Re: 回复: RCU: Question on force_qs_rnp

["Zhang,Qiang" <qiang.zhang@windriver.com>]

On 9/16/20 2:06 AM, Paul E. McKenney wrote:
> On Tue, Sep 15, 2020 at 01:16:39PM +0800, Zhang,Qiang wrote:
>>
>>
>> On 9/15/20 11:41 AM, Paul E. McKenney wrote:
>>> On Tue, Sep 15, 2020 at 03:18:23AM +0000, Zhang, Qiang wrote:
>>>>
>>>>
>>>> ________________________________________
>>>> 发件人: Paul E. McKenney <paulmck@kernel.org>
>>>> 发送时间: 2020年9月15日 4:56
>>>> 收件人: Joel Fernandes
>>>> 抄送: Zhang, Qiang; Uladzislau Rezki; josh@joshtriplett.org; rostedt@goodmis.org; mathieu.desnoyers@efficios.com; Lai Jiangshan; rcu@vger.kernel.org; LKML
>>>> 主题: Re: RCU: Question on force_qs_rnp
>>>>
>>>> On Mon, Sep 14, 2020 at 03:42:08PM -0400, Joel Fernandes wrote:
>>>>> On Mon, Sep 14, 2020 at 07:55:18AM +0000, Zhang, Qiang wrote:
>>>>>> Hello Paul
>>>>>>
>>>>>> I have some questions for you .
>>>>>> in force_qs_rnp func ,  if  "f(rdp)" func return true we will call rcu_report_qs_rnp func
>>>>>> report a quiescent state for this rnp node, and clear grpmask form rnp->qsmask.
>>>>>> after that ,  can we make a check for this rnp->qsmask,  if  rnp->qsmask == 0,
>>>>>> we will check blocked readers in this rnp node,  instead of jumping directly to the next node .
>>>>>
>>>>> Could you clarify what good is this going to do? What problem are you trying to
>>>>> address?
>>>>>
>>>>> You could have a task that is blocked in an RCU leaf node, but the
>>>>> force_qs_rnp() decided to call rcu_report_qs_rnp(). This is perfectly Ok. The
>>>>> CPU could be dyntick-idle and a quiescent state is reported. However, the GP
>>>>> must not end and the rcu leaf node should still be present in its parent
>>>>> intermediate nodes ->qsmask. In this case, the ->qsmask == 0 does not have
>>>>> any relevance.
>>>>>
>>>>> Or am I missing the point of the question?
>>>>
>>>>> Hello, Qiang,
>>>>
>>>>> Another way of making Joel's point is to say that the additional check
>>>>> you are asking for is already being done, but by rcu_report_qs_rnp().
>>>>
>>>>>                                                          Thanx, Paul
>>>>
>>>> Hello Pual,  Joel
>>>>
>>>> What I want to express is as follows :
>>>>
>>>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>>>> index 7623128d0020..beb554539f01 100644
>>>> --- a/kernel/rcu/tree.c
>>>> +++ b/kernel/rcu/tree.c
>>>> @@ -2622,6 +2622,11 @@ static void force_qs_rnp(int (*f)(struct rcu_data *rdp))
>>>>                   if (mask != 0) {
>>>>                           /* Idle/offline CPUs, report (releases rnp->lock). */
>>>>                           rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags);
>>>> +                       raw_spin_lock_irqsave_rcu_node(rnp, flags);
>>>> +                       if (rnp->qsmask == 0 && rcu_preempt_blocked_readers_cgp(rnp))
>>>> +                               rcu_initiate_boost(rnp, flags);
>>>> +                       else
>>>> +                               raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>>>>                   } else {
>>>>                           /* Nothing to do here, so just drop the lock. */
>>>>                           raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>>>
>>> But in that case, why duplicate the code from rcu_initiate_boost()?
>>>
>>> 							Thanx, Paul
>>>
>>
>> Hello Paul
>>
>> When we force a qs for rnp, we first check the leaf node "rnp->qsmask" if it
>> is reached zero, will check if there are some blocked readers in this leaf
>> rnp node, if so we need to priority-boost blocked readers.
>> if not we will check cpu dyntick-idle and report leaf node qs, after this
>> leaf rnp node report qs, there is may be some blocked readers in this node,
>> should we also need to priority-boost blocked readers?
> 
> Yes, but we will do that on the next time around, a few milliseconds
> later.  And by that time, it is quite possible that the reader will have
> completed, which will save us from having to priority-boost it.
> 
> 							Thanx, Paul
> 

Thanks Paul, I see.