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

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Message-ID: <b3f9e3c5-679d-1a15-f797-5e211d543a68@bytedance.com>
Date:   Tue, 15 Feb 2022 20:19:33 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tj@...nel.org, arbn@...dex-team.com, mingo@...hat.com,
        linux-kernel@...r.kernel.org, Minye Zhu <zhuminye@...edance.com>
Subject: Re: [External] Re: [PATCH] sched/cpuacct: fix charge percpu cpuusage

On 2022/2/14 6:09 下午, Peter Zijlstra wrote:
> 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?

Yes, Fixes: 73e6aafd9ea8 ("sched/cpuacct: Simplify the cpuacct code")
I will send a patch v2 to add it.

> 
>> ---
>>  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();
>  }
>  
>  /*

This is much better, I will send an additional patch to include this.

Thanks.