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SubjectRe: [RFC PATCH v5 01/10] tracing: Add array printing helpers
On Thu, 10 Jul 2014 15:18:39 +0100
"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.

Note, please base any patches like this on my for-next branch, because
I have moved the trace_seq code out of trace_print.c. Although, I need
to analyze this a little bit more to see if these still belong in
trace_print.c or if they should go into trace_seq.c.

My repo is at:

git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace.git



>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ingo Molnar <mingo@redhat.com>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
> include/linux/ftrace_event.h | 9 ++++++++
> include/trace/ftrace.h | 17 ++++++++++++++
> kernel/trace/trace_output.c | 55 ++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 81 insertions(+)
>
> diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
> index cff3106ffe2c..919f21a3420b 100644
> --- a/include/linux/ftrace_event.h
> +++ b/include/linux/ftrace_event.h
> @@ -44,6 +44,15 @@ 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_u8_array_seq(struct trace_seq *p,
> + const u8 *buf, int count);
> +const char *ftrace_print_u16_array_seq(struct trace_seq *p,
> + const u16 *buf, int count);
> +const char *ftrace_print_u32_array_seq(struct trace_seq *p,
> + const u32 *buf, int count);
> +const char *ftrace_print_u64_array_seq(struct trace_seq *p,
> + const u64 *buf, int count);
> +
> struct trace_iterator;
> struct trace_event;
>
> diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
> index 26b4f2e13275..15bc5d417aea 100644
> --- a/include/trace/ftrace.h
> +++ b/include/trace/ftrace.h
> @@ -263,6 +263,19 @@
> #undef __print_hex
> #define __print_hex(buf, buf_len) ftrace_print_hex_seq(p, buf, buf_len)
>
> +#undef __print_u8_array
> +#define __print_u8_array(array, count) \
> + ftrace_print_u8_array_seq(p, array, count)
> +#undef __print_u16_array
> +#define __print_u16_array(array, count) \
> + ftrace_print_u16_array_seq(p, array, count)
> +#undef __print_u32_array
> +#define __print_u32_array(array, count) \
> + ftrace_print_u32_array_seq(p, array, count)
> +#undef __print_u64_array
> +#define __print_u64_array(array, count) \
> + ftrace_print_u64_array_seq(p, array, count)

print_hex_seq() is still in trace_output.c, so perhaps it is fine to
keep these there too.

> +
> #undef DECLARE_EVENT_CLASS
> #define DECLARE_EVENT_CLASS(call, proto, args, tstruct, assign, print) \
> static notrace enum print_line_t \
> @@ -676,6 +689,10 @@ static inline void ftrace_test_probe_##call(void) \
> #undef __get_dynamic_array_len
> #undef __get_str
> #undef __get_bitmask
> +#undef __print_u8_array
> +#undef __print_u16_array
> +#undef __print_u32_array
> +#undef __print_u64_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 f3dad80c20b2..b46238e75523 100644
> --- a/kernel/trace/trace_output.c
> +++ b/kernel/trace/trace_output.c
> @@ -454,6 +454,61 @@ ftrace_print_hex_seq(struct trace_seq *p, const unsigned char *buf, int buf_len)
> }
> EXPORT_SYMBOL(ftrace_print_hex_seq);
>
> +static const char *
> +ftrace_print_array_seq(struct trace_seq *p, const void *buf, int buf_len,
> + bool (*iterator)(struct trace_seq *p, const char *prefix,
> + const void **buf, int *buf_len))
> +{
> + const char *ret = p->buffer + p->len;

There is now a helper in my for-next branch that removes the above open
coded code:

const char *ret = trace_seq_buffer_ptr(p);

> + const char *prefix = "";
> +
> + trace_seq_putc(p, '{');
> +
> + if (iterator(p, prefix, &buf, &buf_len)) {
> + prefix = ",";
> +
> + while (iterator(p, prefix, &buf, &buf_len))

The above doesn't even need the prefix variable. But I would keep it
and remove the if and have this:

while (iterator(p, prefix, &buf, &buf_len))
prefix = ",";

and hope gcc just optimizes it. This is not a critical path, don't
micro optimize it and make the code more complex than need be,
especially when gcc should be smart enough to optimize it for you.

> + ;
> + }
> +
> + trace_seq_putc(p, '}');
> + trace_seq_putc(p, 0);

trace_seq is only one page in size, which should be enough. I'm
wondering if we should check the return status of all the trace_seq()
calls and return an error.

> +
> + return ret;
> +}
> +
> +#define DEFINE_PRINT_ARRAY(type, printk_type, format) \
> +static bool \
> +ftrace_print_array_iterator_##type(struct trace_seq *p, const char *prefix, \
> + const void **buf, int *buf_len) \
> +{ \
> + const type *__src = *buf; \
> + \
> + if (*buf_len < sizeof(*__src)) \
> + return false; \
> + \
> + trace_seq_printf(p, "%s" format, prefix, (printk_type)*__src++); \
> + \
> + *buf = __src; \
> + *buf_len -= sizeof(*__src); \
> + \
> + return true; \
> +} \
> + \
> +const char *ftrace_print_##type##_array_seq( \
> + struct trace_seq *p, const type *buf, int count) \
> +{ \
> + return ftrace_print_array_seq(p, buf, (count) * sizeof(type), \
> + ftrace_print_array_iterator_##type); \
> +} \
> + \
> +EXPORT_SYMBOL(ftrace_print_##type##_array_seq);
> +
> +DEFINE_PRINT_ARRAY(u8, unsigned int, "0x%x")

Why not "unsigned char"?

> +DEFINE_PRINT_ARRAY(u16, unsigned int, "0x%x")

Why not "unsigned short"?

> +DEFINE_PRINT_ARRAY(u32, unsigned int, "0x%x")
> +DEFINE_PRINT_ARRAY(u64, unsigned long long, "0x%llx")
> +
> int ftrace_raw_output_prep(struct trace_iterator *iter,
> struct trace_event *trace_event)
> {

-- Steve


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