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Filename : futexes.stp
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#!/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

global entry_times%, uaddrs%

probe syscall.futex {
  if ((op & ~(FUTEX_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME)) != FUTEX_WAIT) next
  entry_times[tid()] = gettimeofday_us()
  uaddrs[tid()] = futex_uaddr
}

probe syscall.futex.return {
  if (!(entry_times[tid()])) next
  elapsed = gettimeofday_us() - entry_times[tid()]
  lock_waits[pid(), uaddrs[tid()]] <<< elapsed
  delete entry_times[tid()]
  delete uaddrs[tid()]
  if (!(pid() in process_names))
    process_names[pid()] = execname()
}

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]))
}