linux-kernel - [PATCH 1/6] rcu: Delete a redundant check in rcu_check_gp_kthread

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Date:   Thu, 19 Oct 2023 14:01:57 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH 1/6] rcu: Delete a redundant check in rcu_check_gp_kthread_starvation()

From: Zhen Lei <thunder.leizhen@...wei.com>

The rcu_check_gp_kthread_starvation() function uses task_cpu() to sample
the last CPU that the grace-period kthread ran on, and task_cpu() samples
the thread_info structure's ->cpu field.  But this field will always
contain a number corresponding to a CPU that was online some time in
the past, thus never a negative number.  This invariant is checked by
a WARN_ON_ONCE() in set_task_cpu().

This means that if the grace-period kthread exists, that is, if the "gpk"
local variable is non-NULL, the "cpu" local variable will be non-negative.
This in turn means that the existing check for non-negative "cpu" is
redundant with the enclosing check for non-NULL "gpk".

This commit threefore removes the redundant check of "cpu".

Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 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 6f06dc12904a..b5ce0580074e 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);
 		}
-- 
2.34.1