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

[Waiman Long <waiman.long@xxxxxx>]

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


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel