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4.3. Profiling

The following sections showcase scripts that profile kernel activity by monitoring function calls.

4.3.1. Counting Function Calls Made

This section describes how to identify how many times the system called a specific kernel function in a 30-second sample. Depending on your use of wildcards, you can also use this script to target multiple kernel functions.

#! /usr/bin/env stap
# The following line command will probe all the functions
# in kernel's memory management code:
# stap  functioncallcount.stp "*@mm/*.c"

probe kernel.function(@1).call {  # probe functions listed on commandline
  called[probefunc()] <<< 1  # add a count efficiently

global called

probe end {
  foreach (fn in called-)  # Sort by call count (in decreasing order)
  #       (fn+ in called)  # Sort by function name
    printf("%s %d\n", fn, @count(called[fn]))

functioncallcount.stp takes the targeted kernel function as an argument. The argument supports wildcards, which enables you to target multiple kernel functions up to a certain extent.
The output of functioncallcount.stp contains the name of the function called and how many times it was called during the sample time (in alphabetical order). Example 4.11, “functioncallcount.stp Sample Output” contains an excerpt from the output of stap functioncallcount.stp "*@mm/*.c":

Example 4.11. functioncallcount.stp Sample Output

__vma_link 97
__vma_link_file 66
__vma_link_list 97
__vma_link_rb 97
__xchg 103
add_page_to_active_list 102
add_page_to_inactive_list 19
add_to_page_cache 19
add_to_page_cache_lru 7
all_vm_events 6
alloc_pages_node 4630
alloc_slabmgmt 67
anon_vma_alloc 62
anon_vma_free 62
anon_vma_lock 66
anon_vma_prepare 98
anon_vma_unlink 97
anon_vma_unlock 66
arch_get_unmapped_area_topdown 94
arch_get_unmapped_exec_area 3
arch_unmap_area_topdown 97
atomic_add 2
atomic_add_negative 97
atomic_dec_and_test 5153
atomic_inc 470
atomic_inc_and_test 1