On Fri, Sep 06, 2019 at 12:57:51PM -0400, Joel Fernandes wrote: > On Fri, Sep 06, 2019 at 08:27:53AM -0700, Paul E. McKenney wrote: > > On Fri, Sep 06, 2019 at 08:21:44AM -0700, Paul E. McKenney wrote: > > > On Fri, Sep 06, 2019 at 11:08:06AM -0400, Joel Fernandes wrote: > > > > On Thu, Sep 05, 2019 at 08:01:37PM -0400, Joel Fernandes wrote: > > > > [snip] > > > > > > > > @@ -3004,7 +3007,7 @@ static int rcu_pending(void) > > > > > > > > return 0; > > > > > > > > > > > > > > > > /* Is the RCU core waiting for a quiescent state from this CPU? */ > > > > > > > > - if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm) > > > > > > > > + if (READ_ONCE(rdp->core_needs_qs) && !rdp->cpu_no_qs.b.norm) > > > > > > > > return 1; > > > > > > > > > > > > > > > > /* Does this CPU have callbacks ready to invoke? */ > > > > > > > > @@ -3244,7 +3247,6 @@ int rcutree_prepare_cpu(unsigned int cpu) > > > > > > > > rdp->gp_seq = rnp->gp_seq; > > > > > > > > rdp->gp_seq_needed = rnp->gp_seq; > > > > > > > > rdp->cpu_no_qs.b.norm = true; > > > > > > > > - rdp->core_needs_qs = false; > > > > > > > > > > > > > > How about calling the new hint-clearing function here as well? Just for > > > > > > > robustness and consistency purposes? > > > > > > > > > > > > This and the next function are both called during a CPU-hotplug online > > > > > > operation, so there is little robustness or consistency to be had by > > > > > > doing it twice. > > > > > > > > > > Ok, sorry I missed you are clearing it below in the next function. That's > > > > > fine with me. > > > > > > > > > > This patch looks good to me and I am Ok with merging of these changes into > > > > > the original patch with my authorship as you mentioned. Or if you wanted to > > > > > be author, that's fine too :) > > > > > > > > Paul, does it make sense to clear these urgency hints in rcu_qs() as well? > > > > After all, we are clearing atleast one urgency hint there: the > > > > rcu_read_unlock_special::need_qs bit. > > Makes sense. > > > > We certainly don't want to turn off the scheduling-clock interrupt until > > > after the quiescent state has been reported to the RCU core. And it might > > > still be useful to have a heavy quiescent state because the grace-period > > > kthread can detect that. Just in case the CPU that just called rcu_qs() > > > is slow about actually reporting that quiescent state to the RCU core. > > > > Hmmm... Should ->need_qs ever be cleared from FQS to begin with? > > I did not see the FQS loop clearing ->need_qs in the rcu_read_unlock_special > union after looking for a few minutes. Could you clarify which path this? > > Or do you mean ->core_needs_qs? If so, I feel the FQS loop should clear it as > I think your patch does, since the FQS loop is essentially doing heavy-weight > RCU core processing right? > > Also, where does FQS loop clear rdp.cpu_no_qs? Shouldn't it clear that as > well for the dyntick-idle CPUs? Synchronization? Thanx, Paul
