[PATCH RFC tip/core/rcu 2/4] rcu-tasks: Use more aggressive polling for RCU Tasks Trace

[paulmck@kernel.org]

From: "Paul E. McKenney" <paulmck@kernel.org>

The RCU Tasks Trace grace periods are too slow, as in 40x slower than
those of RCU Tasks.  This is due to my having assumed a one-second grace
period was OK, and thus not having optimized any further.  This commit
provides the first step in this optimization process, namely by allowing
the task_list scan backoff interval to be specified on a per-flavor basis,
and then speeding up the scans for RCU Tasks Trace.  However, kernels
built with CONFIG_TASKS_TRACE_RCU_READ_MB=y continue to use the old slower
backoff, consistent with that Kconfig option's goal of reducing IPIs.

Link: https://lore.kernel.org/bpf/CAADnVQK_AiX+S_L_A4CQWT11XyveppBbQSQgH_qWGyzu_E8Yeg@mail.gmail.com/
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 978508e..ad8c4f3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -28,6 +28,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
  * @kthread_ptr: This flavor's grace-period/callback-invocation kthread.
  * @gp_func: This flavor's grace-period-wait function.
  * @gp_state: Grace period's most recent state transition (debugging).
+ * @init_fract: Initial backoff sleep interval.
  * @gp_jiffies: Time of last @gp_state transition.
  * @gp_start: Most recent grace-period start in jiffies.
  * @n_gps: Number of grace periods completed since boot.
@@ -48,6 +49,7 @@ struct rcu_tasks {
 	struct wait_queue_head cbs_wq;
 	raw_spinlock_t cbs_lock;
 	int gp_state;
+	int init_fract;
 	unsigned long gp_jiffies;
 	unsigned long gp_start;
 	unsigned long n_gps;
@@ -329,8 +331,10 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 	 */
 	lastreport = jiffies;
 
-	/* Start off with HZ/10 wait and slowly back off to 1 HZ wait. */
-	fract = 10;
+	// Start off with initial wait and slowly back off to 1 HZ wait.
+	fract = rtp->init_fract;
+	if (fract > HZ)
+		fract = HZ;
 
 	for (;;) {
 		bool firstreport;
@@ -553,6 +557,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks);
 
 static int __init rcu_spawn_tasks_kthread(void)
 {
+	rcu_tasks.init_fract = 10;
 	rcu_tasks.pregp_func = rcu_tasks_pregp_step;
 	rcu_tasks.pertask_func = rcu_tasks_pertask;
 	rcu_tasks.postscan_func = rcu_tasks_postscan;
@@ -1163,6 +1168,13 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace);
 
 static int __init rcu_spawn_tasks_trace_kthread(void)
 {
+	if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) {
+		rcu_tasks_trace.init_fract = 10;
+	} else {
+		rcu_tasks_trace.init_fract = HZ / 5;
+		if (rcu_tasks_trace.init_fract <= 0)
+			rcu_tasks_trace.init_fract = 1;
+	}
 	rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step;
 	rcu_tasks_trace.pertask_func = rcu_tasks_trace_pertask;
 	rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan;
-- 
2.9.5