回复: RCU: Question on force_qs_rnp

[Zhang, Qiang]

________________________________________
发件人: Paul E. McKenney <paulmck@xxxxxxxxxx>
发送时间: 2020年9月15日 4:56
收件人: Joel Fernandes
抄送: Zhang, Qiang; Uladzislau Rezki; josh@xxxxxxxxxxxxxxxx; rostedt@xxxxxxxxxxx; mathieu.desnoyers@xxxxxxxxxxxx; Lai Jiangshan; rcu@xxxxxxxxxxxxxxx; 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);


Thanks

Qiang