Preempt-RCU: CPU Hotplug handling
From: Paul E. McKenney <[email protected]>
This patch allows preemptible RCU to tolerate CPU-hotplug operations.
It accomplishes this by maintaining a local copy of a map of online
CPUs, which it accesses under its own lock.
Signed-off-by: Gautham R Shenoy <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
---
kernel/rcupreempt.c | 147 +++++++++++++++++++++++++++++++++++++++++++++++++--
1 files changed, 142 insertions(+), 5 deletions(-)
diff --git a/kernel/rcupreempt.c b/kernel/rcupreempt.c
index a5aabb1..987cfb7 100644
--- a/kernel/rcupreempt.c
+++ b/kernel/rcupreempt.c
@@ -147,6 +147,8 @@ static char *rcu_try_flip_state_names[] =
{ "idle", "waitack", "waitzero", "waitmb" };
#endif /* #ifdef CONFIG_RCU_TRACE */
+static cpumask_t rcu_cpu_online_map __read_mostly = CPU_MASK_NONE;
+
/*
* Enum and per-CPU flag to determine when each CPU has seen
* the most recent counter flip.
@@ -445,7 +447,7 @@ rcu_try_flip_idle(void)
/* Now ask each CPU for acknowledgement of the flip. */
- for_each_possible_cpu(cpu)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map)
per_cpu(rcu_flip_flag, cpu) = rcu_flipped;
return 1;
@@ -461,7 +463,7 @@ rcu_try_flip_waitack(void)
int cpu;
RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);
- for_each_possible_cpu(cpu)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map)
if (per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);
return 0;
@@ -492,7 +494,7 @@ rcu_try_flip_waitzero(void)
/* Check to see if the sum of the "last" counters is zero. */
RCU_TRACE_ME(rcupreempt_trace_try_flip_z1);
- for_each_possible_cpu(cpu)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map)
sum += RCU_DATA_CPU(cpu)->rcu_flipctr[lastidx];
if (sum != 0) {
RCU_TRACE_ME(rcupreempt_trace_try_flip_ze1);
@@ -507,7 +509,7 @@ rcu_try_flip_waitzero(void)
smp_mb(); /* ^^^^^^^^^^^^ */
/* Call for a memory barrier from each CPU. */
- for_each_possible_cpu(cpu)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map)
per_cpu(rcu_mb_flag, cpu) = rcu_mb_needed;
RCU_TRACE_ME(rcupreempt_trace_try_flip_z2);
@@ -525,7 +527,7 @@ rcu_try_flip_waitmb(void)
int cpu;
RCU_TRACE_ME(rcupreempt_trace_try_flip_m1);
- for_each_possible_cpu(cpu)
+ for_each_cpu_mask(cpu, rcu_cpu_online_map)
if (per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
RCU_TRACE_ME(rcupreempt_trace_try_flip_me1);
return 0;
@@ -637,6 +639,98 @@ void rcu_advance_callbacks(int cpu, int user)
spin_unlock_irqrestore(&rdp->lock, flags);
}
+#ifdef CONFIG_HOTPLUG_CPU
+#define rcu_offline_cpu_enqueue(srclist, srctail, dstlist, dsttail) do { \
+ *dsttail = srclist; \
+ if (srclist != NULL) { \
+ dsttail = srctail; \
+ srclist = NULL; \
+ srctail = &srclist;\
+ } \
+ } while (0)
+
+void rcu_offline_cpu(int cpu)
+{
+ int i;
+ struct rcu_head *list = NULL;
+ unsigned long flags;
+ struct rcu_data *rdp = RCU_DATA_CPU(cpu);
+ struct rcu_head **tail = &list;
+
+ /*
+ * Remove all callbacks from the newly dead CPU, retaining order.
+ * Otherwise rcu_barrier() will fail
+ */
+
+ spin_lock_irqsave(&rdp->lock, flags);
+ rcu_offline_cpu_enqueue(rdp->donelist, rdp->donetail, list, tail);
+ for (i = GP_STAGES - 1; i >= 0; i--)
+ rcu_offline_cpu_enqueue(rdp->waitlist[i], rdp->waittail[i],
+ list, tail);
+ rcu_offline_cpu_enqueue(rdp->nextlist, rdp->nexttail, list, tail);
+ spin_unlock_irqrestore(&rdp->lock, flags);
+ rdp->waitlistcount = 0;
+
+ /* Disengage the newly dead CPU from the grace-period computation. */
+
+ spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
+ rcu_check_mb(cpu);
+ if (per_cpu(rcu_flip_flag, cpu) == rcu_flipped) {
+ smp_mb(); /* Subsequent counter accesses must see new value */
+ per_cpu(rcu_flip_flag, cpu) = rcu_flip_seen;
+ smp_mb(); /* Subsequent RCU read-side critical sections */
+ /* seen -after- acknowledgement. */
+ }
+
+ RCU_DATA_ME()->rcu_flipctr[0] += RCU_DATA_CPU(cpu)->rcu_flipctr[0];
+ RCU_DATA_ME()->rcu_flipctr[1] += RCU_DATA_CPU(cpu)->rcu_flipctr[1];
+
+ RCU_DATA_CPU(cpu)->rcu_flipctr[0] = 0;
+ RCU_DATA_CPU(cpu)->rcu_flipctr[1] = 0;
+
+ cpu_clear(cpu, rcu_cpu_online_map);
+
+ spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);
+
+ /*
+ * Place the removed callbacks on the current CPU's queue.
+ * Make them all start a new grace period: simple approach,
+ * in theory could starve a given set of callbacks, but
+ * you would need to be doing some serious CPU hotplugging
+ * to make this happen. If this becomes a problem, adding
+ * a synchronize_rcu() to the hotplug path would be a simple
+ * fix.
+ */
+
+ rdp = RCU_DATA_ME();
+ spin_lock_irqsave(&rdp->lock, flags);
+ *rdp->nexttail = list;
+ if (list)
+ rdp->nexttail = tail;
+ spin_unlock_irqrestore(&rdp->lock, flags);
+}
+
+void __devinit rcu_online_cpu(int cpu)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&rcu_ctrlblk.fliplock, flags);
+ cpu_set(cpu, rcu_cpu_online_map);
+ spin_unlock_irqrestore(&rcu_ctrlblk.fliplock, flags);
+}
+
+#else /* #ifdef CONFIG_HOTPLUG_CPU */
+
+void rcu_offline_cpu(int cpu)
+{
+}
+
+void __devinit rcu_online_cpu(int cpu)
+{
+}
+
+#endif /* #else #ifdef CONFIG_HOTPLUG_CPU */
+
static void rcu_process_callbacks(struct softirq_action *unused)
{
unsigned long flags;
@@ -746,6 +840,32 @@ int rcu_pending(int cpu)
return 0;
}
+static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
+ unsigned long action, void *hcpu)
+{
+ long cpu = (long)hcpu;
+
+ switch (action) {
+ case CPU_UP_PREPARE:
+ case CPU_UP_PREPARE_FROZEN:
+ rcu_online_cpu(cpu);
+ break;
+ case CPU_UP_CANCELED:
+ case CPU_UP_CANCELED_FROZEN:
+ case CPU_DEAD:
+ case CPU_DEAD_FROZEN:
+ rcu_offline_cpu(cpu);
+ break;
+ default:
+ break;
+ }
+ return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata rcu_nb = {
+ .notifier_call = rcu_cpu_notify,
+};
+
void __init __rcu_init(void)
{
int cpu;
@@ -769,6 +889,23 @@ void __init __rcu_init(void)
rdp->rcu_flipctr[0] = 0;
rdp->rcu_flipctr[1] = 0;
}
+ register_cpu_notifier(&rcu_nb);
+
+ /*
+ * We don't need protection against CPU-Hotplug here
+ * since
+ * a) If a CPU comes online while we are iterating over the
+ * cpu_online_map below, we would only end up making a
+ * duplicate call to rcu_online_cpu() which sets the corresponding
+ * CPU's mask in the rcu_cpu_online_map.
+ *
+ * b) A CPU cannot go offline at this point in time since the user
+ * does not have access to the sysfs interface, nor do we
+ * suspend the system.
+ */
+ for_each_online_cpu(cpu)
+ rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE, (void *)(long) cpu);
+
open_softirq(RCU_SOFTIRQ, rcu_process_callbacks, NULL);
}
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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