[PATCH RFC] v7 expedited "big hammer" RCU grace periods

Paul E. McKenney
2009-05-22T19:05:42Z

Seventh cut of "big hammer" expedited RCU grace periods. This leverages
the existing per-CPU migration kthreads, as suggested by Ingo. These
are awakened in a loop, and waited for in a second loop. Not fully
scalable, but removing the extra hop through smp_call_function
reduces latency on systems with moderate numbers of CPUs. The
synchronize_rcu_expedited() and and synchronize_bh_expedited() primitives
invoke synchronize_sched_expedited(), except for CONFIG_PREEMPT_RCU,
where they instead invoke synchronize_rcu() and synchronize_rcu_bh(),
respectively. This will be fixed in the future, after preemptable RCU
is folded into the rcutree implementation.

As before, this does nothing to expedite callbacks already registered
with call_rcu() or call_rcu_bh(), but there is no need to.

Passes 10 hours of rcutorture testing in parallel with a script that
randomly offlines and onlines CPUs. Grace periods take about 28
microseconds on an 8-CPU Power machine, which I believe is good enough
from a performance viewpoint for the near future.

This is not yet for inclusion. This does seem to me to be a good-enough
implementation, but more testing and review is required.

Shortcomings:

o Does not address preemptable RCU.

o Probably not helpful on systems with thousands of CPUs, but likely
quite helpful even on systems with a few hundreds of CPUs.

Changes since v6:

o Moved to using the migration threads, as suggested by Ingo.

Changes since v5:

o Fixed several embarrassing locking bugs, including those
noted by Ingo and Lai.

o Added a missing set of braces.

o Cut out the extra kthread, so that synchronize_sched_expedited()
directly calls smp_call_function() and waits for the quiescent
states.

o Removed some debug code, but promoted one to production.

o Fix a compiler warning.

Changes since v4:

o Use per-CPU kthreads to force the quiescent states in parallel.

Changes since v3:

o Use a kthread that schedules itself on each CPU in turn to
force a grace period. The synchronize_rcu() primitive
wakes up the kthread in order to avoid messing with affinity
masks on user tasks.

o Tried a number of additional variations on the v3 approach, none
of which helped much.

Changes since v2:

o Use reschedule IPIs rather than a softirq.

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/rcuclassic.h | 15 +++
include/linux/rcupdate.h | 24 ++---
include/linux/rcupreempt.h | 10 ++
include/linux/rcutree.h | 12 ++
kernel/rcupdate.c | 27 ++++++
kernel/rcutorture.c | 200 ++++++++++++++++++++++++---------------------
kernel/sched.c | 52 +++++++++++
7 files changed, 233 insertions(+), 107 deletions(-)

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index bfd92e1..b4b4fe1 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -158,14 +158,27 @@ extern struct lockdep_map rcu_lock_map;

#define call_rcu_sched(head, func) call_rcu(head, func)

+static inline void synchronize_rcu_expedited(void)
+{
+ synchronize_sched_expedited();
+}
+
+static inline void synchronize_rcu_bh_expedited(void)
+{
+ synchronize_sched_expedited();
+}
+
extern void __rcu_init(void);
-#define rcu_init_sched() do { } while (0)
extern void rcu_check_callbacks(int cpu, int user);
extern void rcu_restart_cpu(int cpu);

extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

+static inline void rcu_init_sched(void)
+{
+}
+
#define rcu_enter_nohz() do { } while (0)
#define rcu_exit_nohz() do { } while (0)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 15fbb3c..d67dfce 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -51,7 +51,18 @@ struct rcu_head {
void (*func)(struct rcu_head *head);
};

-/* Internal to kernel, but needed by rcupreempt.h. */
+/* Exported common interfaces */
+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_sched_expedited(void);
+extern int sched_expedited_torture_stats(char *page);
+
+/* Internal to kernel */
+extern void rcu_init(void);
+extern void rcu_scheduler_starting(void);
+extern int rcu_needs_cpu(int cpu);
extern int rcu_scheduler_active;

#if defined(CONFIG_CLASSIC_RCU)
@@ -259,15 +270,4 @@ extern void call_rcu(struct rcu_head *head,
extern void call_rcu_bh(struct rcu_head *head,
void (*func)(struct rcu_head *head));

-/* Exported common interfaces */
-extern void synchronize_rcu(void);
-extern void rcu_barrier(void);
-extern void rcu_barrier_bh(void);
-extern void rcu_barrier_sched(void);
-
-/* Internal to kernel */
-extern void rcu_init(void);
-extern void rcu_scheduler_starting(void);
-extern int rcu_needs_cpu(int cpu);
-
#endif /* __LINUX_RCUPDATE_H */
diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h
index fce5227..78117ed 100644
--- a/include/linux/rcupreempt.h
+++ b/include/linux/rcupreempt.h
@@ -74,6 +74,16 @@ extern int rcu_needs_cpu(int cpu);

extern void __synchronize_sched(void);

+static inline void synchronize_rcu_expedited(void)
+{
+ synchronize_rcu(); /* Placeholder for new rcupreempt implementation. */
+}
+
+static inline void synchronize_rcu_bh_expedited(void)
+{
+ synchronize_rcu(); /* Placeholder for new rcupreempt implementation. */
+}
+
extern void __rcu_init(void);
extern void rcu_init_sched(void);
extern void rcu_check_callbacks(int cpu, int user);
diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 58b2aa5..41c23cb 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -279,8 +279,14 @@ static inline void __rcu_read_unlock_bh(void)

#define call_rcu_sched(head, func) call_rcu(head, func)

-static inline void rcu_init_sched(void)
+static inline void synchronize_rcu_expedited(void)
+{
+ synchronize_sched_expedited();
+}
+
+static inline void synchronize_rcu_bh_expedited(void)
{
+ synchronize_sched_expedited();
}

extern void __rcu_init(void);
@@ -290,6 +296,10 @@ extern void rcu_restart_cpu(int cpu);
extern long rcu_batches_completed(void);
extern long rcu_batches_completed_bh(void);

+static inline void rcu_init_sched(void)
+{
+}
+
#ifdef CONFIG_NO_HZ
void rcu_enter_nohz(void);
void rcu_exit_nohz(void);
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index a967c9f..454995f 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -45,6 +45,8 @@
#include <linux/mutex.h>
#include <linux/module.h>
#include <linux/kernel_stat.h>
+#include <linux/delay.h>
+#include <linux/kthread.h>

enum rcu_barrier {
RCU_BARRIER_STD,
@@ -98,6 +100,30 @@ void synchronize_rcu(void)
}
EXPORT_SYMBOL_GPL(synchronize_rcu);

+/**
+ * synchronize_rcu_bh - wait until an rcu_bh grace period has elapsed.
+ *
+ * Control will return to the caller some time after a full rcu_bh grace
+ * period has elapsed, in other words after all currently executing rcu_bh
+ * read-side critical sections have completed. RCU read-side critical
+ * sections are delimited by rcu_read_lock_bh() and rcu_read_unlock_bh(),
+ * and may be nested.
+ */
+void synchronize_rcu_bh(void)
+{
+ struct rcu_synchronize rcu;
+
+ if (rcu_blocking_is_gp())
+ return;
+
+ init_completion(&rcu.completion);
+ /* Will wake me after RCU finished. */
+ call_rcu_bh(&rcu.head, wakeme_after_rcu);
+ /* Wait for it. */
+ wait_for_completion(&rcu.completion);
+}
+EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
+
static void rcu_barrier_callback(struct rcu_head *notused)
{
if (atomic_dec_and_test(&rcu_barrier_cpu_count))
@@ -129,6 +155,7 @@ static void rcu_barrier_func(void *type)
static inline void wait_migrated_callbacks(void)
{
wait_event(rcu_migrate_wq, !atomic_read(&rcu_migrate_type_count));
+ smp_mb(); /* In case we didn't sleep. */
}

/*
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 9b4a975..d3a1e56 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 +432,33 @@ 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"
};

/*
@@ -530,17 +530,17 @@ 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 +574,47 @@ 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"
+};
+
+static struct rcu_torture_ops sched_expedited_ops = {
+ .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 = synchronize_sched_expedited,
+ .cb_barrier = NULL,
+ .stats = NULL,
+ .irq_capable = 1,
+ .name = "sched_expedited"
};

/*
@@ -635,7 +650,7 @@ 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 +715,7 @@ 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 +753,11 @@ 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 +772,7 @@ 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 +793,7 @@ 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 +1093,7 @@ 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,
+ &sched_expedited_ops,
&srcu_ops, &sched_ops, &sched_ops_sync, };

mutex_lock(&fullstop_mutex);
diff --git a/kernel/sched.c b/kernel/sched.c
index 26efa47..d6d4fc3 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -6772,7 +6772,8 @@ static int migration_thread(void *data)
list_del_init(head->next);

spin_unlock(&rq->lock);
- __migrate_task(req->task, cpu, req->dest_cpu);
+ if (req->task != NULL)
+ __migrate_task(req->task, cpu, req->dest_cpu);
local_irq_enable();

complete(&req->done);
@@ -10239,3 +10240,52 @@ struct cgroup_subsys cpuacct_subsys = {
.subsys_id = cpuacct_subsys_id,
};
#endif /* CONFIG_CGROUP_CPUACCT */
+
+#ifndef CONFIG_SMP
+
+void synchronize_sched_expedited(void)
+{
+}
+EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
+
+#else /* #ifndef CONFIG_SMP */
+
+static DEFINE_PER_CPU(struct migration_req, rcu_migration_req);
+static DEFINE_MUTEX(rcu_sched_expedited_mutex);
+
+/*
+ * Wait for an rcu-sched grace period to elapse, but use "big hammer"
+ * approach to force grace period to end quickly. This consumes
+ * significant time on all CPUs, and is thus not recommended for
+ * any sort of common-case code.
+ */
+void synchronize_sched_expedited(void)
+{
+ int cpu;
+ unsigned long flags;
+ struct rq *rq;
+ struct migration_req *req;
+
+ mutex_lock(&rcu_sched_expedited_mutex);
+ get_online_cpus();
+ for_each_online_cpu(cpu) {
+ rq = cpu_rq(cpu);
+ req = &per_cpu(rcu_migration_req, cpu);
+ init_completion(&req->done);
+ req->task = NULL;
+ req->dest_cpu = -1;
+ spin_lock_irqsave(&rq->lock, flags);
+ list_add(&req->list, &rq->migration_queue);
+ spin_unlock_irqrestore(&rq->lock, flags);
+ wake_up_process(rq->migration_thread);
+ }
+ for_each_online_cpu(cpu) {
+ req = &per_cpu(rcu_migration_req, cpu);
+ wait_for_completion(&req->done);
+ }
+ put_online_cpus();
+ mutex_unlock(&rcu_sched_expedited_mutex);
+}
+EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
+
+#endif /* #else #ifndef CONFIG_SMP */