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Date: Sat, 11 Dec 2010 01:15:17 +0100
From: Frederic Weisbecker <fweisbec@...il.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 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?
Thanks.
> 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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