On Thu, 16 Apr 2009, Frederic Weisbecker wrote:

quoted text
> Impact: less memory usage for tracing
> 
> This patch provides the support for dynamic size strings on
> event tracing.
> 
> The key concept is to use a structure with an ending char array field of
> undefined size and use such ability to allocate the minimal size on the ring
> buffer to make the entry fit inside as opposite to a fixed length strings with
> upper bound.
> 
> This patch provides two new macros:
> 
> -__ending_string(name)
> 
> This one declares the string to the structure inside TP_STRUCT__entry.
> Only the name is needed.
> Two constraints: only one __ending_string() per TRACE_EVENT can be added and
> it must be the last field to be declared. Hence the __ending prefix.
> 
> - open_string_assign(call, dst, src)
> 
> This one does the copy inside TP__fast_assign() of the source
> string to the destination. The name of the tracepoint (call) must be provided
> for now. Hopefully I will find a solution to avoid it later.
> 
> Two constraint: can be used only once and always on the beginning because
> it needs to manage the ring buffer reservation by itself. Hence the open prefix.
> 
> How does it works?
> 
> A new has_ending_string field has been added to struct ftrace_event_call and is
> false by default.
> Once a TRACE_EVENT uses an __ending_string field, it is set to 1.
> 
> Until now, the ring buffer reservation was done in ftrace_raw_event_##call().
> It is still the case if we don't have an __ending_string() field. If we have one,
> open_string_assign() will manage it itself to allocate the appropriate size,
> depending of the size of the string to be copied for each trace.
> 
> The choice between the usual static allocation and the new dynamic one depends
> on the "has_ending_string" value.
> 
> It also support filtering because these strings behave essentially
> like usual fixed length string.

And I thought I was the only one that could master CPP wackiness ;-)

BTW, I was confused on IRC, because I was thinking this was printf like, 
but no, we only need to be concerned about a string.

But I have another idea:


quoted text
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> ---
>  include/linux/ftrace_event.h |    1 +
>  include/trace/ftrace.h       |   62 ++++++++++++++++++++++++++++++++++++-----
>  2 files changed, 55 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index 75f3ac0..b49bfbf 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -100,6 +100,7 @@ struct ftrace_event_call {
>  	int			n_preds;
>  	struct filter_pred	**preds;
>  	void			*mod;
> +	bool			has_ending_string;
>  
>  #ifdef CONFIG_EVENT_PROFILE
>  	atomic_t	profile_count;
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 60c5323..8ea750e 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -27,6 +27,9 @@
>  #undef __field
>  #define __field(type, item)		type	item;
>  
> +#undef __ending_string
> +#define __ending_string(item)			char	item[];
> +
>  #undef TP_STRUCT__entry
>  #define TP_STRUCT__entry(args...) args
>  
> @@ -146,12 +149,23 @@ ftrace_raw_output_##call(struct trace_iterator *iter, int flags)	\
>  	if (!ret)							\
>  		return 0;
>  
> +#undef __ending_string
> +#define __ending_string(item)						      \
> +	ret = trace_seq_printf(s, "\tfield: char " #item "[];\t"	      \
> +			       "offset:%u;\n",				      \
> +			       (unsigned int)offsetof(typeof(field), item));   \
> +	if (!ret)							      \
> +		return 0;
> +
>  #undef __entry
>  #define __entry REC
>  
>  #undef TP_printk
>  #define TP_printk(fmt, args...) "%s, %s\n", #fmt, __stringify(args)
>  
> +#undef open_string_assign
> +#define open_string_assign(call, dst, src)	strcpy(__entry->dst, src)
> +
>  #undef TP_fast_assign
>  #define TP_fast_assign(args...) args
>  
> @@ -189,6 +203,19 @@ ftrace_format_##call(struct trace_seq *s)				\
>  	if (ret)							\
>  		return ret;
>  
> +/*
> + * We choose a size of MAX_FILTER_STR_VAL, then we behave like
> + * a usual string with the maximum size to keep being filterable.
> + */
> +#undef __ending_string
> +#define __ending_string(item)						\
> +	ret = trace_define_field(event_call, "char []", #item,		\
> +				 offsetof(typeof(field), item),		\
> +				 MAX_FILTER_STR_VAL);			\
> +	if (ret)							\
> +		return ret;						\
> +	event_call->has_ending_string = true;
> +
>  #undef TRACE_EVENT
>  #define TRACE_EVENT(call, proto, args, tstruct, func, print)		\
>  int									\
> @@ -409,6 +436,22 @@ __attribute__((section("_ftrace_events"))) event_##call = {		\
>  #undef __entry
>  #define __entry entry
>  
> +/*
> + * If we have and ending undefined string size, then the size of
> + * the entry is dynamic. In such case we override the ring buffer
> + * reservation to manage it ourselves with our dynamic string size.
> + */
> +#undef open_string_assign
> +#define open_string_assign(call, dst, src)				       \
> +	event = trace_current_buffer_lock_reserve(			       \
> +			event_##call.id,				       \
> +			sizeof(struct ftrace_raw_##call) + strlen(src) + 1,    \
> +			irq_flags, pc);					       \
> +	if (!event)							       \
> +		return;							       \
> +	entry	= ring_buffer_event_data(event);			       \
> +	strcpy(entry->dst, src);
> +
>  #undef TRACE_EVENT
>  #define TRACE_EVENT(call, proto, args, tstruct, assign, print)		\
>  _TRACE_PROFILE(call, PARAMS(proto), PARAMS(args))			\
> @@ -418,20 +461,23 @@ static struct ftrace_event_call event_##call;				\
>  static void ftrace_raw_event_##call(proto)				\
>  {									\
>  	struct ftrace_event_call *call = &event_##call;			\
> -	struct ring_buffer_event *event;				\
> -	struct ftrace_raw_##call *entry;				\
> +	struct ring_buffer_event *event = NULL;				\
> +	struct ftrace_raw_##call *entry = NULL;				\
>  	unsigned long irq_flags;					\
>  	int pc;								\
>  									\
>  	local_save_flags(irq_flags);					\
>  	pc = preempt_count();						\
>  									\
> -	event = trace_current_buffer_lock_reserve(event_##call.id,	\
> -				  sizeof(struct ftrace_raw_##call),	\
> -				  irq_flags, pc);			\
> -	if (!event)							\
> -		return;							\
> -	entry	= ring_buffer_event_data(event);			\
> +	if (!call->has_ending_string) {					\
> +		event = trace_current_buffer_lock_reserve(		\
> +				event_##call.id,			\
> +				sizeof(struct ftrace_raw_##call),	\
> +				irq_flags, pc);				\
> +		if (!event)						\
> +			return;						\
> +		entry	= ring_buffer_event_data(event);		\
> +	}								\
>  									\
>  	assign;								\
>  									\


We can get rid of the has_ending_string and open_string_assign and have 
this:

 
        TP_STRUCT__entry(
                __field(unsigned long, wait_usec)
                __field(unsigned long, wait_nsec_rem)
                __ending_string(name)
        ),
        TP_fast_assign(
                __entry->wait_nsec_rem = do_div(waittime, NSEC_PER_USEC);
                __entry->wait_usec = (unsigned long) waittime;
                strcpy(__entry->name, lock->name);
        ),


Then we could do the following in the print the assign:

#undef __field
#define __field(a,b)
#undef __array
#define __array(a,b,c)
#undef __ending_sting
#define __ending_string(name) __str_size__ += strlen(name) + 1;

#define TRACE_EVENT(call, proto, args, tstruct, assign, print)
static void ftrace_raw_event_##call(proto)
{
	int __str_size__ = 0;

	tstruct

	event = trace_current_buffer_lock_reserve(event_##call.id,
				sizeof(struct ftrace_raw_##call) +
				__str_size__,
				irq_flags, pc)
	if (!event)
		return;

	entry = ring_buffer_event_data(event);

	assign
}


Yes, I did not add other declarations nor the backslashes for the macro, 
but you get what I'm doing, right?

-- Steve

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