• Byungchul Park's avatar
    tracing: Add additional marks to signal very large time deltas · 8e1e1df2
    Byungchul Park authored
    Currently, function graph tracer prints "!" or "+" just before
    function execution time to signal a function overhead, depending
    on the time. And some tracers tracing latency also print "!" or
    "+" just after time to signal overhead, depending on the interval
    between events. Even it is usually enough to do that, we sometimes
    need to signal 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 large
    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.
    
    Link: http://lkml.kernel.org/r/1416789259-24038-1-git-send-email-byungchul.park@lge.comSigned-off-by: default avatarByungchul Park <byungchul.park@lge.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    8e1e1df2
ftrace.txt 105 KB