refperf: Tune reader measurement interval

This commit moves a printk() out of the measurement interval, converts
a atomic_dec()/atomic_read() pair to atomic_dec_and_test(), and adds
a smp_mb__before_atomic() to avoid potential wake/wait hangs.  These
changes have the added benefit of reducing the number of loops required
for amortizing loop overhead for CONFIG_PREEMPT=n RCU measurements from
1,000,000 to 10,000.  This reduction in turn shortens the test, reducing
the probability of interference.

Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
index 0a900f3ae151cc7809a4ae4ac55a1f83e085cdb6..8815ccfb6f98595d18f2450b36e2cbe7a31449ab 100644 (file)
--- a/kernel/rcu/refperf.c
+++ b/kernel/rcu/refperf.c
@@ -252,15 +252,16 @@ repeat:
        // Make sure that the CPU is affinitized appropriately during testing.
        WARN_ON_ONCE(smp_processor_id() != me);
 
+       smp_mb__before_atomic();
        atomic_dec(&rt->start);
 
+       VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);
+
        // To prevent noise, keep interrupts disabled. This also has the
        // effect of preventing entries into slow path for rcu_read_unlock().
        local_irq_save(flags);
        start = ktime_get_mono_fast_ns();
 
-       VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d started", me, exp_idx);
-
        cur_ops->readsection(loops);
 
        duration = ktime_get_mono_fast_ns() - start;
@@ -268,14 +269,12 @@ repeat:
 
        rt->last_duration_ns = WARN_ON_ONCE(duration < 0) ? 0 : duration;
 
-       atomic_dec(&nreaders_exp);
+       if (atomic_dec_and_test(&nreaders_exp))
+               wake_up(&main_wq);
 
        VERBOSE_PERFOUT("ref_perf_reader %ld: experiment %d ended, (readers remaining=%d)",
                        me, exp_idx, atomic_read(&nreaders_exp));
 
-       if (!atomic_read(&nreaders_exp))
-               wake_up(&main_wq);
-
        if (!torture_must_stop())
                goto repeat;
 end: