futexes.stp

#!/usr/bin/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 FUTEX_PRIVATE_FLAG = 128 /* linux 2.6.22+ */
global FUTEX_CLOCK_REALTIME = 256 /* linux 2.6.29+ */

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_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME)) != FUTEX_WAIT) next
  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]))
}