Re: [PATCH] ring_buffer: enlarge RB_MAX_SMALL_DATA

[Steven Rostedt]

On Mon, 2009-04-13 at 11:00 +0800, Lai Jiangshan wrote:
> When I am writing userspace tools for ftrace, I found
> RB_MAX_SMALL_DATA is too small, some events waste an 'u32'
> to save the actually length.

Although I like the idea, I want to look at something else.

2^27 is: 134,217,728
2^26 is: 67,108,864

That is the count in nanoseconds. Thus we go from 134 millisecs to 67
millisecs before we must add an extended counter.

I guess that is not an issue, since 67ms is still quite big. For sparse
tracing, it could add more extended counters where none were needed. But
this I doubt this is an issue.

> 
> This fix will break previous userspace tools,
> so complaints are also welcome.
> --------------
> 
> Subject: [PATCH] ring_buffer: enlarge RB_MAX_SMALL_DATA
> 
> Impact: save memory in ring_buffer
> 
> I found 28bytes for an event data is sometimes too small,
> it will waste an 'u32' to save the actually length for
> events which data size > 28.
> 
> 'len=5' in the next logs means the event size is sizeof(u32)*5=20 bytes,
> it includes event head and event data. This logs are collected
> in 32bits system.
> 
> syscall tracer:
> time_stamp=391042354662405, len=10
> time_stamp=391042354679741, len=5
> time_stamp=391042354686278, len=9
> time_stamp=391042354691465, len=5
> time_stamp=391042354695348, len=9
> time_stamp=391042354698631, len=5
> time_stamp=391042354703472, len=7
> time_stamp=391042354756153, len=5
> time_stamp=391042354766952, len=7
> time_stamp=391042354790733, len=5
> time_stamp=391042355124535, len=10
> time_stamp=391042355136474, len=5
> time_stamp=391042355141883, len=9
> time_stamp=391042355146048, len=5
> time_stamp=391042355149837, len=9
> time_stamp=391042355152939, len=5
> time_stamp=391042355157944, len=7
> time_stamp=391042355194303, len=5
> time_stamp=391042355200947, len=10
> time_stamp=391042355690428, len=5
> time_stamp=391042355700443, len=9
> 
> kmemtrace tracer:
> time_stamp=391116237929771, len=6
> time_stamp=391116237935560, len=6
> time_stamp=391116237937269, len=6
> time_stamp=391116649408762, len=11
> time_stamp=391116649415541, len=11
> time_stamp=391116649421702, len=11
> time_stamp=391116649428121, len=6
> time_stamp=391116649612696, len=6
> time_stamp=391116649616327, len=6
> time_stamp=391116649948694, len=11
> time_stamp=391116650097324, len=11
> time_stamp=391116650111344, len=11
> time_stamp=391116650116344, len=6
> time_stamp=391116656377385, len=11
> time_stamp=391116656383258, len=11
> time_stamp=391116656399858, len=11
> time_stamp=391116656406757, len=6
> time_stamp=391116657165045, len=11
> time_stamp=391116657172126, len=11
> time_stamp=391116657178716, len=11
> time_stamp=391116657185031, len=6
> time_stamp=391116657268110, len=6
> time_stamp=391116657273307, len=6
> time_stamp=391116657275341, len=6
> time_stamp=391116657276638, len=6
> time_stamp=391116657283869, len=6
> 
> sched_switch tracer:
> time_stamp=391250280621660, len=9
> time_stamp=391250281277013, len=9
> time_stamp=391250281328332, len=9
> time_stamp=391250600954274, len=9
> time_stamp=391250601092171, len=9
> time_stamp=391250601276548, len=9
> time_stamp=391250601342968, len=9
> time_stamp=391250601616099, len=9
> time_stamp=391250602099709, len=9
> time_stamp=391250603414452, len=9
> time_stamp=391250605580652, len=9
> time_stamp=391250605598567, len=9
> 
> function_graph tracer:
> time_stamp=391216875265827, len=11
> time_stamp=391216875270031, len=5
> time_stamp=391216875272943, len=5
> time_stamp=391216875275169, len=5
> time_stamp=391216875279253, len=11
> time_stamp=391216875282684, len=5
> time_stamp=391216875291602, len=11
> time_stamp=391216875294898, len=5
> 
> After this fix applied, function_graph tracer:
> 
> time_stamp=395685660789931, len=5
> time_stamp=395685660792751, len=10
> time_stamp=395685660795807, len=5
> time_stamp=395685660798260, len=10
> time_stamp=395685660801224, len=10
> time_stamp=395685660804164, len=10
> time_stamp=395685660807068, len=5
> time_stamp=395685660809314, len=5
> time_stamp=395685660812001, len=10
> time_stamp=395685660815589, len=10
> time_stamp=395685660823151, len=5
> time_stamp=395685660825215, len=5
> time_stamp=395685660827697, len=5
> time_stamp=395685660829589, len=10
> time_stamp=395685660833558, len=10
> time_stamp=395685660836630, len=10
> 
> Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> ---
> diff --git a/include/linux/ring_buffer.h b/include/linux/ring_buffer.h
> index 3d3ab00..5298ee1 100644
> --- a/include/linux/ring_buffer.h
> +++ b/include/linux/ring_buffer.h
> @@ -13,7 +13,7 @@ struct ring_buffer_iter;
>   */
>  struct ring_buffer_event {
>  	kmemcheck_define_bitfield(bitfield, {
> -		u32		type:2, len:3, time_delta:27;
> +		u32		type:2, len:4, time_delta:26;
>  	});
>  
>  	u32		array[];
> @@ -31,7 +31,7 @@ struct ring_buffer_event {
>   *				  everything else same as RINGBUF_TYPE_DATA
>   *
>   * @RINGBUF_TYPE_TIME_EXTEND:	Extend the time delta
> - *				 array[0] = time delta (28 .. 59)
> + *				 array[0] = time delta (27 .. 58)
>   *				 size = 8 bytes

Unfortunately, this changes the API to userspace. For those parsers that
do this in binary. I think the answer is, before we add this, we export
the format of the ring buffer headers just like we do for other formats.
This way, a user tool can default to the old way if the format file does
not exist, and can know the current format with the file.

I'll work on adding that format file sometime this week.

-- Steve

>   *
>   * @RINGBUF_TYPE_TIME_STAMP:	Sync time stamp with external clock
> @@ -64,7 +64,7 @@ void *ring_buffer_event_data(struct ring_buffer_event *event);
>   * ring_buffer_event_time_delta - return the delta timestamp of the event
>   * @event: the event to get the delta timestamp of
>   *
> - * The delta timestamp is the 27 bit timestamp since the last event.
> + * The delta timestamp is the 26 bit timestamp since the last event.
>   */
>  static inline unsigned
>  ring_buffer_event_time_delta(struct ring_buffer_event *event)
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index e773c57..33bdde2 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -183,7 +183,7 @@ EXPORT_SYMBOL_GPL(tracing_is_on);
>  
>  #define RB_EVNT_HDR_SIZE (offsetof(struct ring_buffer_event, array))
>  #define RB_ALIGNMENT		4U
> -#define RB_MAX_SMALL_DATA	28
> +#define RB_MAX_SMALL_DATA	60
>  
>  enum {
>  	RB_LEN_TIME_EXTEND = 8,
> @@ -285,7 +285,7 @@ EXPORT_SYMBOL_GPL(ring_buffer_event_data);
>  #define for_each_buffer_cpu(buffer, cpu)		\
>  	for_each_cpu(cpu, buffer->cpumask)
>  
> -#define TS_SHIFT	27
> +#define TS_SHIFT	26
>  #define TS_MASK		((1ULL << TS_SHIFT) - 1)
>  #define TS_DELTA_TEST	(~TS_MASK)
>  
> @@ -330,7 +330,7 @@ static void free_buffer_page(struct buffer_page *bpage)
>  }
>  
>  /*
> - * We need to fit the time_stamp delta into 27 bits.
> + * We need to fit the time_stamp delta into 26 bits.
>   */
>  static inline int test_time_stamp(u64 delta)
>  {
> @@ -1299,7 +1299,7 @@ rb_add_time_stamp(struct ring_buffer_per_cpu *cpu_buffer,
>  	static int once;
>  	int ret;
>  
> -	if (unlikely(*delta > (1ULL << 59) && !once++)) {
> +	if (unlikely(*delta > (1ULL << 58) && !once++)) {
>  		printk(KERN_WARNING "Delta way too big! %llu"
>  		       " ts=%llu write stamp = %llu\n",
>  		       (unsigned long long)*delta,
> 
> 

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