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refperf: Tune reader measurement interval

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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>
This commit is contained in:
Paul E. McKenney 2020-05-26 10:57:34 -07:00
parent 2990750bce
commit b864f89ff6
1 changed files with 5 additions and 6 deletions
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11
kernel/rcu/refperf.c
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@ -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: