Commit b864f89f authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 2990750b

kernel/rcu/refperf.c

+5 −6
Original line number Diff line number Diff line
@@ -252,15 +252,16 @@ ref_perf_reader(void *arg) 
	// 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 @@ ref_perf_reader(void *arg) 


	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: