Messages in this thread | | | Date | Fri, 10 Dec 2010 22:36:38 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized |
| |
On Sat, Dec 11, 2010 at 02:21:04AM +0100, Frederic Weisbecker wrote: > On Fri, Dec 10, 2010 at 04:58:27PM -0800, Paul E. McKenney wrote: > > On Sat, Dec 11, 2010 at 01:15:17AM +0100, Frederic Weisbecker wrote: > > > On Fri, Dec 10, 2010 at 04:04:51PM -0800, Paul E. McKenney wrote: > > > > On Sat, Dec 11, 2010 at 12:47:11AM +0100, Frederic Weisbecker wrote: > > > > > On Fri, Dec 10, 2010 at 03:39:20PM -0800, Paul E. McKenney wrote: > > > > > > On Fri, Dec 10, 2010 at 03:02:00PM -0800, Paul E. McKenney wrote: > > > > > > > On Fri, Dec 10, 2010 at 10:11:11PM +0100, Frederic Weisbecker wrote: > > > > > > > > When a CPU that was in an extended quiescent state wakes > > > > > > > > up and catches up with grace periods that remote CPUs > > > > > > > > completed on its behalf, we update the completed field > > > > > > > > but not the gpnum that keeps a stale value of a backward > > > > > > > > grace period ID. > > > > > > > > > > > > > > > > Later, note_new_gpnum() will interpret the shift between > > > > > > > > the local CPU and the node grace period ID as some new grace > > > > > > > > period to handle and will then start to hunt quiescent state. > > > > > > > > > > > > > > > > But if every grace periods have already been completed, this > > > > > > > > interpretation becomes broken. And we'll be stuck in clusters > > > > > > > > of spurious softirqs because rcu_report_qs_rdp() will make > > > > > > > > this broken state run into infinite loop. > > > > > > > > > > > > > > > > The solution, as suggested by Lai Jiangshan, is to ensure that > > > > > > > > the gpnum and completed fields are well synchronized when we catch > > > > > > > > up with completed grace periods on their behalf by other cpus. > > > > > > > > This way we won't start noting spurious new grace periods. > > > > > > > > > > > > > > Also good, queued! > > > > > > > > > > > > > > One issue -- this approach is vulnerable to overflow. I therefore > > > > > > > followed up with a patch that changes the condition to > > > > > > > > > > > > > > if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) > > > > > > > > > > > > And here is the follow-up patch, FWIW. > > > > > > > > > > > > Thanx, Paul > > > > > > > > > > Hmm, it doesn't apply on top of my two patches. It seems you have > > > > > kept my two previous patches, which makes it fail as it lacks them > > > > > as a base. > > > > > > > > > > Did you intend to keep them? I hope they are quite useless now, otherwise > > > > > it means there is other cases I forgot. > > > > > > > > One is indeed useless, while the other is useful in combinations of > > > > dyntick-idle and force_quiescent_state(). > > > > > > I don't see how. > > > > > > Before we call __note_new_gpnum(), we always have the opportunity > > > to resync gpnum and completed as __rcu_process_gp_end() is called > > > before. > > > > > > Am I missing something? > > > > If the CPU is already aware of the end of the previous grace period, > > then __rcu_process_gp_end() will return without doing anything. But if > > force_quiescent_state() already took care of this CPU, there is no point > > in its looking for another quiescent state. This can happen as follows: > > > > o CPU 0 notes the end of the previous grace period and then > > enters dyntick-idle mode. > > > > o CPU 2 enters a very long RCU read-side critical section. > > > > o CPU 1 starts a new grace period. > > > > o CPU 0 does not check in because it is in dyntick-idle mode. > > > > o CPU 1 eventually calls force_quiescent_state() a few times, > > and sees that CPU 0 is in dyntick-idle mode, so tells RCU > > that CPU 0 is in an extended quiescent state. But the > > grace period cannot end because CPU 2 is still in its > > RCU read-side critical section. > > > > o CPU 0 comes out of dyntick-idle mode, and sees the new > > grace period. The old code would nevertheless look for > > a quiescent state, and the new code would avoid doing so. > > > > Unless I am missing something, of course... > > > > Thanx, Paul > > Aah, so in your scenario, CPU 0, 1 et 2 are the same node (rnp), > we have not updated rnp->completed because we still wait for CPU 2. > > Then __rcu_process_gp_end() won't increase the gpnum either > because rnp->completed is still equal to rdp->completed. > > And later on we call note_new_gpnum() that thinks it has a new > gp to handle but it's wrong. > > Hence the need to look at the mask level there.
CPUs 0, 1, et 2 are not necessarily on the same node, but other than that, you have it exactly. The trick is that force_quiescent_state() takes global action, so CPU 1 does not need to be on the same node as CPU 0. Furthermore, an RCU read-side critical section anywhere in the system will prevent any subsequent grace period from completing, so CPU 2 can be on yet another node.
> This makes all sense!
Thank you for the confirmation, will try testing it out more thoroughly.
Thanx, Paul
> Thanks! > > > > > > Thanks. > > > > > > > I rebased your earlier two > > > > out and reworked mine, please see below. Work better? > > > > > > > > Thanx, Paul > > > > > > > > ------------------------------------------------------------------------ > > > > > > > > commit c808bedd1b1d7c720546a6682fca44c66703af4e > > > > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > Date: Fri Dec 10 15:02:47 2010 -0800 > > > > > > > > rcu: fine-tune grace-period begin/end checks > > > > > > > > Use the CPU's bit in rnp->qsmask to determine whether or not the CPU > > > > should try to report a quiescent state. Handle overflow in the check > > > > for rdp->gpnum having fallen behind. > > > > > > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > > > > > > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > > > > index 368be76..530cdcd 100644 > > > > --- a/kernel/rcutree.c > > > > +++ b/kernel/rcutree.c > > > > @@ -616,9 +616,17 @@ static void __init check_cpu_stall_init(void) > > > > static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp) > > > > { > > > > if (rdp->gpnum != rnp->gpnum) { > > > > - rdp->qs_pending = 1; > > > > - rdp->passed_quiesc = 0; > > > > + /* > > > > + * If the current grace period is waiting for this CPU, > > > > + * set up to detect a quiescent state, otherwise don't > > > > + * go looking for one. > > > > + */ > > > > rdp->gpnum = rnp->gpnum; > > > > + if (rnp->qsmask & rdp->grpmask) { > > > > + rdp->qs_pending = 1; > > > > + rdp->passed_quiesc = 0; > > > > + } else > > > > + rdp->qs_pending = 0; > > > > } > > > > } > > > > > > > > @@ -680,19 +688,20 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat > > > > > > > > /* > > > > * If we were in an extended quiescent state, we may have > > > > - * missed some grace periods that others CPUs took care on > > > > + * missed some grace periods that others CPUs handled on > > > > * our behalf. Catch up with this state to avoid noting > > > > - * spurious new grace periods. > > > > + * spurious new grace periods. If another grace period > > > > + * has started, then rnp->gpnum will have advanced, so > > > > + * we will detect this later on. > > > > */ > > > > - if (rdp->completed > rdp->gpnum) > > > > + if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) > > > > rdp->gpnum = rdp->completed; > > > > > > > > /* > > > > - * If another CPU handled our extended quiescent states and > > > > - * we have no more grace period to complete yet, then stop > > > > - * chasing quiescent states. > > > > + * If RCU does not need a quiescent state from this CPU, > > > > + * then make sure that this CPU doesn't go looking for one. > > > > */ > > > > - if (rdp->completed == rnp->gpnum) > > > > + if (rnp->qsmask & rdp->grpmask) > > > > rdp->qs_pending = 0; > > > > } > > > > }
| |