Spinlock thrashing with libthr

I ran a modified version of the thr1 stress test on a quad amd64,
which spawns 256 threads (using libthr), each of which loops doing
getpid() 10000 times.  The system spends essentially 100% of the time
in the kernel.  Profiling with hwpmc shows that the system is spending
about 60% time in _mtx_lock_spin().

I made an extremely stupid set of changes to call different functions
based on certain different spin lock types (so that the spinning mutex
is visible to profiling), and discovered the following:

  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 42.0  200076.00 200076.00        0  100.00%           _turnstile_lock_spin [1]
 20.9  299496.00 99420.00        0  100.00%           sched_lock_spin [2]
  6.2  328880.00 29384.00        0  100.00%           syscall [3]
  5.3  354262.00 25382.00        0  100.00%           Xfast_syscall [4]
  4.4  375267.00 21005.00        0  100.00%           bcopy [5]
  3.1  389831.00 14564.00        0  100.00%           getpid [6]
  2.8  403386.00 13555.00        0  100.00%           spinlock_enter [7]
...

_turnstile_lock_spin comes via the following in subr_turnstile.c:

        turnstile_lock_spin(&tc->tc_lock);

TURNSTILE_PROFILING shows debug.turnstile.max_depth: 1
so it's not overloading chains.

Even with only 4 threads (on 4 CPUs) it still thrashes:

  %   cumulative   self              self     total
 time   seconds   seconds    calls  ms/call  ms/call  name
 30.7  133288.00 133288.00        0  100.00%           _turnstile_lock_spin [1]
 27.0  250336.00 117048.00        0  100.00%           sched_lock_spin [2]

But running the same test with libpthread shows nearly 0 spin lock
contention (and system time is correspondingly much lower):

        61.16 real         2.57 user        60.83 sys <-- libpthread
        60.27 real         7.57 user       212.28 sys <-- libthr

# gprof -l /boot/kernel/kernel kernel.gmon | grep spin
time is in ticks, not seconds
  0.5   20105.00   100.00        0  100.00%           spinlock_exit [17]
  0.2   20622.00    37.00        0  100.00%           spinlock_enter [26]
  0.2   20692.00    35.00        0  100.00%           sched_lock_spin [28]

What is going on?

Kris