linux-kernel - [PATCH v2 1/2] rcu: Delete a redundant check in rcu_check_gp_kthread

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Date:   Wed, 12 Jul 2023 23:15:56 +0800
From:   Zhen Lei <thunder.leizhen@...wei.com>
To:     "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Zqiang <qiang.zhang1211@...il.com>, <rcu@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
CC:     Zhen Lei <thunder.leizhen@...wei.com>
Subject: [PATCH v2 1/2] rcu: Delete a redundant check in rcu_check_gp_kthread_starvation()

The kernel provides the only legal way, that is, function set_task_cpu(),
to dynamically change the value of task_cpu(p). And in which the validity
of the CPU ID is checked.
set_task_cpu():
	WARN_ON_ONCE(!cpu_online(new_cpu));
	__set_task_cpu(p, new_cpu);

Therefore, in normal cases, the value of task_cpu(gpk) is always valid.
Then for the following snippet in rcu_check_gp_kthread_starvation():
	cpu = gpk ? task_cpu(gpk) : -1;
	if (gpk) {
		if (cpu >= 0) {

The above condition "if (gpk)" already ensures that gp_kthread is created,
so the local variable 'cpu' cannot be negative here.

Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
---
 kernel/rcu/tree_stall.h | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index b10b8349bb2a48b..dcfaa3d5db2cbc7 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -537,13 +537,11 @@ static void rcu_check_gp_kthread_starvation(void)
 			pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
 			pr_err("RCU grace-period kthread stack dump:\n");
 			sched_show_task(gpk);
-			if (cpu >= 0) {
-				if (cpu_is_offline(cpu)) {
-					pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
-				} else  {
-					pr_err("Stack dump where RCU GP kthread last ran:\n");
-					dump_cpu_task(cpu);
-				}
+			if (cpu_is_offline(cpu)) {
+				pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
+			} else  {
+				pr_err("Stack dump where RCU GP kthread last ran:\n");
+				dump_cpu_task(cpu);
 			}
 			wake_up_process(gpk);
 		}
-- 
2.25.1