsigned long ip, unsigned int cnt)
>  	if (softirq_count() == cnt)
>  		trace_softirqs_off(ip);
>  	raw_local_irq_restore(flags);
> -
> +#ifdef CONFIG_PREEMPT_TRACER
>  	if (preempt_count() == cnt)
>  		trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));
> +#endif
>  }
>  #else /* !CONFIG_TRACE_IRQFLAGS */
>  static inline void __local_bh_disable(unsigned long ip, unsigned int cnt)


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