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Date: Fri, 10 Dec 2010 16:04:51 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
Thomas Gleixner <tglx@...utronix.de>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Steven Rostedt <rostedt@...dmis.org>, laijs@...fujitsu.com
Subject: Re: [PATCH 2/2] rcu: Keep gpnum and completed fields synchronized
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 rebased your earlier two
out and reworked mine, please see below. Work better?
Thanx, Paul
------------------------------------------------------------------------
commit c808bedd1b1d7c720546a6682fca44c66703af4e
Author: Paul E. McKenney <paulmck@...ux.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@...ux.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;
}
}
--
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