On Wed, Jun 27, 2018 at 11:46:33AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 27, 2018 at 11:11:06AM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 26, 2018 at 04:40:04PM -0700, Paul E. McKenney wrote:
> > > The options I have considered are as follows:
> >
> > > 2. Stick with the no-failsafe approach, but rely on RCU's
> > > grace-period
> > > kthread to wake up later due to its timed wait during the
> > > force-quiescent-state process. This would be a bit obnoxious,
> > > as it requires passing a don't-wake flag (or some such) up the
> > > quiescent-state reporting mechanism. It would also needlessly
> > > delay grace-period ends, especially on large systems (RCU scales
> > > up the FQS delay on larger systems to maintain limited CPU
> > > consumption per unit time).
> > >
> > > 3. Stick with the no-failsafe approach, but have the
> > > quiescent-state
> > > reporting code hand back a value indicating that a wakeup is needed.
> > > Also a bit obnoxious, as this value would need to be threaded up
> > > the reporting code's return path. Simple in theory, but a bit
> > > of an ugly change, especially for the many places in the code that
> > > currently expect quiescent-state reporting to be an unconditional
> > > fire-and-forget operation.
> >
> > Here's a variant on 2+3, instead of propagating the state back, we
> > completely ignore if we needed a wakeup or not, and then unconditionally
> > wake the GP kthread on the managing CPU's rcutree_migrate_callbacks()
> > invocation.
> >
> > Hotplug is rare (or should damn well be), doing a spurious wake of the
> > GP thread shouldn't matter here.
>
> Another variant, which simply skips the wakeup whever ran on an offline
> CPU, relying on the wakeup from rcutree_migrate_callbacks() right after
> the CPU really is dead.
Cute! ;-)
And a much smaller change.
However, this means that if someone indirectly and erroneously causes
rcu_report_qs_rsp() to be invoked from an offline CPU, the result is an
intermittent and difficult-to-debug grace-period hang. A lockdep splat
whose stack trace directly implicates the culprit is much better.
But your point about CPU hotplug being rare is a good one. I should
at the very least move ->ofl_lock to sit right beside ->lock, ditching
the ____cacheline_internodealigned_in_smp.
Thanx, Paul
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 7832dd556490..417496a03259 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -104,7 +104,6 @@ struct rcu_state sname##_state = { \
> .abbr = sabbr, \
> .exp_mutex = __MUTEX_INITIALIZER(sname##_state.exp_mutex), \
> .exp_wake_mutex = __MUTEX_INITIALIZER(sname##_state.exp_wake_mutex), \
> - .ofl_lock = __SPIN_LOCK_UNLOCKED(sname##_state.ofl_lock), \
> }
>
> RCU_STATE_INITIALIZER(rcu_sched, 's', call_rcu_sched);
> @@ -1928,13 +1927,11 @@ static bool rcu_gp_init(struct rcu_state *rsp)
> */
> rsp->gp_state = RCU_GP_ONOFF;
> rcu_for_each_leaf_node(rsp, rnp) {
> - spin_lock(&rsp->ofl_lock);
> raw_spin_lock_irq_rcu_node(rnp);
> if (rnp->qsmaskinit == rnp->qsmaskinitnext &&
> !rnp->wait_blkd_tasks) {
> /* Nothing to do on this leaf rcu_node structure. */
> raw_spin_unlock_irq_rcu_node(rnp);
> - spin_unlock(&rsp->ofl_lock);
> continue;
> }
>
> @@ -1970,7 +1967,6 @@ static bool rcu_gp_init(struct rcu_state *rsp)
> }
>
> raw_spin_unlock_irq_rcu_node(rnp);
> - spin_unlock(&rsp->ofl_lock);
> }
> rcu_gp_slow(rsp, gp_preinit_delay); /* Races with CPU hotplug. */
>
> @@ -2250,11 +2246,19 @@ static int __noreturn rcu_gp_kthread(void *arg)
> static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
> __releases(rcu_get_root(rsp)->lock)
> {
> + int cpu = smp_processor_id();
> +
> raw_lockdep_assert_held_rcu_node(rcu_get_root(rsp));
> WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
> WRITE_ONCE(rsp->gp_flags, READ_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS);
> raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
> - rcu_gp_kthread_wake(rsp);
> +
> + /*
> + * When our @cpu is offline, we'll get a wakeup from
> + * rcutree_migrate_callbacks.
> + */
> + if (cpu_online(cpu))
> + rcu_gp_kthread_wake(rsp);
> }
>
> /*
> @@ -3768,18 +3772,15 @@ static void rcu_cleanup_dying_idle_cpu(int cpu,
> struct rcu_state *rsp)
>
> /* Remove outgoing CPU from mask in the leaf rcu_node structure. */
> mask = rdp->grpmask;
> - spin_lock(&rsp->ofl_lock);
> raw_spin_lock_irqsave_rcu_node(rnp, flags); /* Enforce GP memory-order
> guarantee. */
> rdp->rcu_ofl_gp_seq = READ_ONCE(rsp->gp_seq);
> rdp->rcu_ofl_gp_flags = READ_ONCE(rsp->gp_flags);
> + rnp->qsmaskinitnext &= ~mask;
> if (rnp->qsmask & mask) { /* RCU waiting on outgoing CPU? */
> - /* Report quiescent state -before- changing ->qsmaskinitnext! */
> rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
> raw_spin_lock_irqsave_rcu_node(rnp, flags);
> }
> - rnp->qsmaskinitnext &= ~mask;
> raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
> - spin_unlock(&rsp->ofl_lock);
> }
>
> /*
> @@ -3849,6 +3850,12 @@ void rcutree_migrate_callbacks(int cpu)
> {
> struct rcu_state *rsp;
>
> + /*
> + * Just in case the outgoing CPU needed to wake the GP kthread
> + * do so here.
> + */
> + rcu_gp_kthread_wake(rsp);
> +
> for_each_rcu_flavor(rsp)
> rcu_migrate_callbacks(cpu, rsp);
> }
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 4e74df768c57..8dab71838141 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -367,10 +367,6 @@ struct rcu_state {
> const char *name; /* Name of structure. */
> char abbr; /* Abbreviated name. */
> struct list_head flavors; /* List of RCU flavors. */
> -
> - spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
> - /* Synchronize offline with */
> - /* GP pre-initialization. */
> };
>
> /* Values for rcu_state structure's gp_flags field. */
>
