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Subject[PATCH RFC] rcu: Make rcu_barrier() less disruptive
The rcu_barrier() primitive interrupts each and every CPU, registering
a callback on every CPU. Once all of these callbacks have been invoked,
rcu_barrier() knows that every callback that was registered before
the call to rcu_barrier() has also been invoked.

However, there is no point in registering a callback on a CPU that
currently has no callbacks, most especially if that CPU is in a
deep idle state. This commit therefore makes rcu_barrier() avoid
interrupting CPUs that have no callbacks. Doing this requires reworking
the handling of orphaned callbacks, otherwise callbacks could slip through
rcu_barrier()'s net by being orphaned from a CPU that rcu_barrier() had
not yet interrupted to a CPU that rcu_barrier() had already interrupted.
This reworking was needed anyway to take a first step towards weaning
RCU from the CPU_DYING notifier's use of stop_cpu().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 403306b..8269656 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -75,6 +75,8 @@ static struct lock_class_key rcu_node_class[NUM_RCU_LVLS];
.gpnum = -300, \
.completed = -300, \
.onofflock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.onofflock), \
+ .orphan_nxttail = &structname##_state.orphan_nxtlist, \
+ .orphan_donetail = &structname##_state.orphan_donelist, \
.fqslock = __RAW_SPIN_LOCK_UNLOCKED(&structname##_state.fqslock), \
.n_force_qs = 0, \
.n_force_qs_ngp = 0, \
@@ -145,6 +147,13 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
unsigned long rcutorture_testseq;
unsigned long rcutorture_vernum;

+/* State information for rcu_barrier() and friends. */
+
+static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
+static atomic_t rcu_barrier_cpu_count;
+static DEFINE_MUTEX(rcu_barrier_mutex);
+static DECLARE_WAIT_QUEUE_HEAD(rcu_barrier_wq);
+
/*
* Return true if an RCU grace period is in progress. The ACCESS_ONCE()s
* permit this function to be invoked without holding the root rcu_node
@@ -1311,7 +1320,60 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
#ifdef CONFIG_HOTPLUG_CPU

/*
- * Move a dying CPU's RCU callbacks to online CPU's callback list.
+ * Adopt the RCU callbacks from the specified rcu_state structure's
+ * orphanage. The caller must hold the ->onofflock.
+ */
+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+ int i;
+ struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
+
+ /*
+ * If there is an rcu_barrier() operation in progress, then
+ * only the task doing that operation is permitted to adopt
+ * callbacks. To do otherwise breaks rcu_barrier() and friends
+ * by causing them to fail to wait for the callbacks in the
+ * orphanage.
+ */
+ if (rsp->rcu_barrier_in_progress &&
+ rsp->rcu_barrier_in_progress != current)
+ return;
+
+ /* Do the accounting first. */
+ rdp->qlen_lazy += rsp->qlen_lazy;
+ rdp->qlen += rsp->qlen;
+ rdp->n_cbs_adopted += rsp->qlen;
+ rsp->qlen_lazy = 0;
+ rsp->qlen = 0;
+
+ /*
+ * We do not need a memory barrier here because the only way we
+ * can get here if there is an rcu_barrier() in flight is if
+ * we are the task doing the rcu_barrier().
+ */
+
+ /* First adopt the ready-to-invoke callbacks. */
+ if (rsp->orphan_donelist != NULL) {
+ *rsp->orphan_donetail = *rdp->nxttail[RCU_DONE_TAIL];
+ *rdp->nxttail[RCU_DONE_TAIL] = rsp->orphan_donelist;
+ for (i = RCU_NEXT_SIZE - 1; i >= RCU_DONE_TAIL; i--)
+ if (rdp->nxttail[i] == rdp->nxttail[RCU_DONE_TAIL])
+ rdp->nxttail[i] = rsp->orphan_donetail;
+ rsp->orphan_donelist = NULL;
+ rsp->orphan_donetail = &rsp->orphan_donelist;
+ }
+
+ /* And then adopt the callbacks that still need a grace period. */
+ if (rsp->orphan_nxtlist != NULL) {
+ *rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxtlist;
+ rdp->nxttail[RCU_NEXT_TAIL] = rsp->orphan_nxttail;
+ rsp->orphan_nxtlist = NULL;
+ rsp->orphan_nxttail = &rsp->orphan_nxtlist;
+ }
+}
+
+/*
+ * Move a dying CPU's RCU callbacks to the rcu_state structure's orphanage.
* Also record a quiescent state for this CPU for the current grace period.
* Synchronization and interrupt disabling are not required because
* this function executes in stop_machine() context. Therefore, cleanup
@@ -1325,64 +1387,67 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
int i;
+ unsigned long flags;
unsigned long mask;
- int receive_cpu = cpumask_any(cpu_online_mask);
+ bool orphaned = 0;
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
- struct rcu_data *receive_rdp = per_cpu_ptr(rsp->rda, receive_cpu);
RCU_TRACE(struct rcu_node *rnp = rdp->mynode); /* For dying CPU. */

- /* First, adjust the counts. */
+ /* Move the callbacks to the orphanage under ->onofflock protection. */
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+
+ /* First adjust the counts. */
if (rdp->nxtlist != NULL) {
- receive_rdp->qlen_lazy += rdp->qlen_lazy;
- receive_rdp->qlen += rdp->qlen;
+ rsp->qlen_lazy += rdp->qlen_lazy;
+ rsp->qlen += rdp->qlen;
+ rdp->n_cbs_orphaned += rdp->qlen;
rdp->qlen_lazy = 0;
rdp->qlen = 0;
+ orphaned = 1;
}

/*
- * Next, move ready-to-invoke callbacks to be invoked on some
- * other CPU. These will not be required to pass through another
- * grace period: They are done, regardless of CPU.
+ * Next, move those callbacks still needing a grace period to
+ * the orphanage, where some other CPU will pick them up.
+ * Some of the callbacks might have gone partway through a grace
+ * period, but that is too bad. They get to start over because we
+ * cannot assume that grace periods are synchronized across CPUs.
+ * We don't bother updating the ->nxttail[] array yet, instead
+ * we just reset the whole thing later on.
*/
- if (rdp->nxtlist != NULL &&
- rdp->nxttail[RCU_DONE_TAIL] != &rdp->nxtlist) {
- struct rcu_head *oldhead;
- struct rcu_head **oldtail;
- struct rcu_head **newtail;
-
- oldhead = rdp->nxtlist;
- oldtail = receive_rdp->nxttail[RCU_DONE_TAIL];
- rdp->nxtlist = *rdp->nxttail[RCU_DONE_TAIL];
- *rdp->nxttail[RCU_DONE_TAIL] = *oldtail;
- *receive_rdp->nxttail[RCU_DONE_TAIL] = oldhead;
- newtail = rdp->nxttail[RCU_DONE_TAIL];
- for (i = RCU_DONE_TAIL; i < RCU_NEXT_SIZE; i++) {
- if (receive_rdp->nxttail[i] == oldtail)
- receive_rdp->nxttail[i] = newtail;
- if (rdp->nxttail[i] == newtail)
- rdp->nxttail[i] = &rdp->nxtlist;
- }
+ if (*rdp->nxttail[RCU_DONE_TAIL] != NULL) {
+ *rsp->orphan_nxttail = *rdp->nxttail[RCU_DONE_TAIL];
+ rsp->orphan_nxttail = rdp->nxttail[RCU_NEXT_TAIL];
+ *rdp->nxttail[RCU_DONE_TAIL] = NULL;
}

/*
- * Finally, put the rest of the callbacks at the end of the list.
- * The ones that made it partway through get to start over: We
- * cannot assume that grace periods are synchronized across CPUs.
- * (We could splice RCU_WAIT_TAIL into RCU_NEXT_READY_TAIL, but
- * this does not seem compelling. Not yet, anyway.)
+ * Then move the ready-to-invoke callbacks to the orphanage,
+ * where some other CPU will pick them up. These will not be
+ * required to pass though another grace period: They are done.
*/
if (rdp->nxtlist != NULL) {
- *receive_rdp->nxttail[RCU_NEXT_TAIL] = rdp->nxtlist;
- receive_rdp->nxttail[RCU_NEXT_TAIL] =
- rdp->nxttail[RCU_NEXT_TAIL];
- receive_rdp->n_cbs_adopted += rdp->qlen;
- rdp->n_cbs_orphaned += rdp->qlen;
-
- rdp->nxtlist = NULL;
- for (i = 0; i < RCU_NEXT_SIZE; i++)
- rdp->nxttail[i] = &rdp->nxtlist;
+ *rsp->orphan_donetail = rdp->nxtlist;
+ rsp->orphan_donetail = rdp->nxttail[RCU_DONE_TAIL];
}

+ /* Finally, initialize the rcu_data structure's list to empty. */
+ rdp->nxtlist = NULL;
+ for (i = 0; i < RCU_NEXT_SIZE; i++)
+ rdp->nxttail[i] = &rdp->nxtlist;
+
+ /*
+ * Wait up the rcu_barrier() task if there is one and if we
+ * actually sent anything to the orphanage. Except that we
+ * must delay the wakeup until ->onofflock is released to
+ * avoid deadlock.
+ */
+ if (!rsp->rcu_barrier_in_progress)
+ orphaned = 0;
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+ if (orphaned)
+ wake_up(&rcu_barrier_wq);
+
/*
* Record a quiescent state for the dying CPU. This is safe
* only because we have already cleared out the callbacks.
@@ -1415,11 +1480,14 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)
rcu_stop_cpu_kthread(cpu);
rcu_node_kthread_setaffinity(rnp, -1);

- /* Remove the dying CPU from the bitmasks in the rcu_node hierarchy. */
+ /* Remove the dead CPU from the bitmasks in the rcu_node hierarchy. */

/* Exclude any attempts to start a new grace period. */
raw_spin_lock_irqsave(&rsp->onofflock, flags);

+ /* Collect the dead CPU's callbacks. */
+ rcu_adopt_orphan_cbs(rsp);
+
/* Remove the outgoing CPU from the masks in the rcu_node hierarchy. */
mask = rdp->grpmask; /* rnp->grplo is constant. */
do {
@@ -1456,6 +1524,10 @@ static void rcu_cleanup_dead_cpu(int cpu, struct rcu_state *rsp)

#else /* #ifdef CONFIG_HOTPLUG_CPU */

+static void rcu_adopt_orphan_cbs(struct rcu_state *rsp)
+{
+}
+
static void rcu_cleanup_dying_cpu(struct rcu_state *rsp)
{
}
@@ -1524,9 +1596,6 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
rcu_is_callbacks_kthread());

/* Update count, and requeue any remaining callbacks. */
- rdp->qlen_lazy -= count_lazy;
- rdp->qlen -= count;
- rdp->n_cbs_invoked += count;
if (list != NULL) {
*tail = rdp->nxtlist;
rdp->nxtlist = list;
@@ -1536,6 +1605,10 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
else
break;
}
+ smp_mb(); /* List handling before counting for rcu_barrier(). */
+ rdp->qlen_lazy -= count_lazy;
+ rdp->qlen -= count;
+ rdp->n_cbs_invoked += count;

/* Reinstate batch limit if we have worked down the excess. */
if (rdp->blimit == LONG_MAX && rdp->qlen <= qlowmark)
@@ -1824,13 +1897,14 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu),
rdp = this_cpu_ptr(rsp->rda);

/* Add the callback to our list. */
- *rdp->nxttail[RCU_NEXT_TAIL] = head;
- rdp->nxttail[RCU_NEXT_TAIL] = &head->next;
rdp->qlen++;
if (lazy)
rdp->qlen_lazy++;
else
rcu_idle_count_callbacks_posted();
+ smp_mb(); /* Count before adding callback for rcu_barrier(). */
+ *rdp->nxttail[RCU_NEXT_TAIL] = head;
+ rdp->nxttail[RCU_NEXT_TAIL] = &head->next;

if (__is_kfree_rcu_offset((unsigned long)func))
trace_rcu_kfree_callback(rsp->name, head, (unsigned long)func,
@@ -2169,15 +2243,10 @@ static int rcu_cpu_has_callbacks(int cpu)
rcu_preempt_cpu_has_callbacks(cpu);
}

-static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};
-static atomic_t rcu_barrier_cpu_count;
-static DEFINE_MUTEX(rcu_barrier_mutex);
-static struct completion rcu_barrier_completion;
-
static void rcu_barrier_callback(struct rcu_head *notused)
{
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
- complete(&rcu_barrier_completion);
+ wake_up(&rcu_barrier_wq);
}

/*
@@ -2203,27 +2272,121 @@ static void _rcu_barrier(struct rcu_state *rsp,
void (*call_rcu_func)(struct rcu_head *head,
void (*func)(struct rcu_head *head)))
{
- BUG_ON(in_interrupt());
+ int cpu;
+ unsigned long flags;
+ struct rcu_data *rdp;
+ struct rcu_head rh;
+
+ init_rcu_head_on_stack(&rh);
+
/* Take mutex to serialize concurrent rcu_barrier() requests. */
mutex_lock(&rcu_barrier_mutex);
- init_completion(&rcu_barrier_completion);
+
+ smp_mb(); /* Prevent any prior operations from leaking in. */
+
/*
- * Initialize rcu_barrier_cpu_count to 1, then invoke
- * rcu_barrier_func() on each CPU, so that each CPU also has
- * incremented rcu_barrier_cpu_count. Only then is it safe to
- * decrement rcu_barrier_cpu_count -- otherwise the first CPU
- * might complete its grace period before all of the other CPUs
- * did their increment, causing this function to return too
- * early. Note that on_each_cpu() disables irqs, which prevents
- * any CPUs from coming online or going offline until each online
- * CPU has queued its RCU-barrier callback.
+ * Initialize the count to one rather than to zero in order to
+ * avoid a too-soon return to zero in case of a short grace period
+ * (or preemption of this task). Also flag this task as doing
+ * an rcu_barrier(). This will prevent anyone else from adopting
+ * orphaned callbacks, which could cause otherwise failure if a
+ * CPU went offline and quickly came back online. To see this,
+ * consider the following sequence of events:
+ *
+ * 1. We cause CPU 0 to post an rcu_barrier_callback() callback.
+ * 2. CPU 1 goes offline, orphaning its callbacks.
+ * 3. CPU 0 adopts CPU 1's orphaned callbacks.
+ * 4. CPU 1 comes back online.
+ * 5. We cause CPU 1 to post an rcu_barrier_callback() callback.
+ * 6. Both rcu_barrier_callback() callbacks are invoked, awakening
+ * us -- but before CPU 1's orphaned callbacks are invoked!!!
*/
atomic_set(&rcu_barrier_cpu_count, 1);
- on_each_cpu(rcu_barrier_func, (void *)call_rcu_func, 1);
- if (atomic_dec_and_test(&rcu_barrier_cpu_count))
- complete(&rcu_barrier_completion);
- wait_for_completion(&rcu_barrier_completion);
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rsp->rcu_barrier_in_progress = current;
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+
+ /*
+ * Force every CPU with callbacks to register a new callback
+ * that will tell us when all the preceding callbacks have
+ * been invoked. If an offline CPU has callbacks, wait for
+ * it to either come back online or to finish orphaning those
+ * callbacks.
+ */
+ for_each_possible_cpu(cpu) {
+ preempt_disable();
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ if (cpu_is_offline(cpu)) {
+ preempt_enable();
+ while (cpu_is_offline(cpu) &&
+ ACCESS_ONCE(rdp->qlen))
+ schedule_timeout_interruptible(1);
+ } else if (ACCESS_ONCE(rdp->qlen)) {
+ smp_call_function_single(cpu, rcu_barrier_func,
+ (void *)call_rcu_func, 1);
+ preempt_enable();
+ }
+ }
+
+ /*
+ * Force any ongoing CPU-hotplug operations to complete,
+ * so that any callbacks from the outgoing CPUs are now in
+ * the orphanage.
+ */
+ cpu_maps_update_begin();
+ cpu_maps_update_done();
+
+ /*
+ * Now that all online CPUs have rcu_barrier_callback() callbacks
+ * posted, we can adopt all of the orphaned callbacks and place
+ * an rcu_barrier_callback() callback after them. When that is done,
+ * we are guaranteed to have an rcu_barrier_callback() callback
+ * following every callback that could possibly have been
+ * registered before _rcu_barrier() was called.
+ */
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rcu_adopt_orphan_cbs(rsp);
+ atomic_inc(&rcu_barrier_cpu_count);
+ call_rcu_func(&rh, rcu_barrier_callback);
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+
+ /*
+ * Now that we have an rcu_barrier_callback() callback on each
+ * CPU, and thus each counted, remove the initial count.
+ */
+ atomic_dec(&rcu_barrier_cpu_count);
+ smp_mb__after_atomic_dec();
+
+ /*
+ * Loop waiting for all rcu_barrier_callback() callbacks to be
+ * invoked. Adopt any orphaned callbacks in the meantime, just
+ * in case one of the rcu_barrier_callback() callbacks is orphaned.
+ */
+ while (atomic_read(&rcu_barrier_cpu_count) > 0) {
+ wait_event(rcu_barrier_wq,
+ atomic_read(&rcu_barrier_cpu_count) == 0 ||
+ ACCESS_ONCE(rsp->qlen));
+ if (ACCESS_ONCE(rsp->qlen)) {
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rcu_adopt_orphan_cbs(rsp);
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+ }
+ }
+
+ /*
+ * Done, so let others adopt orphaned callbacks. But avoid
+ * indefinite postponement of any additional orphans by adopting
+ * one more time.
+ */
+ raw_spin_lock_irqsave(&rsp->onofflock, flags);
+ rcu_adopt_orphan_cbs(rsp);
+ rsp->rcu_barrier_in_progress = NULL;
+ raw_spin_unlock_irqrestore(&rsp->onofflock, flags);
+
+ /* Other rcu_barrier() invocations can now safely proceed. */
mutex_unlock(&rcu_barrier_mutex);
+
+ destroy_rcu_head_on_stack(&rh);
}

/**
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 36ca28e..1e49c56 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -371,6 +371,17 @@ struct rcu_state {

raw_spinlock_t onofflock; /* exclude on/offline and */
/* starting new GP. */
+ struct rcu_head *orphan_nxtlist; /* Orphaned callbacks that */
+ /* need a grace period. */
+ struct rcu_head **orphan_nxttail; /* Tail of above. */
+ struct rcu_head *orphan_donelist; /* Orphaned callbacks that */
+ /* are ready to invoke. */
+ struct rcu_head **orphan_donetail; /* Tail of above. */
+ long qlen_lazy; /* Number of lazy callbacks. */
+ long qlen; /* Total number of callbacks. */
+ struct task_struct *rcu_barrier_in_progress;
+ /* Task doing rcu_barrier(), */
+ /* or NULL if no barrier. */
raw_spinlock_t fqslock; /* Only one task forcing */
/* quiescent states. */
unsigned long jiffies_force_qs; /* Time at which to invoke */
diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index ed459ed..d4bc16d 100644
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -271,13 +271,13 @@ static void print_one_rcu_state(struct seq_file *m, struct rcu_state *rsp)

gpnum = rsp->gpnum;
seq_printf(m, "c=%lu g=%lu s=%d jfq=%ld j=%x "
- "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu\n",
+ "nfqs=%lu/nfqsng=%lu(%lu) fqlh=%lu oqlen=%ld/%ld\n",
rsp->completed, gpnum, rsp->fqs_state,
(long)(rsp->jiffies_force_qs - jiffies),
(int)(jiffies & 0xffff),
rsp->n_force_qs, rsp->n_force_qs_ngp,
rsp->n_force_qs - rsp->n_force_qs_ngp,
- rsp->n_force_qs_lh);
+ rsp->n_force_qs_lh, rsp->qlen_lazy, rsp->qlen);
for (rnp = &rsp->node[0]; rnp - &rsp->node[0] < NUM_RCU_NODES; rnp++) {
if (rnp->level != level) {
seq_puts(m, "\n");


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