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

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Date:   Mon, 20 Jun 2022 15:54:05 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com,
        rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        KP Singh <kpsingh@...nel.org>
Subject: [PATCH rcu 26/32] rcu-tasks: Stop RCU Tasks Trace from scanning full tasks list

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@...nel.org>
Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Alexei Starovoitov <ast@...nel.org>
Cc: Andrii Nakryiko <andrii@...nel.org>
Cc: Martin KaFai Lau <kafai@...com>
Cc: KP Singh <kpsingh@...nel.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