Re: [PATCH v5 3/8] qspinlock, x86: Add x86 specific optimization for 2 contending tasks

[Waiman Long <waiman.long@hp.com>]

Peter,

I was trying to implement the generic queue code exchange code using
cmpxchg as suggested by you. However, when I gathered the performance
data, the code performed worse than I expected at a higher contention
level. Below were the execution time of the benchmark tool that I sent
you:

                 [xchg]        [cmpxchg]
   # of tasks    Ticket lock     Queue lock      Queue Lock
   ----------    -----------     -----------     ----------
        1          135            135              135
        2          732           1315            1102
        3         1827           2372            2681
        4         2689           2934             5392
        5         3736           3658             7696
        6         4942           4434            9876
        7         6304           5176           11901
        8         7736           5955           14551

Below is the code that I used:

static inline u32 queue_code_xchg(struct qspinlock *lock, u32 *ocode, 
u32 ncode)
{
         while (true) {
                 u32 qlcode = atomic_read(&lock->qlcode);

                 if (qlcode == 0) {
                         /*
                          * Try to get the lock
                          */
                         if (atomic_cmpxchg(&lock->qlcode, 0,
                                            _QSPINLOCK_LOCKED) == 0)
                                 return 1;
                 } else if (qlcode & _QSPINLOCK_LOCKED) {
                         *ocode = atomic_cmpxchg(&lock->qlcode, qlcode,
                                                 ncode | _QSPINLOCK_LOCKED);
                         if (*ocode == qlcode) {
                                 /* Clear lock bit before return */
                                 *ocode &= ~_QSPINLOCK_LOCKED;
                                 return 0;
                         }
                 }
                 /*
                  * Wait if atomic_cmpxchg() fails or lock is 
temporarily free.
                  */
                 arch_mutex_cpu_relax();
         }
}

My cmpxchg code is not optimal, and I can probably tune the code to
make it perform better. Given the trend that I was seeing, however,
I think I will keep the current xchg code, but I will package it in
an inline function.

-Longman