[PATCH tip/core/rcu 13/30] refperf: Tune reader measurement interval

paulmck
2020-06-23T00:30:43Z

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>
---
kernel/rcu/refperf.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
index 0a900f3..8815ccf 100644
--- a/kernel/rcu/refperf.c
+++ b/kernel/rcu/refperf.c
@@ -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:
--
2.9.5