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Date: Wed, 27 Jun 2018 11:11:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...nel.org,
jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 13/22] rcu: Fix grace-period hangs due to
race with CPU offline
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.
The extra argument isn't really pretty but not nearly as bad as feared.
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 7832dd556490..d4c38d8d3621 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);
@@ -160,7 +159,8 @@ static int rcu_scheduler_fully_active __read_mostly;
static void
rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
- struct rcu_node *rnp, unsigned long gps, unsigned long flags);
+ struct rcu_node *rnp, unsigned long gps,
+ unsigned long flags, bool no_wakeup);
static void rcu_init_new_rnp(struct rcu_node *rnp_leaf);
static void rcu_cleanup_dead_rnp(struct rcu_node *rnp_leaf);
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu);
@@ -1928,13 +1928,11 @@ 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 +1968,6 @@ 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. */
@@ -2004,7 +2001,7 @@ static bool rcu_gp_init(struct rcu_state *rsp)
mask = rnp->qsmask & ~rnp->qsmaskinitnext;
rnp->rcu_gp_init_mask = mask;
if ((mask || rnp->wait_blkd_tasks) && rcu_is_leaf_node(rnp))
- rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
+ rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags, false);
else
raw_spin_unlock_irq_rcu_node(rnp);
cond_resched_tasks_rcu_qs();
@@ -2247,14 +2244,17 @@ static int __noreturn rcu_gp_kthread(void *arg)
* just-completed grace period. Note that the caller must hold rnp->lock,
* which is released before return.
*/
-static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
+static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags,
+ bool no_wakeup)
__releases(rcu_get_root(rsp)->lock)
{
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);
+
+ if (!no_wakeup)
+ rcu_gp_kthread_wake(rsp);
}
/*
@@ -2273,7 +2273,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
*/
static void
rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
- struct rcu_node *rnp, unsigned long gps, unsigned long flags)
+ struct rcu_node *rnp, unsigned long gps,
+ unsigned long flags, bool no_wakeup)
__releases(rnp->lock)
{
unsigned long oldmask = 0;
@@ -2326,7 +2327,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp,
* state for this grace period. Invoke rcu_report_qs_rsp()
* to clean up and start the next grace period if one is needed.
*/
- rcu_report_qs_rsp(rsp, flags); /* releases rnp->lock. */
+ rcu_report_qs_rsp(rsp, flags, no_wakeup); /* releases rnp->lock. */
}
/*
@@ -2361,7 +2362,7 @@ rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
* Only one rcu_node structure in the tree, so don't
* try to report up to its nonexistent parent!
*/
- rcu_report_qs_rsp(rsp, flags);
+ rcu_report_qs_rsp(rsp, flags, false);
return;
}
@@ -2370,7 +2371,7 @@ rcu_report_unblock_qs_rnp(struct rcu_state *rsp,
mask = rnp->grpmask;
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
raw_spin_lock_rcu_node(rnp_p); /* irqs already disabled. */
- rcu_report_qs_rnp(mask, rsp, rnp_p, gps, flags);
+ rcu_report_qs_rnp(mask, rsp, rnp_p, gps, flags, false);
}
/*
@@ -2413,7 +2414,7 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
*/
needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
- rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
+ rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags, false);
/* ^^^ Released rnp->lock */
if (needwake)
rcu_gp_kthread_wake(rsp);
@@ -2711,7 +2712,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *rsp))
}
if (mask != 0) {
/* Idle/offline CPUs, report (releases rnp->lock). */
- rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
+ rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags, false);
} else {
/* Nothing to do here, so just drop the lock. */
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
@@ -3745,7 +3746,7 @@ void rcu_cpu_starting(unsigned int cpu)
rdp->rcu_onl_gp_flags = READ_ONCE(rsp->gp_flags);
if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
/* Report QS -after- changing ->qsmaskinitnext! */
- rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags);
+ rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags, false);
} else {
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
@@ -3768,18 +3769,15 @@ 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);
+ rcu_report_qs_rnp(mask, rsp, rnp, rnp->gp_seq, flags, true);
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 +3847,12 @@ 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. */
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