Re: [PATCH] tracing: Cleanup the convoluted softirq tracepoints

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Thomas Gleixner (tglx@linutronix.de) wrote:
> With the addition of trace_softirq_raise() the softirq tracepoint got
> even more convoluted. Why the tracepoints take two pointers to assign
> an integer is beyond my comprehension.
> 
> But adding an extra case which treats the first pointer as an unsigned
> long when the second pointer is NULL including the back and forth
> type casting is just horrible.
> 
> Convert the softirq tracepoints to take a single unsigned int argument
> for the softirq vector number and fix the call sites.

Well, there was originally a reason for this oddness. The in __do_softirq(),
"h - softirq_ve"c computation was not needed outside of the tracepoint handler
in the past, but it now seems to be required with the new inlined
"kstat_incr_softirqs_this_cpu()".

So yes, thanks to this recent change, it now makes sense to pull this
computation out of the tracepoints and do it unconditionally in the kernel code.

Feel free to put my:
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

Thanks,

Mathieu

> 
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>  include/linux/interrupt.h  |    2 -
>  include/trace/events/irq.h |   54 ++++++++++++++++-----------------------------
>  kernel/softirq.c           |   14 ++++++-----
>  3 files changed, 29 insertions(+), 41 deletions(-)
> 
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 531495d..0ac1949 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -410,7 +410,7 @@ extern void open_softirq(int nr, void (*action)(struct softirq_action *));
>  extern void softirq_init(void);
>  static inline void __raise_softirq_irqoff(unsigned int nr)
>  {
> -	trace_softirq_raise((struct softirq_action *)(unsigned long)nr, NULL);
> +	trace_softirq_raise(nr);
>  	or_softirq_pending(1UL << nr);
>  }
>  
> diff --git a/include/trace/events/irq.h b/include/trace/events/irq.h
> index 6fa7cba..1c09820 100644
> --- a/include/trace/events/irq.h
> +++ b/include/trace/events/irq.h
> @@ -86,76 +86,62 @@ TRACE_EVENT(irq_handler_exit,
>  
>  DECLARE_EVENT_CLASS(softirq,
>  
> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
> +	TP_PROTO(unsigned int vec_nr),
>  
> -	TP_ARGS(h, vec),
> +	TP_ARGS(vec_nr),
>  
>  	TP_STRUCT__entry(
> -		__field(	int,	vec			)
> +		__field(	unsigned int,	vec	)
>  	),
>  
>  	TP_fast_assign(
> -		if (vec)
> -			__entry->vec = (int)(h - vec);
> -		else
> -			__entry->vec = (int)(long)h;
> +		__entry->vec = vec_nr;
>  	),
>  
> -	TP_printk("vec=%d [action=%s]", __entry->vec,
> +	TP_printk("vec=%u [action=%s]", __entry->vec,
>  		  show_softirq_name(__entry->vec))
>  );
>  
>  /**
>   * softirq_entry - called immediately before the softirq handler
> - * @h: pointer to struct softirq_action
> - * @vec: pointer to first struct softirq_action in softirq_vec array
> + * @vec_nr:  softirq vector number
>   *
> - * The @h parameter, contains a pointer to the struct softirq_action
> - * which has a pointer to the action handler that is called. By subtracting
> - * the @vec pointer from the @h pointer, we can determine the softirq
> - * number. Also, when used in combination with the softirq_exit tracepoint
> - * we can determine the softirq latency.
> + * When used in combination with the softirq_exit tracepoint
> + * we can determine the softirq handler runtine.
>   */
>  DEFINE_EVENT(softirq, softirq_entry,
>  
> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
> +	TP_PROTO(unsigned int vec_nr),
>  
> -	TP_ARGS(h, vec)
> +	TP_ARGS(vec_nr)
>  );
>  
>  /**
>   * softirq_exit - called immediately after the softirq handler returns
> - * @h: pointer to struct softirq_action
> - * @vec: pointer to first struct softirq_action in softirq_vec array
> + * @vec_nr:  softirq vector number
>   *
> - * The @h parameter contains a pointer to the struct softirq_action
> - * that has handled the softirq. By subtracting the @vec pointer from
> - * the @h pointer, we can determine the softirq number. Also, when used in
> - * combination with the softirq_entry tracepoint we can determine the softirq
> - * latency.
> + * When used in combination with the softirq_entry tracepoint
> + * we can determine the softirq handler runtine.
>   */
>  DEFINE_EVENT(softirq, softirq_exit,
>  
> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
> +	TP_PROTO(unsigned int vec_nr),
>  
> -	TP_ARGS(h, vec)
> +	TP_ARGS(vec_nr)
>  );
>  
>  /**
>   * softirq_raise - called immediately when a softirq is raised
> - * @h: pointer to struct softirq_action
> - * @vec: pointer to first struct softirq_action in softirq_vec array
> + * @vec_nr:  softirq vector number
>   *
> - * The @h parameter contains a pointer to the softirq vector number which is
> - * raised. @vec is NULL and it means @h includes vector number not
> - * softirq_action. When used in combination with the softirq_entry tracepoint
> - * we can determine the softirq raise latency.
> + * When used in combination with the softirq_entry tracepoint
> + * we can determine the softirq raise to run latency.
>   */
>  DEFINE_EVENT(softirq, softirq_raise,
>  
> -	TP_PROTO(struct softirq_action *h, struct softirq_action *vec),
> +	TP_PROTO(unsigned int vec_nr),
>  
> -	TP_ARGS(h, vec)
> +	TP_ARGS(vec_nr)
>  );
>  
>  #endif /*  _TRACE_IRQ_H */
> diff --git a/kernel/softirq.c b/kernel/softirq.c
> index 07b4f1b..c0a9ea5 100644
> --- a/kernel/softirq.c
> +++ b/kernel/softirq.c
> @@ -212,18 +212,20 @@ restart:
>  
>  	do {
>  		if (pending & 1) {
> +			unsigned int vec_nr = h - softirq_vec;
>  			int prev_count = preempt_count();
> -			kstat_incr_softirqs_this_cpu(h - softirq_vec);
>  
> -			trace_softirq_entry(h, softirq_vec);
> +			kstat_incr_softirqs_this_cpu(vec_nr);
> +
> +			trace_softirq_entry(vec_nr);
>  			h->action(h);
> -			trace_softirq_exit(h, softirq_vec);
> +			trace_softirq_exit(vec_nr);
>  			if (unlikely(prev_count != preempt_count())) {
>  				printk(KERN_ERR "huh, entered softirq %td %s %p"
>  				       "with preempt_count %08x,"
> -				       " exited with %08x?\n", h - softirq_vec,
> -				       softirq_to_name[h - softirq_vec],
> -				       h->action, prev_count, preempt_count());
> +				       " exited with %08x?\n", vec_nr,
> +				       softirq_to_name[vec_nr], h->action,
> +				       prev_count, preempt_count());
>  				preempt_count() = prev_count;
>  			}
>  

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com