Re: 回复: RCU: Question on force_qs_rnp

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

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?

Thanks

Qiang