linux-kernel - failure to boot after dc6e0818bc9a "sched/cpuacct: Optimize away RCU read lock"

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Date:   Wed, 16 Mar 2022 13:43:24 -0400
From:   bfields@...ldses.org (J. Bruce Fields)
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: failure to boot after dc6e0818bc9a "sched/cpuacct: Optimize away RCU
 read lock"

One of my test VMs has been failing to boot linux-next recently.  I
finally got around to a bisect this morning, and it landed on the below.

What other information would be useful to debug this?

--b.

commit dc6e0818bc9a
Author: Chengming Zhou <zhouchengming@...edance.com>
Date:   Sun Feb 20 13:14:25 2022 +0800

    sched/cpuacct: Optimize away RCU read lock
    
    Since cpuacct_charge() is called from the scheduler update_curr(),
    we must already have rq lock held, then the RCU read lock can
    be optimized away.
    
    And do the same thing in it's wrapper cgroup_account_cputime(),
    but we can't use lockdep_assert_rq_held() there, which defined
    in kernel/sched/sched.h.
    
    Suggested-by: Peter Zijlstra (Intel) <peterz@...radead.org>
    Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
    Link: https://lore.kernel.org/r/20220220051426.5274-2-zhouchengming@bytedance.com

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 75c151413fda..9a109c6ac0e0 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -791,11 +791,9 @@ static inline void cgroup_account_cputime(struct task_struct *task,
 
 	cpuacct_charge(task, delta_exec);
 
-	rcu_read_lock();
 	cgrp = task_dfl_cgroup(task);
 	if (cgroup_parent(cgrp))
 		__cgroup_account_cputime(cgrp, delta_exec);
-	rcu_read_unlock();
 }
 
 static inline void cgroup_account_cputime_field(struct task_struct *task,
diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
index 307800586ac8..f79f88456d72 100644
--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -337,12 +337,10 @@ void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 	unsigned int cpu = task_cpu(tsk);
 	struct cpuacct *ca;
 
-	rcu_read_lock();
+	lockdep_assert_rq_held(cpu_rq(cpu));
 
 	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))
 		*per_cpu_ptr(ca->cpuusage, cpu) += cputime;
-
-	rcu_read_unlock();
 }
 
 /*