Re: 回复: RCU: Question on force_qs_rnp

["Paul E. McKenney" <paulmck@kernel.org>]

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