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4.4. Identifying Contended User-Space Locks

This section describes how to identify contended user-space locks throughout the system within a specific time period. The ability to identify contended user-space locks can help you investigate hangs that you suspect may be caused by futex contentions.
Simply put, a futex contention occurs when multiple processes are trying to access the same region of memory. In some cases, this can result in a deadlock between the processes in contention, thereby appearing as an application hang.
To do this, futexes.stp probes the futex system call.
futexes.stp
#! /usr/bin/env stap

# This script tries to identify contended user-space locks by hooking
# into the futex system call.

global FUTEX_WAIT = 0 /*, FUTEX_WAKE = 1 */

global lock_waits # long-lived stats on (tid,lock) blockage elapsed time
global process_names # long-lived pid-to-execname mapping

probe syscall.futex.return {  
  if ($op != FUTEX_WAIT) next # don't care about WAKE event originator
  process_names[pid()] = execname()
  elapsed = gettimeofday_us() - @entry(gettimeofday_us())
  lock_waits[pid(), $uaddr] <<< elapsed
}

probe end {
  foreach ([pid+, lock] in lock_waits) 
    printf ("%s[%d] lock %p contended %d times, %d avg us\n",
            process_names[pid], pid, lock, @count(lock_waits[pid,lock]),
            @avg(lock_waits[pid,lock]))
}

futexes.stp needs to be manually stopped; upon exit, it prints the following information:
Example 4.19, “futexes.stp Sample Output” contains an excerpt from the output of futexes.stp upon exiting the script (after approximately 20 seconds).
[...]	
automount[2825] lock 0x00bc7784 contended 18 times, 999931 avg us
synergyc[3686] lock 0x0861e96c contended 192 times, 101991 avg us
synergyc[3758] lock 0x08d98744 contended 192 times, 101990 avg us
synergyc[3938] lock 0x0982a8b4 contended 192 times, 101997 avg us
[...]
Example 4.19. futexes.stp Sample Output