• Steven Rostedt (VMware)'s avatar
    ftrace: Allow enabling of filters via index of available_filter_functions · f79b3f33
    Steven Rostedt (VMware) authored
    Enabling of large number of functions by echoing in a large subset of the
    functions in available_filter_functions can take a very long time. The
    process requires testing all functions registered by the function tracer
    (which is in the 10s of thousands), and doing a kallsyms lookup to convert
    the ip address into a name, then comparing that name with the string passed
    in.
    
    When a function causes the function tracer to crash the system, a binary
    bisect of the available_filter_functions can be done to find the culprit.
    But this requires passing in half of the functions in
    available_filter_functions over and over again, which makes it basically a
    O(n^2) operation. With 40,000 functions, that ends up bing 1,600,000,000
    opertions! And enabling this can take over 20 minutes.
    
    As a quick speed up, if a number is passed into one of the filter files,
    instead of doing a search, it just enables the function at the corresponding
    line of the available_filter_functions file. That is:
    
     # echo 50 > set_ftrace_filter
     # cat set_ftrace_filter
     x86_pmu_commit_txn
    
     # head -50 available_filter_functions | tail -1
     x86_pmu_commit_txn
    
    This allows setting of half the available_filter_functions to take place in
    less than a second!
    
     # time seq 20000 > set_ftrace_filter
     real    0m0.042s
     user    0m0.005s
     sys     0m0.015s
    
     # wc -l set_ftrace_filter
     20000 set_ftrace_filter
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    f79b3f33
ftrace.rst 126 KB