[PATCH RFC] v2 expedited "big hammer" RCU grace periods

Paul E. McKenney
2009-04-26T05:23:59Z

Second cut of "big hammer" expedited RCU grace periods, but only for
rcu_bh. This creates another softirq vector, so that entering this
softirq vector will have forced an rcu_bh quiescent state (as noted by
Dave Miller). Use smp_call_function() to invoke raise_softirq() on all
CPUs in order to cause this to happen. Track the CPUs that have passed
through a quiescent state (or gone offline) with a cpumask.

Does nothing to expedite callbacks already registered with call_rcu_bh(),
but there is no need to.

Passes light rcutorture testing. Grace periods take about 28 microseconds
on an 8CPU 1.5GHZ Power box.

Shortcomings:

o Needs more testing, still not for inclusion.

o Does not handle rcu, only rcu_bh.

Changes since v1:

o Added rcutorture support, and added exports required by
rcutorture.

o Added comment stating that smp_call_function() implies a
memory barrier, suggested by Mathieu.

o Added #include for delay.h.

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

include/linux/interrupt.h | 1
include/linux/rcupdate.h | 2
kernel/rcupdate.c | 115 +++++++++++++++++++++++++
kernel/rcutorture.c | 205 +++++++++++++++++++++++++---------------------
4 files changed, 231 insertions(+), 92 deletions(-)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 91bb76f..b7b58cc 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -338,6 +338,7 @@ enum
TASKLET_SOFTIRQ,
SCHED_SOFTIRQ,
HRTIMER_SOFTIRQ,
+ RCU_EXPEDITED_SOFTIRQ,
RCU_SOFTIRQ, /* Preferable RCU should always be the last softirq */

NR_SOFTIRQS
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 15fbb3c..a606b42 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -264,6 +264,8 @@ extern void synchronize_rcu(void);
extern void rcu_barrier(void);
extern void rcu_barrier_bh(void);
extern void rcu_barrier_sched(void);
+extern void synchronize_rcu_bh_expedited(void);
+extern long rcu_batches_completed_bh_expedited(void);

/* Internal to kernel */
extern void rcu_init(void);
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index a967c9f..e995e73 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -45,6 +45,7 @@
#include <linux/mutex.h>
#include <linux/module.h>
#include <linux/kernel_stat.h>
+#include <linux/delay.h>

enum rcu_barrier {
RCU_BARRIER_STD,
@@ -217,10 +218,124 @@ static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
return NOTIFY_OK;
}

+static DEFINE_MUTEX(synchronize_rcu_bh_mutex);
+static long synchronize_rcu_bh_completed; /* Expedited-grace-period count. */
+
+long rcu_batches_completed_bh_expedited(void)
+{
+ return synchronize_rcu_bh_completed;
+}
+EXPORT_SYMBOL_GPL(rcu_batches_completed_bh_expedited);
+
+#ifndef CONFIG_SMP
+
+static void __init synchronize_rcu_expedited_init(void)
+{
+}
+
+void synchronize_rcu_bh_expedited(void)
+{
+ mutex_lock(&synchronize_rcu_bh_mutex);
+ synchronize_rcu_bh_completed++;
+ mutex_unlock(&synchronize_rcu_bh_mutex);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_bh_expedited);
+
+#else /* #ifndef CONFIG_SMP */
+
+static DEFINE_PER_CPU(int, rcu_bh_need_qs);
+static cpumask_var_t rcu_bh_waiting_map;
+
+static void synchronize_rcu_bh_expedited_help(struct softirq_action *unused)
+{
+ if (__get_cpu_var(rcu_bh_need_qs)) {
+ smp_mb();
+ __get_cpu_var(rcu_bh_need_qs) = 0;
+ smp_mb();
+ }
+}
+
+static void rcu_bh_fast_qs(void *unused)
+{
+ raise_softirq(RCU_EXPEDITED_SOFTIRQ);
+}
+
+static void __init synchronize_rcu_expedited_init(void)
+{
+ open_softirq(RCU_EXPEDITED_SOFTIRQ, synchronize_rcu_bh_expedited_help);
+ alloc_bootmem_cpumask_var(&rcu_bh_waiting_map);
+}
+
+void synchronize_rcu_bh_expedited(void)
+{
+ int cpu;
+ int done;
+ int times = 0;
+
+ mutex_lock(&synchronize_rcu_bh_mutex);
+
+ /* Take snapshot of online CPUs, blocking CPU hotplug. */
+ preempt_disable();
+ cpumask_copy(rcu_bh_waiting_map, &cpu_online_map);
+ preempt_enable();
+
+ /* Mark each online CPU as needing a quiescent state. */
+ for_each_cpu(cpu, rcu_bh_waiting_map)
+ per_cpu(rcu_bh_need_qs, cpu) = 1;
+
+ /* Call for a quiescent state on each online CPU. */
+ preempt_disable();
+ cpumask_clear_cpu(smp_processor_id(), rcu_bh_waiting_map);
+ smp_call_function(rcu_bh_fast_qs, NULL, 1); /* implies smp_mb(). */
+ preempt_enable();
+
+ /*
+ * Loop waiting for each CPU to either pass through a quiescent
+ * state or to go offline. We don't care which.
+ */
+ for (;;) {
+
+ /* Ignore CPUs that have gone offline, blocking CPU hotplug. */
+ preempt_disable();
+ cpumask_and(rcu_bh_waiting_map, rcu_bh_waiting_map,
+ &cpu_online_map);
+ cpumask_clear_cpu(smp_processor_id(), rcu_bh_waiting_map);
+ preempt_enable();
+
+ /* Check if any CPUs still need a quiescent state. */
+ done = 1;
+ for_each_cpu(cpu, rcu_bh_waiting_map) {
+ if (per_cpu(rcu_bh_need_qs, cpu)) {
+ done = 0;
+ break;
+ }
+ cpumask_clear_cpu(cpu, rcu_bh_waiting_map);
+ }
+ if (done)
+ break;
+
+ /*
+ * Wait a bit. If we have already waited a fair
+ * amount of time, sleep.
+ */
+ if (++times < 10)
+ udelay(10 * times);
+ else
+ schedule_timeout_uninterruptible(1);
+ }
+
+ synchronize_rcu_bh_completed++;
+ mutex_unlock(&synchronize_rcu_bh_mutex);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_bh_expedited);
+
+#endif /* #else #ifndef CONFIG_SMP */
+
void __init rcu_init(void)
{
__rcu_init();
hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);
+ synchronize_rcu_expedited_init();
}

void rcu_scheduler_starting(void)
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 9b4a975..8845936 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -257,14 +257,14 @@ struct rcu_torture_ops {
void (*init)(void);
void (*cleanup)(void);
int (*readlock)(void);
- void (*readdelay)(struct rcu_random_state *rrsp);
+ void (*read_delay)(struct rcu_random_state *rrsp);
void (*readunlock)(int idx);
int (*completed)(void);
- void (*deferredfree)(struct rcu_torture *p);
+ void (*deferred_free)(struct rcu_torture *p);
void (*sync)(void);
void (*cb_barrier)(void);
int (*stats)(char *page);
- int irqcapable;
+ int irq_capable;
char *name;
};
static struct rcu_torture_ops *cur_ops = NULL;
@@ -320,7 +320,7 @@ rcu_torture_cb(struct rcu_head *p)
rp->rtort_mbtest = 0;
rcu_torture_free(rp);
} else
- cur_ops->deferredfree(rp);
+ cur_ops->deferred_free(rp);
}

static void rcu_torture_deferred_free(struct rcu_torture *p)
@@ -329,18 +329,18 @@ static void rcu_torture_deferred_free(struct rcu_torture *p)
}

static struct rcu_torture_ops rcu_ops = {
- .init = NULL,
- .cleanup = NULL,
- .readlock = rcu_torture_read_lock,
- .readdelay = rcu_read_delay,
- .readunlock = rcu_torture_read_unlock,
- .completed = rcu_torture_completed,
- .deferredfree = rcu_torture_deferred_free,
- .sync = synchronize_rcu,
- .cb_barrier = rcu_barrier,
- .stats = NULL,
- .irqcapable = 1,
- .name = "rcu"
+ .init = NULL,
+ .cleanup = NULL,
+ .readlock = rcu_torture_read_lock,
+ .read_delay = rcu_read_delay,
+ .readunlock = rcu_torture_read_unlock,
+ .completed = rcu_torture_completed,
+ .deferred_free = rcu_torture_deferred_free,
+ .sync = synchronize_rcu,
+ .cb_barrier = rcu_barrier,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "rcu"
};

static void rcu_sync_torture_deferred_free(struct rcu_torture *p)
@@ -370,18 +370,18 @@ static void rcu_sync_torture_init(void)
}

static struct rcu_torture_ops rcu_sync_ops = {
- .init = rcu_sync_torture_init,
- .cleanup = NULL,
- .readlock = rcu_torture_read_lock,
- .readdelay = rcu_read_delay,
- .readunlock = rcu_torture_read_unlock,
- .completed = rcu_torture_completed,
- .deferredfree = rcu_sync_torture_deferred_free,
- .sync = synchronize_rcu,
- .cb_barrier = NULL,
- .stats = NULL,
- .irqcapable = 1,
- .name = "rcu_sync"
+ .init = rcu_sync_torture_init,
+ .cleanup = NULL,
+ .readlock = rcu_torture_read_lock,
+ .read_delay = rcu_read_delay,
+ .readunlock = rcu_torture_read_unlock,
+ .completed = rcu_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = synchronize_rcu,
+ .cb_barrier = NULL,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "rcu_sync"
};

/*
@@ -432,33 +432,53 @@ static void rcu_bh_torture_synchronize(void)
}

static struct rcu_torture_ops rcu_bh_ops = {
- .init = NULL,
- .cleanup = NULL,
- .readlock = rcu_bh_torture_read_lock,
- .readdelay = rcu_read_delay, /* just reuse rcu's version. */
- .readunlock = rcu_bh_torture_read_unlock,
- .completed = rcu_bh_torture_completed,
- .deferredfree = rcu_bh_torture_deferred_free,
- .sync = rcu_bh_torture_synchronize,
- .cb_barrier = rcu_barrier_bh,
- .stats = NULL,
- .irqcapable = 1,
- .name = "rcu_bh"
+ .init = NULL,
+ .cleanup = NULL,
+ .readlock = rcu_bh_torture_read_lock,
+ .read_delay = rcu_read_delay, /* just reuse rcu's version. */
+ .readunlock = rcu_bh_torture_read_unlock,
+ .completed = rcu_bh_torture_completed,
+ .deferred_free = rcu_bh_torture_deferred_free,
+ .sync = rcu_bh_torture_synchronize,
+ .cb_barrier = rcu_barrier_bh,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "rcu_bh"
};

static struct rcu_torture_ops rcu_bh_sync_ops = {
- .init = rcu_sync_torture_init,
- .cleanup = NULL,
- .readlock = rcu_bh_torture_read_lock,
- .readdelay = rcu_read_delay, /* just reuse rcu's version. */
- .readunlock = rcu_bh_torture_read_unlock,
- .completed = rcu_bh_torture_completed,
- .deferredfree = rcu_sync_torture_deferred_free,
- .sync = rcu_bh_torture_synchronize,
- .cb_barrier = NULL,
- .stats = NULL,
- .irqcapable = 1,
- .name = "rcu_bh_sync"
+ .init = rcu_sync_torture_init,
+ .cleanup = NULL,
+ .readlock = rcu_bh_torture_read_lock,
+ .read_delay = rcu_read_delay, /* just reuse rcu's version. */
+ .readunlock = rcu_bh_torture_read_unlock,
+ .completed = rcu_bh_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = rcu_bh_torture_synchronize,
+ .cb_barrier = NULL,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "rcu_bh_sync"
+};
+
+static int rcu_bh_expedited_torture_completed(void)
+{
+ return rcu_batches_completed_bh_expedited();
+}
+
+static struct rcu_torture_ops rcu_bh_expedited_ops = {
+ .init = rcu_sync_torture_init,
+ .cleanup = NULL,
+ .readlock = rcu_bh_torture_read_lock,
+ .read_delay = rcu_read_delay, /* just reuse rcu's version. */
+ .readunlock = rcu_bh_torture_read_unlock,
+ .completed = rcu_bh_expedited_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = synchronize_rcu_bh_expedited,
+ .cb_barrier = NULL,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "rcu_bh_expedited"
};

/*
@@ -530,17 +550,17 @@ static int srcu_torture_stats(char *page)
}

static struct rcu_torture_ops srcu_ops = {
- .init = srcu_torture_init,
- .cleanup = srcu_torture_cleanup,
- .readlock = srcu_torture_read_lock,
- .readdelay = srcu_read_delay,
- .readunlock = srcu_torture_read_unlock,
- .completed = srcu_torture_completed,
- .deferredfree = rcu_sync_torture_deferred_free,
- .sync = srcu_torture_synchronize,
- .cb_barrier = NULL,
- .stats = srcu_torture_stats,
- .name = "srcu"
+ .init = srcu_torture_init,
+ .cleanup = srcu_torture_cleanup,
+ .readlock = srcu_torture_read_lock,
+ .read_delay = srcu_read_delay,
+ .readunlock = srcu_torture_read_unlock,
+ .completed = srcu_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = srcu_torture_synchronize,
+ .cb_barrier = NULL,
+ .stats = srcu_torture_stats,
+ .name = "srcu"
};

/*
@@ -574,32 +594,32 @@ static void sched_torture_synchronize(void)
}

static struct rcu_torture_ops sched_ops = {
- .init = rcu_sync_torture_init,
- .cleanup = NULL,
- .readlock = sched_torture_read_lock,
- .readdelay = rcu_read_delay, /* just reuse rcu's version. */
- .readunlock = sched_torture_read_unlock,
- .completed = sched_torture_completed,
- .deferredfree = rcu_sched_torture_deferred_free,
- .sync = sched_torture_synchronize,
- .cb_barrier = rcu_barrier_sched,
- .stats = NULL,
- .irqcapable = 1,
- .name = "sched"
+ .init = rcu_sync_torture_init,
+ .cleanup = NULL,
+ .readlock = sched_torture_read_lock,
+ .read_delay = rcu_read_delay, /* just reuse rcu's version. */
+ .readunlock = sched_torture_read_unlock,
+ .completed = sched_torture_completed,
+ .deferred_free = rcu_sched_torture_deferred_free,
+ .sync = sched_torture_synchronize,
+ .cb_barrier = rcu_barrier_sched,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "sched"
};

static struct rcu_torture_ops sched_ops_sync = {
- .init = rcu_sync_torture_init,
- .cleanup = NULL,
- .readlock = sched_torture_read_lock,
- .readdelay = rcu_read_delay, /* just reuse rcu's version. */
- .readunlock = sched_torture_read_unlock,
- .completed = sched_torture_completed,
- .deferredfree = rcu_sync_torture_deferred_free,
- .sync = sched_torture_synchronize,
- .cb_barrier = NULL,
- .stats = NULL,
- .name = "sched_sync"
+ .init = rcu_sync_torture_init,
+ .cleanup = NULL,
+ .readlock = sched_torture_read_lock,
+ .read_delay = rcu_read_delay, /* just reuse rcu's version. */
+ .readunlock = sched_torture_read_unlock,
+ .completed = sched_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = sched_torture_synchronize,
+ .cb_barrier = NULL,
+ .stats = NULL,
+ .name = "sched_sync"
};

/*
@@ -635,7 +655,7 @@ rcu_torture_writer(void *arg)
i = RCU_TORTURE_PIPE_LEN;
atomic_inc(&rcu_torture_wcount[i]);
old_rp->rtort_pipe_count++;
- cur_ops->deferredfree(old_rp);
+ cur_ops->deferred_free(old_rp);
}
rcu_torture_current_version++;
oldbatch = cur_ops->completed();
@@ -700,7 +720,7 @@ static void rcu_torture_timer(unsigned long unused)
if (p->rtort_mbtest == 0)
atomic_inc(&n_rcu_torture_mberror);
spin_lock(&rand_lock);
- cur_ops->readdelay(&rand);
+ cur_ops->read_delay(&rand);
n_rcu_torture_timers++;
spin_unlock(&rand_lock);
preempt_disable();
@@ -738,11 +758,11 @@ rcu_torture_reader(void *arg)

VERBOSE_PRINTK_STRING("rcu_torture_reader task started");
set_user_nice(current, 19);
- if (irqreader && cur_ops->irqcapable)
+ if (irqreader && cur_ops->irq_capable)
setup_timer_on_stack(&t, rcu_torture_timer, 0);

do {
- if (irqreader && cur_ops->irqcapable) {
+ if (irqreader && cur_ops->irq_capable) {
if (!timer_pending(&t))
mod_timer(&t, 1);
}
@@ -757,7 +777,7 @@ rcu_torture_reader(void *arg)
}
if (p->rtort_mbtest == 0)
atomic_inc(&n_rcu_torture_mberror);
- cur_ops->readdelay(&rand);
+ cur_ops->read_delay(&rand);
preempt_disable();
pipe_count = p->rtort_pipe_count;
if (pipe_count > RCU_TORTURE_PIPE_LEN) {
@@ -778,7 +798,7 @@ rcu_torture_reader(void *arg)
} while (!kthread_should_stop() && fullstop == FULLSTOP_DONTSTOP);
VERBOSE_PRINTK_STRING("rcu_torture_reader task stopping");
rcutorture_shutdown_absorb("rcu_torture_reader");
- if (irqreader && cur_ops->irqcapable)
+ if (irqreader && cur_ops->irq_capable)
del_timer_sync(&t);
while (!kthread_should_stop())
schedule_timeout_uninterruptible(1);
@@ -1078,6 +1098,7 @@ rcu_torture_init(void)
int firsterr = 0;
static struct rcu_torture_ops *torture_ops[] =
{ &rcu_ops, &rcu_sync_ops, &rcu_bh_ops, &rcu_bh_sync_ops,
+ &rcu_bh_expedited_ops,
&srcu_ops, &sched_ops, &sched_ops_sync, };

mutex_lock(&fullstop_mutex);