Re: 3.0-git15 Atomic scheduling in pidmap_init

Paul E. McKenney
2011-08-18T23:01:13Z

On Thu, Aug 18, 2011 at 06:21:08PM -0400, Josh Boyer wrote:
> On Thu, Aug 18, 2011 at 02:55:40PM -0700, Paul E. McKenney wrote:
> > And here is this patch ported to v3.1-rc2, FYI.
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > rcu: Avoid having just-onlined CPU resched itself when RCU is idle
> >
> > CPUs set rdp->qs_pending when coming online to resolve races with
> > grace-period start. However, this means that if RCU is idle, the
> > just-onlined CPU might needlessly send itself resched IPIs. Adjust the
> > online-CPU initialization to avoid this, and also to correctly cause
> > the CPU to respond to the current grace period if needed.
> >
> > This patch is believed to fix or otherwise suppress problems in
> > https://bugzilla.redhat.com/show_bug.cgi?id=726877, however, the
> > relationship is not apparent to this patch's author.
> >
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>
> This is what has been in rawhide for a while now.
>
> Tested-by: Josh Boyer <jwboyer@redhat.com>

Thank you, I have added your Tested-by.

Thanx, Paul

> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index ba06207..6986d34 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -1865,8 +1865,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> >
> > /* Set up local state, ensuring consistent view of global state. */
> > raw_spin_lock_irqsave(&rnp->lock, flags);
> > - rdp->passed_quiesc = 0; /* We could be racing with new GP, */
> > - rdp->qs_pending = 1; /* so set up to respond to current GP. */
> > rdp->beenonline = 1; /* We have now been online. */
> > rdp->preemptible = preemptible;
> > rdp->qlen_last_fqs_check = 0;
> > @@ -1891,8 +1889,15 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
> > rnp->qsmaskinit |= mask;
> > mask = rnp->grpmask;
> > if (rnp == rdp->mynode) {
> > - rdp->gpnum = rnp->completed; /* if GP in progress... */
> > + /*
> > + * If there is a grace period in progress, we will
> > + * set up to wait for it next time we run the
> > + * RCU core code.
> > + */
> > + rdp->gpnum = rnp->completed;
> > rdp->completed = rnp->completed;
> > + rdp->passed_quiesc = 0;
> > + rdp->qs_pending = 1;
> > rdp->passed_quiesc_completed = rnp->completed - 1;
> > }
> > raw_spin_unlock(&rnp->lock); /* irqs already disabled. */