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McKenney wrote: > On Tue, Sep 15, 2020 at 03:18:23AM +0000, Zhang, Qiang wrote: >> >> >> ________________________________________ >> 发件人: Paul E. McKenney >> 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