[Xen-devel] Re: PROBLEM: 3.0-rc kernels unbootable since -rc3

["Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>]

On Tue, Jul 12, 2011 at 10:04:48PM +0100, Julie Sullivan wrote:
> On Tue, Jul 12, 2011 at 9:46 PM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Jul 12, 2011 at 03:57:32PM -0400, Konrad Rzeszutek Wilk wrote:
> >> On Tue, Jul 12, 2011 at 09:10:36PM +0200, Peter Zijlstra wrote:
> >> > On Tue, 2011-07-12 at 11:59 -0700, Paul E. McKenney wrote:
> >> > > OK, so the infinite loop in task_waking_fair() happens even if RCU 
> >> > > callbacks
> >> > > are deferred until after the scheduler is fully initialized.  Sounds 
> >> > > like
> >> > > one for the scheduler guys.  ;-)
> >> >
> >> > https://lkml.org/lkml/2011/7/12/150
> >>
> >> Such a simple patch. And yes, it fixes the issue. You can add
> >> Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> if it hasn't yet
> >> showed up in Ingo's tree.
> >>
> >> Paul, thanks for help on this and providing ideas to test!
> >
> > Konrad, thank you for all the testing!
> >
> > Julie, if you apply Peter's patch,
> 
> But this is for 32-bit , right?

Indeed it is, please accept my apologies for my confusion.

> > +#ifndef CONFIG_64BIT
> > +   cfs_rq->min_vruntime_copy = cfs_rq->min_vruntime;
> > +#endif
>  }
> 
> I'm using 64-bit...
> 
> Would you still like me to try the below patch?

Could you please?  It restores the exact behavior of the patch that
worked for you, but in a form that can go upstream.

So I am very much hoping that it works for you.

                                                        Thanx, Paul

> Cheers
> Julie
> 
> 
> > do you also need the patch shown
> > below?
> >
> > Ravi, could you please retest with the patch below as well?
> >
> >                                                        Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index 7e59ffb..ba06207 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -84,9 +84,32 @@ DEFINE_PER_CPU(struct rcu_data, rcu_bh_data);
> >
> >  static struct rcu_state *rcu_state;
> >
> > +/*
> > + * The rcu_scheduler_active variable transitions from zero to one just
> > + * before the first task is spawned.  So when this variable is zero, RCU
> > + * can assume that there is but one task, allowing RCU to (for example)
> > + * optimized synchronize_sched() to a simple barrier().  When this variable
> > + * is one, RCU must actually do all the hard work required to detect real
> > + * grace periods.  This variable is also used to suppress boot-time false
> > + * positives from lockdep-RCU error checking.
> > + */
> >  int rcu_scheduler_active __read_mostly;
> >  EXPORT_SYMBOL_GPL(rcu_scheduler_active);
> >
> > +/*
> > + * The rcu_scheduler_fully_active variable transitions from zero to one
> > + * during the early_initcall() processing, which is after the scheduler
> > + * is capable of creating new tasks.  So RCU processing (for example,
> > + * creating tasks for RCU priority boosting) must be delayed until after
> > + * rcu_scheduler_fully_active transitions from zero to one.  We also
> > + * currently delay invocation of any RCU callbacks until after this point.
> > + *
> > + * It might later prove better for people registering RCU callbacks during
> > + * early boot to take responsibility for these callbacks, but one step at
> > + * a time.
> > + */
> > +static int rcu_scheduler_fully_active __read_mostly;
> > +
> >  #ifdef CONFIG_RCU_BOOST
> >
> >  /*
> > @@ -98,7 +121,6 @@ DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_status);
> >  DEFINE_PER_CPU(int, rcu_cpu_kthread_cpu);
> >  DEFINE_PER_CPU(unsigned int, rcu_cpu_kthread_loops);
> >  DEFINE_PER_CPU(char, rcu_cpu_has_work);
> > -static char rcu_kthreads_spawnable;
> >
> >  #endif /* #ifdef CONFIG_RCU_BOOST */
> >
> > @@ -1467,6 +1489,8 @@ static void rcu_process_callbacks(struct 
> > softirq_action *unused)
> >  */
> >  static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data 
> > *rdp)
> >  {
> > +       if (unlikely(!ACCESS_ONCE(rcu_scheduler_fully_active)))
> > +               return;
> >        if (likely(!rsp->boost)) {
> >                rcu_do_batch(rsp, rdp);
> >                return;
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index 14dc7dd..75113cb 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -1532,7 +1532,7 @@ static int __cpuinit rcu_spawn_one_cpu_kthread(int 
> > cpu)
> >        struct sched_param sp;
> >        struct task_struct *t;
> >
> > -       if (!rcu_kthreads_spawnable ||
> > +       if (!rcu_scheduler_fully_active ||
> >            per_cpu(rcu_cpu_kthread_task, cpu) != NULL)
> >                return 0;
> >        t = kthread_create(rcu_cpu_kthread, (void *)(long)cpu, "rcuc%d", 
> > cpu);
> > @@ -1639,7 +1639,7 @@ static int __cpuinit 
> > rcu_spawn_one_node_kthread(struct rcu_state *rsp,
> >        struct sched_param sp;
> >        struct task_struct *t;
> >
> > -       if (!rcu_kthreads_spawnable ||
> > +       if (!rcu_scheduler_fully_active ||
> >            rnp->qsmaskinit == 0)
> >                return 0;
> >        if (rnp->node_kthread_task == NULL) {
> > @@ -1665,7 +1665,7 @@ static int __init rcu_spawn_kthreads(void)
> >        int cpu;
> >        struct rcu_node *rnp;
> >
> > -       rcu_kthreads_spawnable = 1;
> > +       rcu_scheduler_fully_active = 1;
> >        for_each_possible_cpu(cpu) {
> >                per_cpu(rcu_cpu_has_work, cpu) = 0;
> >                if (cpu_online(cpu))
> > @@ -1687,7 +1687,7 @@ static void __cpuinit rcu_prepare_kthreads(int cpu)
> >        struct rcu_node *rnp = rdp->mynode;
> >
> >        /* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
> > -       if (rcu_kthreads_spawnable) {
> > +       if (rcu_scheduler_fully_active) {
> >                (void)rcu_spawn_one_cpu_kthread(cpu);
> >                if (rnp->node_kthread_task == NULL)
> >                        (void)rcu_spawn_one_node_kthread(rcu_state, rnp);
> > @@ -1726,6 +1726,13 @@ static void rcu_cpu_kthread_setrt(int cpu, int to_rt)
> >  {
> >  }
> >
> > +static int __init rcu_scheduler_really_started(void)
> > +{
> > +       rcu_scheduler_fully_active = 1;
> > +       return 0;
> > +}
> > +early_initcall(rcu_scheduler_really_started);
> > +
> >  static void __cpuinit rcu_prepare_kthreads(int cpu)
> >  {
> >  }
> >

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