Re: [PATCH v4 1/3] tracing: Add array printing helpers

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 26 Jan 2015 12:11:49 +0000
Javi Merino <javi.merino@arm.com> wrote:

> From: Dave Martin <Dave.Martin@arm.com>
> 
> If a trace event contains an array, there is currently no standard
> way to format this for text output.  Drivers are currently hacking
> around this by a) local hacks that use the trace_seq functionailty
> directly, or b) just not printing that information.  For fixed size
> arrays, formatting of the elements can be open-coded, but this gets
> cumbersome for arrays of non-trivial size.
> 
> These approaches result in non-standard content of the event format
> description delivered to userspace, so userland tools needs to be
> taught to understand and parse each array printing method
> individually.
> 
> This patch implements common __print_<type>_array() helpers that
> tracepoint implementations can use instead of reinventing them.  A
> simple C-style syntax is used to delimit the array and its elements
> {like,this}.
> 
> So that the helpers can be used with large static arrays as well as
> dynamic arrays, they take a pointer and element count: they can be
> used with __get_dynamic_array() for use with dynamic arrays.
> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> Signed-off-by: Javi Merino <javi.merino@arm.com>
> ---
>  include/linux/ftrace_event.h |  4 ++++
>  include/trace/ftrace.h       |  9 +++++++++
>  kernel/trace/trace_output.c  | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 57 insertions(+)
> 
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index 0bebb5c348b8..5aa4a9269547 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -44,6 +44,10 @@ const char *ftrace_print_bitmask_seq(struct trace_seq *p, void *bitmask_ptr,
>  const char *ftrace_print_hex_seq(struct trace_seq *p,
>  				 const unsigned char *buf, int len);
>  
> +const char *ftrace_print_array_seq(struct trace_seq *p,
> +				   const void *buf, int buf_len,
> +				   size_t el_size);
> +
>  struct trace_iterator;
>  struct trace_event;
>  
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 139b5067345b..36afd0ed3458 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -263,6 +263,14 @@
>  #undef __print_hex
>  #define __print_hex(buf, buf_len) ftrace_print_hex_seq(p, buf, buf_len)
>  
> +#undef __print_array
> +#define __print_array(array, count, el_size)				\
> +	({								\
> +		BUILD_BUG_ON(el_size != 8 && el_size != 16 &&		\
> +			el_size != 32 && el_size != 64);		\

I tried testing this patch by writing a print_array myself, and I kept
hitting this BUILD_BUG_ON, and was wondering WTF? Then it dawned on me.

el_size should not be based on bits, it should be based on bytes. I
passed in "sizeof()" which doesn't work with bits.

Please update to "el_size < 1 || el_size > 8".

and adjust the rest accordingly.

Thanks,

-- Steve


> +		ftrace_print_array_seq(p, array, count, el_size);	\
> +	})
> +
>  #undef DECLARE_EVENT_CLASS
>  #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print)	\
>  static notrace enum print_line_t					\
> @@ -674,6 +682,7 @@ static inline void ftrace_test_probe_##call(void)			\
>  #undef __get_dynamic_array_len
>  #undef __get_str
>  #undef __get_bitmask
> +#undef __print_array
>  
>  #undef TP_printk
>  #define TP_printk(fmt, args...) "\"" fmt "\", "  __stringify(args)
> diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
> index b77b9a697619..c06bd6e4ae0e 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -177,6 +177,50 @@ ftrace_print_hex_seq(struct trace_seq *p, const unsigned char *buf, int buf_len)
>  }
>  EXPORT_SYMBOL(ftrace_print_hex_seq);
>  
> +const char *
> +ftrace_print_array_seq(struct trace_seq *p, const void *buf, int buf_len,
> +		       size_t el_size)
> +{
> +	const char *ret = trace_seq_buffer_ptr(p);
> +	const char *prefix = "";
> +	void *ptr = (void *)buf;
> +
> +	trace_seq_putc(p, '{');
> +
> +	while (ptr < buf + buf_len) {
> +		switch (el_size) {
> +		case 8:
> +			trace_seq_printf(p, "%s0x%x", prefix,
> +					 *(u8 *)ptr);
> +			break;
> +		case 16:
> +			trace_seq_printf(p, "%s0x%x", prefix,
> +					 *(u16 *)ptr);
> +			break;
> +		case 32:
> +			trace_seq_printf(p, "%s0x%x", prefix,
> +					 *(u32 *)ptr);
> +			break;
> +		case 64:
> +			trace_seq_printf(p, "%s0x%llx", prefix,
> +					 *(u64 *)ptr);
> +			break;
> +		default:
> +			trace_seq_printf(p, "BAD SIZE:%zu 0x%x", el_size,
> +					 *(u8 *)ptr);
> +			el_size = 8;
> +		}
> +		prefix = ",";
> +		ptr += el_size / 8;
> +	}
> +
> +	trace_seq_putc(p, '}');
> +	trace_seq_putc(p, 0);
> +
> +	return ret;
> +}
> +EXPORT_SYMBOL(ftrace_print_array_seq);
> +
>  int ftrace_raw_output_prep(struct trace_iterator *iter,
>  			   struct trace_event *trace_event)
>  {