linux-kernel - [PATCH v3 2/3] sched/cpuacct: optimize away RCU read lock

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Date:   Sun, 20 Feb 2022 13:14:25 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        bristot@...hat.com, zhaolei@...fujitsu.com, tj@...nel.org,
        lizefan.x@...edance.com, hannes@...xchg.org
Cc:     linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH v3 2/3] 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>
---
 include/linux/cgroup.h | 2 --
 kernel/sched/cpuacct.c | 4 +---
 2 files changed, 1 insertion(+), 5 deletions(-)

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();
 }
 
 /*
-- 
2.20.1