linux-kernel - Re: [PATCH] sched/cpuacct: fix charge percpu cpuusage

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Message-ID: <YgoqPT67g2NcV/eH@hirez.programming.kicks-ass.net>
Date:   Mon, 14 Feb 2022 11:09:01 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     tj@...nel.org, arbn@...dex-team.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, Minye Zhu <zhuminye@...edance.com>
Subject: Re: [PATCH] sched/cpuacct: fix charge percpu cpuusage

On Sun, Feb 13, 2022 at 08:01:18PM +0800, Chengming Zhou wrote:
> The cpuacct_account_field() is always called by the current task
> itself, so it's ok to use __this_cpu_add() to charge the tick time.
> 
> But cpuacct_charge() maybe called by update_curr() in load_balance()
> on a random CPU, different from the CPU on which the task is running.
> So __this_cpu_add() will charge that cputime to a random incorrect CPU.
> 
> Reported-by: Minye Zhu <zhuminye@...edance.com>
> Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>

Can I get a Fixes: tag for this?

> ---
>  kernel/sched/cpuacct.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/cpuacct.c b/kernel/sched/cpuacct.c
> index 3d06c5e4220d..75fbc212cb71 100644
> --- a/kernel/sched/cpuacct.c
> +++ b/kernel/sched/cpuacct.c
> @@ -335,11 +335,12 @@ static struct cftype files[] = {
>  void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>  {
>  	struct cpuacct *ca;
> +	unsigned int cpu = task_cpu(tsk);
>  
>  	rcu_read_lock();
>  
>  	for (ca = task_ca(tsk); ca; ca = parent_ca(ca))
> -		__this_cpu_add(*ca->cpuusage, cputime);
> +		*per_cpu_ptr(ca->cpuusage, cpu) += cputime;
>  
>  	rcu_read_unlock();
>  }

Also, while we there, what about this as an additional patch?

--- a/kernel/sched/cpuacct.c
+++ b/kernel/sched/cpuacct.c
@@ -334,15 +334,13 @@ static struct cftype files[] = {
  */
 void cpuacct_charge(struct task_struct *tsk, u64 cputime)
 {
-	struct cpuacct *ca;
 	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();
 }
 
 /*