#!/usr/bin/stap
# This script tries to identify contended user-space locks by hooking
# into the futex system call.
/* Bits indicating sharing in futex_key.both.offset */
global FUT_OFF_INODE = 1 /* We set bit 0 if key has a reference on inode */
global FUT_OFF_MMSHARED = 2 /* We set bit 1 if key has a reference on mm */
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 wait_keys # per-thread key strings for waiting futexes
global lock_waits # long-lived stats on (tid,lock) blockage elapsed time
global process_names # long-lived pid-to-execname mapping
@define KEY %( @cast(key, "union futex_key") %)
probe kernel.function("futex_wait_setup").return {
if ($return != 0) next
# NB: this is the entry value
# Further @KEY dereferences happen at return
key = &$q->key
offset = @KEY->both->offset
masked_offset = offset & ~(FUT_OFF_INODE | FUT_OFF_MMSHARED)
if (offset & FUT_OFF_INODE) {
address = @KEY->shared->pgoff * mem_page_size() + masked_offset
path = inode_path(@KEY->shared->inode)
wait_keys[tid()] = sprintf("INODE:%s+%p", path, address)
} else if (offset & FUT_OFF_MMSHARED) {
address = @KEY->private->address + masked_offset
wait_keys[tid()] = sprintf("MMSHARED:%p", address)
}
}
probe syscall.futex.return {
if (!(tid() in wait_keys)) next
key = wait_keys[tid()]
delete wait_keys[tid()]
cmd = $op & ~(FUTEX_PRIVATE_FLAG|FUTEX_CLOCK_REALTIME);
if (cmd != FUTEX_WAIT) next
process_names[pid()] = execname()
elapsed = gettimeofday_us() - @entry(gettimeofday_us())
lock_waits[pid(), key] <<< elapsed
}
probe end {
foreach ([pid+, lock] in lock_waits)
printf ("%s[%d] lock %s contended %d times, %d avg us\n",
process_names[pid], pid, lock, @count(lock_waits[pid,lock]),
@avg(lock_waits[pid,lock]))
}
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