[PATCH rcu 26/32] rcu-tasks: Stop RCU Tasks Trace from scanning full tasks list

["Paul E. McKenney" <paulmck@kernel.org>]

This commit takes off the training wheels and relies only on scanning
currently running tasks and tasks that have blocked or been preempted
within their current RCU Tasks Trace read-side critical section.

Before this commit, the time complexity of an RCU Tasks Trace grace
period is O(T), where T is the number of tasks.  After this commit,
this time complexity is O(C+B), where C is the number of CPUs and B
is the number of tasks that have blocked (or been preempted) at least
once during their current RCU Tasks Trace read-side critical sections.
Of course, if all tasks have blocked (or been preempted) at least once
during their current RCU Tasks Trace read-side critical sections, this is
still O(T), but current expectations are that RCU Tasks Trace read-side
critical section will be short and that there will normally not be large
numbers of tasks blocked within such a critical section.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
---
 kernel/rcu/tasks.h | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 272c905995e56..fe0552086ccfc 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -670,10 +670,12 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 	 * and make a list of them in holdouts.
 	 */
 	set_tasks_gp_state(rtp, RTGS_SCAN_TASKLIST);
-	rcu_read_lock();
-	for_each_process_thread(g, t)
-		rtp->pertask_func(t, &holdouts);
-	rcu_read_unlock();
+	if (rtp->pertask_func) {
+		rcu_read_lock();
+		for_each_process_thread(g, t)
+			rtp->pertask_func(t, &holdouts);
+		rcu_read_unlock();
+	}
 
 	set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
 	rtp->postscan_func(&holdouts);
@@ -1746,7 +1748,6 @@ static int __init rcu_spawn_tasks_trace_kthread(void)
 			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;
 	rcu_tasks_trace.holdouts_func = check_all_holdout_tasks_trace;
 	rcu_tasks_trace.postgp_func = rcu_tasks_trace_postgp;
-- 
2.31.1.189.g2e36527f23