linux-kernel - [PATCH rcu 16/32] rcu-tasks: Make RCU Tasks Trace stall warning handle idle offline tasks

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Date:   Mon, 20 Jun 2022 15:53:55 -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>,
        Peter Zijlstra <peterz@...radead.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 16/32] rcu-tasks: Make RCU Tasks Trace stall warning handle idle offline tasks

When a CPU is offline, its idle task can appear to be running, but it
cannot be doing anything while CPU-hotplug operations are excluded.
This commit takes advantage of that fact by making trc_check_slow_task()
check for task_curr(t) && cpu_online(task_cpu(t)), and recording
full information in that case.

Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.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 | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 5eefbab7f2edb..64eb4d7b142e3 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1476,7 +1476,7 @@ static int trc_check_slow_task(struct task_struct *t, void *arg)
 {
 	struct trc_stall_chk_rdr *trc_rdrp = arg;
 
-	if (task_curr(t))
+	if (task_curr(t) && cpu_online(task_cpu(t)))
 		return false; // It is running, so decline to inspect it.
 	trc_rdrp->nesting = READ_ONCE(t->trc_reader_nesting);
 	trc_rdrp->ipi_to_cpu = READ_ONCE(t->trc_ipi_to_cpu);
-- 
2.31.1.189.g2e36527f23