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Date: Wed, 28 Sep 2011 12:22:58 -0300
From: Glauber Costa <glommer@parallels.com>
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To: Peter Zijlstra <a.p.zijlstra@chello.nl>
CC: Balbir Singh <bsingharora@gmail.com>, <linux-kernel@vger.kernel.org>,
        <paul@paulmenage.org>, <lizf@cn.fujitsu.com>, <daniel.lezcano@free.fr>,
        <jbottomley@parallels.com>
Subject: Re: [RFD 3/9] Display /proc/stat information per cgroup
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On 09/27/2011 07:21 PM, Peter Zijlstra wrote:
> On Tue, 2011-09-27 at 15:42 -0300, Glauber Costa wrote:
>>>> +static inline void task_cgroup_account_field(struct task_struct *p,
>>>> +                                            cputime64_t tmp, int index)
>>>> +{
>>>> +       struct kernel_stat *kstat;
>>>> +       struct task_group *tg = task_group(p);
>>>> +
>>>> +       do {
>>>> +               kstat = this_cpu_ptr(tg->cpustat);
>>>> +               kstat->cpustat[index] = cputime64_add(kstat->cpustat[index],
>>>> +                                                     tmp);
>>>> +               tg = tg->parent;
>>>> +       } while (tg);
>>>> +}
>>>
>>> What protects the walk (tg = tg->parent)? Could you please document it
>> I think that the fact that the hierarchy only grows down, thus parent
>> never changes (or am I wrong?)
>>
>> And since we run all this with preempt disabled and with the runqueue
>> locked, we should have no problems.
>>
>> Do you agree?
>
> Right, so the tg can't be destroyed unless its empty, us finding this
> task in it means its not empty, we require rq->lock or p->pi_lock to
> move the task.
>
> However, afaict we don't actually have any of those locks.
>
> That said, it should be sufficient to wrap the whole thing in
> rcu_read_lock(), accounting one tick funny because a cgroup move race
> isn't the end of the world and its really no different than moving the
> task a little later anyway.
>
> task_group() should complain about this if you compile a kernel with
> CONFIG_PROVE_RCU.


Yeah, wrapping it around rcu_read_lock locks fine.