Re: [PATCH v3 2/3] tracing, function_graph: add additional marks to signal very large function execution time

[Namhyung Kim <namhyung@kernel.org>]

Hi Byungchul,

On Mon, 17 Nov 2014 09:41:57 +0900, byungchul park wrote:
> From: Byungchul Park <byungchul.park@lge.com>
>
> Currently, function graph tracer prints "!" or "+" just before
> function execution time to signal a function overhead, depending
> on the time. Even it is usually enough to do that, we sometimes
> need to be signaled for bigger execution time than 100 micro seconds.
>
> For example, I used function graph tracer to detect if there is
> any case that exit_mm() takes too much time. I did following steps
> in /sys/kernel/debug/tracing. It was easier to detect very big
> excution time with patched kernel than with original kernel.
>
> $ echo exit_mm > set_graph_function
> $ echo function_graph > current_tracer
> $ echo > trace
> $ cat trace_pipe > $LOGFILE
>  ... (do something and terminate logging)
> $ grep "\\$" $LOGFILE
>  3) $ 22082032 us |                      } /* kernel_map_pages */
>  3) $ 22082040 us |                    } /* free_pages_prepare */
>  3) $ 22082113 us |                  } /* free_hot_cold_page */
>  3) $ 22083455 us |                } /* free_hot_cold_page_list */
>  3) $ 22083895 us |              } /* release_pages */
>  3) $ 22177873 us |            } /* free_pages_and_swap_cache */
>  3) $ 22178929 us |          } /* unmap_single_vma */
>  3) $ 22198885 us |        } /* unmap_vmas */
>  3) $ 22206949 us |      } /* exit_mmap */
>  3) $ 22207659 us |    } /* mmput */
>  3) $ 22207793 us |  } /* exit_mm */
>
> And then, it was easy to find out that a schedule-out occured by
> sub_preempt_count() within kernel_map_pages().
>
> To detect very large function exection time caused by either problematic
> function implementation or scheduling issues, this patch can be useful.
>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> ---
>  Documentation/trace/ftrace.txt       |    2 ++
>  kernel/trace/trace.h                 |   19 +++++++++++++++++++
>  kernel/trace/trace_functions_graph.c |   20 ++++++++++++++------
>  3 files changed, 35 insertions(+), 6 deletions(-)
>
> diff --git a/Documentation/trace/ftrace.txt b/Documentation/trace/ftrace.txt
> index 4da4261..f827e2f 100644
> --- a/Documentation/trace/ftrace.txt
> +++ b/Documentation/trace/ftrace.txt
> @@ -1951,6 +1951,8 @@ want, depending on your needs.
>  
>    + means that the function exceeded 10 usecs.
>    ! means that the function exceeded 100 usecs.
> +  # means that the function exceeded 1000 usecs.
> +  $ means that the function exceeded 1 sec.
>  
>  
>  - The task/pid field displays the thread cmdline and pid which
> diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
> index 385391f..d89868a 100644
> --- a/kernel/trace/trace.h
> +++ b/kernel/trace/trace.h
> @@ -331,6 +331,25 @@ struct tracer_flags {
>  /* Makes more easy to define a tracer opt */
>  #define TRACER_OPT(s, b)	.name = #s, .bit = b
>  
> +/* trace overhead mark */
> +struct trace_mark {
> +	unsigned long long	val; /* unit: nsec */
> +	char			sym;
> +};
> +
> +#define MARK(v, s) {.val = v, .sym = s}
> +
> +static inline char trace_duration_mark(unsigned long long d,
> +				       const struct trace_mark m[],
> +				       int size)
> +{
> +	int idx = size;
> +
> +	if (size <= 0) return ' ';
> +	while (d <= m[--idx].val && idx > 0);
> +
> +	return m[idx].sym;
> +}

I think it'd be simpler if you arrange the mark array in an opposity
order:

static const struct trace_mark mark[] = {
	MARK(1000000000ULL	, '$'), /* 1 sec */
	MARK(1000000ULL		, '#'), /* 1000 usecs */
	MARK(100000ULL		, '!'), /* 100 usecs */
	MARK(10000ULL		, '+'), /* 10 usecs */
	MARK(0ULL		, ' '), /* 0 usecs */
};

static inline char trace_duration_mark(unsigned long long d,
				       const struct trace_mark m[],
				       int size)
{
	int i;

	for (i = 0; i < size; i++) {
		if (d >= m[i].val)
			break;
	}

	return m[i].sym;
}

Thanks,
Namhyung


>  
>  /**
>   * struct tracer - a specific tracer and its callbacks to interact with debugfs
> diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
> index cb33f46..d9ac09f 100644
> --- a/kernel/trace/trace_functions_graph.c
> +++ b/kernel/trace/trace_functions_graph.c
> @@ -797,6 +797,19 @@ trace_print_graph_duration(unsigned long long duration, struct trace_seq *s)
>  	return TRACE_TYPE_HANDLED;
>  }
>  
> +static const struct trace_mark mark[] = {
> +	MARK(0ULL		, ' '), /* 0 usecs */
> +	MARK(10000ULL		, '+'), /* 10 usecs */
> +	MARK(100000ULL		, '!'), /* 100 usecs */
> +	MARK(1000000ULL		, '#'), /* 1000 usecs */
> +	MARK(1000000000ULL	, '$'), /* 1 sec */
> +};
> +
> +static inline char find_trace_mark(unsigned long long d)
> +{
> +	return trace_duration_mark(d, mark, ARRAY_SIZE(mark));
> +}
> +
>  static enum print_line_t
>  print_graph_duration(unsigned long long duration, struct trace_seq *s,
>  		     u32 flags)
> @@ -822,12 +835,7 @@ print_graph_duration(unsigned long long duration, struct trace_seq *s,
>  
>  	/* Signal a overhead of time execution to the output */
>  	if (flags & TRACE_GRAPH_PRINT_OVERHEAD) {
> -		/* Duration exceeded 100 usecs */
> -		if (duration > 100000ULL)
> -			ret = trace_seq_puts(s, "! ");
> -		/* Duration exceeded 10 usecs */
> -		else if (duration > 10000ULL)
> -			ret = trace_seq_puts(s, "+ ");
> +		ret = trace_seq_printf(s, "%c ", find_trace_mark(duration));
>  	}
>  
>  	/*