linux-kernel - [PATCH rcu 25/32] rcu-tasks: Stop RCU Tasks Trace from scanning idle tasks

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Date:   Mon, 20 Jun 2022 15:54:04 -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 25/32] rcu-tasks: Stop RCU Tasks Trace from scanning idle tasks

Now that RCU scans both running tasks and tasks that have blocked within
their current RCU Tasks Trace read-side critical section, there is no
need for it to scan the idle tasks.  After all, an idle loop should not
be remain within an RCU Tasks Trace read-side critical section across
exit from idle, and from a BPF viewpoint, functions invoked from the
idle loop should not sleep.  So only running idle tasks can be within
RCU Tasks Trace read-side critical sections.

This commit therefore removes the scan of the idle tasks from the
rcu_tasks_trace_postscan() function.

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 | 8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index d318cdfd2309c..272c905995e56 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1533,16 +1533,10 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
 }
 
 /*
- * Do intermediate processing between task and holdout scans and
- * pick up the idle tasks.
+ * Do intermediate processing between task and holdout scans.
  */
 static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
-	int cpu;
-
-	for_each_online_cpu(cpu)
-		rcu_tasks_trace_pertask(idle_task(cpu), hop);
-
 	// Re-enable CPU hotplug now that the tasklist scan has completed.
 	cpus_read_unlock();
 
-- 
2.31.1.189.g2e36527f23