Commit-ID: 931ea9d1a6e06a5e3af03aa4aaaa7c7fd90e163f
Gitweb: http://git.kernel.org/tip/931ea9d1a6e06a5e3af03aa4aaaa7c7fd90e163f
Author: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
AuthorDate: Mon, 19 Mar 2012 16:12:13 +0800
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon, 30 Apr 2012 10:48:25 -0700
rcu: Implement per-domain single-threaded call_srcu() state machine
This commit implements an SRCU state machine in support of call_srcu().
The state machine is preemptible, light-weight, and single-threaded,
minimizing synchronization overhead. In particular, there is no longer
any need for synchronize_srcu() to be guarded by a mutex.
Expedited processing is handled, at least in the absence of concurrent
grace-period operations on that same srcu_struct structure, by having
the synchronize_srcu_expedited() thread take on the role of the
workqueue thread for one iteration.
There is a reasonable probability that a given SRCU callback will
be invoked on the same CPU that registered it, however, there is no
guarantee. Concurrent SRCU grace-period primitives can cause callbacks
to be executed elsewhere, even in absence of CPU-hotplug operations.
Callbacks execute in process context, but under the influence of
local_bh_disable(), so it is illegal to sleep in an SRCU callback
function.
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
Acked-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
include/linux/srcu.h | 37 +++++-
kernel/srcu.c | 362 +++++++++++++++++++++++++++++++++++++++++---------
2 files changed, 336 insertions(+), 63 deletions(-)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index e5ce804..55a5c52 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -29,16 +29,30 @@
#include <linux/mutex.h>
#include <linux/rcupdate.h>
+#include <linux/workqueue.h>
struct srcu_struct_array {
unsigned long c[2];
unsigned long seq[2];
};
+struct rcu_batch {
+ struct rcu_head *head, **tail;
+};
+
struct srcu_struct {
unsigned completed;
struct srcu_struct_array __percpu *per_cpu_ref;
- struct mutex mutex;
+ spinlock_t queue_lock; /* protect ->batch_queue, ->running */
+ bool running;
+ /* callbacks just queued */
+ struct rcu_batch batch_queue;
+ /* callbacks try to do the first check_zero */
+ struct rcu_batch batch_check0;
+ /* callbacks done with the first check_zero and the flip */
+ struct rcu_batch batch_check1;
+ struct rcu_batch batch_done;
+ struct delayed_work work;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
@@ -62,12 +76,33 @@ int init_srcu_struct(struct srcu_struct *sp);
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
+/**
+ * call_srcu() - Queue a callback for invocation after an SRCU grace period
+ * @sp: srcu_struct in queue the callback
+ * @head: structure to be used for queueing the SRCU callback.
+ * @func: function to be invoked after the SRCU grace period
+ *
+ * The callback function will be invoked some time after a full SRCU
+ * grace period elapses, in other words after all pre-existing SRCU
+ * read-side critical sections have completed. However, the callback
+ * function might well execute concurrently with other SRCU read-side
+ * critical sections that started after call_srcu() was invoked. SRCU
+ * read-side critical sections are delimited by srcu_read_lock() and
+ * srcu_read_unlock(), and may be nested.
+ *
+ * The callback will be invoked from process context, but must nevertheless
+ * be fast and must not block.
+ */
+void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
+ void (*func)(struct rcu_head *head));
+
void cleanup_srcu_struct(struct srcu_struct *sp);
int __srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
void __srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
void synchronize_srcu(struct srcu_struct *sp);
void synchronize_srcu_expedited(struct srcu_struct *sp);
long srcu_batches_completed(struct srcu_struct *sp);
+void srcu_barrier(struct srcu_struct *sp);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
diff --git a/kernel/srcu.c b/kernel/srcu.c
index b908852..2095be3 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -34,10 +34,77 @@
#include <linux/delay.h>
#include <linux/srcu.h>
+/*
+ * Initialize an rcu_batch structure to empty.
+ */
+static inline void rcu_batch_init(struct rcu_batch *b)
+{
+ b->head = NULL;
+ b->tail = &b->head;
+}
+
+/*
+ * Enqueue a callback onto the tail of the specified rcu_batch structure.
+ */
+static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
+{
+ *b->tail = head;
+ b->tail = &head->next;
+}
+
+/*
+ * Is the specified rcu_batch structure empty?
+ */
+static inline bool rcu_batch_empty(struct rcu_batch *b)
+{
+ return b->tail == &b->head;
+}
+
+/*
+ * Remove the callback at the head of the specified rcu_batch structure
+ * and return a pointer to it, or return NULL if the structure is empty.
+ */
+static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
+{
+ struct rcu_head *head;
+
+ if (rcu_batch_empty(b))
+ return NULL;
+
+ head = b->head;
+ b->head = head->next;
+ if (b->tail == &head->next)
+ rcu_batch_init(b);
+
+ return head;
+}
+
+/*
+ * Move all callbacks from the rcu_batch structure specified by "from" to
+ * the structure specified by "to".
+ */
+static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
+{
+ if (!rcu_batch_empty(from)) {
+ *to->tail = from->head;
+ to->tail = from->tail;
+ rcu_batch_init(from);
+ }
+}
+
+/* single-thread state-machine */
+static void process_srcu(struct work_struct *work);
+
static int init_srcu_struct_fields(struct srcu_struct *sp)
{
sp->completed = 0;
- mutex_init(&sp->mutex);
+ spin_lock_init(&sp->queue_lock);
+ sp->running = false;
+ rcu_batch_init(&sp->batch_queue);
+ rcu_batch_init(&sp->batch_check0);
+ rcu_batch_init(&sp->batch_check1);
+ rcu_batch_init(&sp->batch_done);
+ INIT_DELAYED_WORK(&sp->work, process_srcu);
sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
return sp->per_cpu_ref ? 0 : -ENOMEM;
}
@@ -266,43 +333,86 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
* we repeatedly block for 1-millisecond time periods. This approach
* has done well in testing, so there is no need for a config parameter.
*/
-#define SYNCHRONIZE_SRCU_READER_DELAY 5
+#define SRCU_RETRY_CHECK_DELAY 5
#define SYNCHRONIZE_SRCU_TRYCOUNT 2
#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
/*
- * Wait until all pre-existing readers complete. Such readers
+ * @@@ Wait until all pre-existing readers complete. Such readers
* will have used the index specified by "idx".
+ * the caller should ensures the ->completed is not changed while checking
+ * and idx = (->completed & 1) ^ 1
*/
-static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
+static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
{
- /*
- * SRCU read-side critical sections are normally short, so wait
- * a small amount of time before possibly blocking.
- */
- if (!srcu_readers_active_idx_check(sp, idx)) {
- udelay(SYNCHRONIZE_SRCU_READER_DELAY);
- while (!srcu_readers_active_idx_check(sp, idx)) {
- if (trycount > 0) {
- trycount--;
- udelay(SYNCHRONIZE_SRCU_READER_DELAY);
- } else
- schedule_timeout_interruptible(1);
- }
+ for (;;) {
+ if (srcu_readers_active_idx_check(sp, idx))
+ return true;
+ if (--trycount <= 0)
+ return false;
+ udelay(SRCU_RETRY_CHECK_DELAY);
}
}
+/*
+ * Increment the ->completed counter so that future SRCU readers will
+ * use the other rank of the ->c[] and ->seq[] arrays. This allows
+ * us to wait for pre-existing readers in a starvation-free manner.
+ */
static void srcu_flip(struct srcu_struct *sp)
{
sp->completed++;
}
/*
+ * Enqueue an SRCU callback on the specified srcu_struct structure,
+ * initiating grace-period processing if it is not already running.
+ */
+void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
+ void (*func)(struct rcu_head *head))
+{
+ unsigned long flags;
+
+ head->next = NULL;
+ head->func = func;
+ spin_lock_irqsave(&sp->queue_lock, flags);
+ rcu_batch_queue(&sp->batch_queue, head);
+ if (!sp->running) {
+ sp->running = true;
+ queue_delayed_work(system_nrt_wq, &sp->work, 0);
+ }
+ spin_unlock_irqrestore(&sp->queue_lock, flags);
+}
+EXPORT_SYMBOL_GPL(call_srcu);
+
+struct rcu_synchronize {
+ struct rcu_head head;
+ struct completion completion;
+};
+
+/*
+ * Awaken the corresponding synchronize_srcu() instance now that a
+ * grace period has elapsed.
+ */
+static void wakeme_after_rcu(struct rcu_head *head)
+{
+ struct rcu_synchronize *rcu;
+
+ rcu = container_of(head, struct rcu_synchronize, head);
+ complete(&rcu->completion);
+}
+
+static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
+static void srcu_reschedule(struct srcu_struct *sp);
+
+/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
{
- int busy_idx;
+ struct rcu_synchronize rcu;
+ struct rcu_head *head = &rcu.head;
+ bool done = false;
rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
!lock_is_held(&rcu_bh_lock_map) &&
@@ -310,50 +420,32 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
!lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
- mutex_lock(&sp->mutex);
- busy_idx = sp->completed & 0X1UL;
-
- /*
- * If we recently flipped the index, there will be some readers
- * using idx=0 and others using idx=1. Therefore, two calls to
- * wait_idx()s suffice to ensure that all pre-existing readers
- * have completed:
- *
- * __synchronize_srcu() {
- * wait_idx(sp, 0, trycount);
- * wait_idx(sp, 1, trycount);
- * }
- *
- * Starvation is prevented by the fact that we flip the index.
- * While we wait on one index to clear out, almost all new readers
- * will be using the other index. The number of new readers using the
- * index we are waiting on is sharply bounded by roughly the number
- * of CPUs.
- *
- * How can new readers possibly using the old pre-flip value of
- * the index? Consider the following sequence of events:
- *
- * Suppose that during the previous grace period, a reader
- * picked up the old value of the index, but did not increment
- * its counter until after the previous instance of
- * __synchronize_srcu() did the counter summation and recheck.
- * That previous grace period was OK because the reader did
- * not start until after the grace period started, so the grace
- * period was not obligated to wait for that reader.
- *
- * However, this sequence of events is quite improbable, so
- * this call to wait_idx(), which waits on really old readers
- * describe in this comment above, will almost never need to wait.
- */
- wait_idx(sp, 1 - busy_idx, trycount);
-
- /* Flip the index to avoid reader-induced starvation. */
- srcu_flip(sp);
-
- /* Wait for recent pre-existing readers. */
- wait_idx(sp, busy_idx, trycount);
+ init_completion(&rcu.completion);
+
+ head->next = NULL;
+ head->func = wakeme_after_rcu;
+ spin_lock_irq(&sp->queue_lock);
+ if (!sp->running) {
+ /* steal the processing owner */
+ sp->running = true;
+ rcu_batch_queue(&sp->batch_check0, head);
+ spin_unlock_irq(&sp->queue_lock);
+
+ srcu_advance_batches(sp, trycount);
+ if (!rcu_batch_empty(&sp->batch_done)) {
+ BUG_ON(sp->batch_done.head != head);
+ rcu_batch_dequeue(&sp->batch_done);
+ done = true;
+ }
+ /* give the processing owner to work_struct */
+ srcu_reschedule(sp);
+ } else {
+ rcu_batch_queue(&sp->batch_queue, head);
+ spin_unlock_irq(&sp->queue_lock);
+ }
- mutex_unlock(&sp->mutex);
+ if (!done)
+ wait_for_completion(&rcu.completion);
}
/**
@@ -398,15 +490,161 @@ void synchronize_srcu_expedited(struct srcu_struct *sp)
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
/**
+ * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
+ */
+void srcu_barrier(struct srcu_struct *sp)
+{
+ synchronize_srcu(sp);
+}
+EXPORT_SYMBOL_GPL(srcu_barrier);
+
+/**
* srcu_batches_completed - return batches completed.
* @sp: srcu_struct on which to report batch completion.
*
* Report the number of batches, correlated with, but not necessarily
* precisely the same as, the number of grace periods that have elapsed.
*/
-
long srcu_batches_completed(struct srcu_struct *sp)
{
return sp->completed;
}
EXPORT_SYMBOL_GPL(srcu_batches_completed);
+
+#define SRCU_CALLBACK_BATCH 10
+#define SRCU_INTERVAL 1
+
+/*
+ * Move any new SRCU callbacks to the first stage of the SRCU grace
+ * period pipeline.
+ */
+static void srcu_collect_new(struct srcu_struct *sp)
+{
+ if (!rcu_batch_empty(&sp->batch_queue)) {
+ spin_lock_irq(&sp->queue_lock);
+ rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
+ spin_unlock_irq(&sp->queue_lock);
+ }
+}
+
+/*
+ * Core SRCU state machine. Advance callbacks from ->batch_check0 to
+ * ->batch_check1 and then to ->batch_done as readers drain.
+ */
+static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
+{
+ int idx = 1 ^ (sp->completed & 1);
+
+ /*
+ * Because readers might be delayed for an extended period after
+ * fetching ->completed for their index, at any point in time there
+ * might well be readers using both idx=0 and idx=1. We therefore
+ * need to wait for readers to clear from both index values before
+ * invoking a callback.
+ */
+
+ if (rcu_batch_empty(&sp->batch_check0) &&
+ rcu_batch_empty(&sp->batch_check1))
+ return; /* no callbacks need to be advanced */
+
+ if (!try_check_zero(sp, idx, trycount))
+ return; /* failed to advance, will try after SRCU_INTERVAL */
+
+ /*
+ * The callbacks in ->batch_check1 have already done with their
+ * first zero check and flip back when they were enqueued on
+ * ->batch_check0 in a previous invocation of srcu_advance_batches().
+ * (Presumably try_check_zero() returned false during that
+ * invocation, leaving the callbacks stranded on ->batch_check1.)
+ * They are therefore ready to invoke, so move them to ->batch_done.
+ */
+ rcu_batch_move(&sp->batch_done, &sp->batch_check1);
+
+ if (rcu_batch_empty(&sp->batch_check0))
+ return; /* no callbacks need to be advanced */
+ srcu_flip(sp);
+
+ /*
+ * The callbacks in ->batch_check0 just finished their
+ * first check zero and flip, so move them to ->batch_check1
+ * for future checking on the other idx.
+ */
+ rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
+
+ /*
+ * SRCU read-side critical sections are normally short, so check
+ * at least twice in quick succession after a flip.
+ */
+ trycount = trycount < 2 ? 2 : trycount;
+ if (!try_check_zero(sp, idx^1, trycount))
+ return; /* failed to advance, will try after SRCU_INTERVAL */
+
+ /*
+ * The callbacks in ->batch_check1 have now waited for all
+ * pre-existing readers using both idx values. They are therefore
+ * ready to invoke, so move them to ->batch_done.
+ */
+ rcu_batch_move(&sp->batch_done, &sp->batch_check1);
+}
+
+/*
+ * Invoke a limited number of SRCU callbacks that have passed through
+ * their grace period. If there are more to do, SRCU will reschedule
+ * the workqueue.
+ */
+static void srcu_invoke_callbacks(struct srcu_struct *sp)
+{
+ int i;
+ struct rcu_head *head;
+
+ for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
+ head = rcu_batch_dequeue(&sp->batch_done);
+ if (!head)
+ break;
+ local_bh_disable();
+ head->func(head);
+ local_bh_enable();
+ }
+}
+
+/*
+ * Finished one round of SRCU grace period. Start another if there are
+ * more SRCU callbacks queued, otherwise put SRCU into not-running state.
+ */
+static void srcu_reschedule(struct srcu_struct *sp)
+{
+ bool pending = true;
+
+ if (rcu_batch_empty(&sp->batch_done) &&
+ rcu_batch_empty(&sp->batch_check1) &&
+ rcu_batch_empty(&sp->batch_check0) &&
+ rcu_batch_empty(&sp->batch_queue)) {
+ spin_lock_irq(&sp->queue_lock);
+ if (rcu_batch_empty(&sp->batch_done) &&
+ rcu_batch_empty(&sp->batch_check1) &&
+ rcu_batch_empty(&sp->batch_check0) &&
+ rcu_batch_empty(&sp->batch_queue)) {
+ sp->running = false;
+ pending = false;
+ }
+ spin_unlock_irq(&sp->queue_lock);
+ }
+
+ if (pending)
+ queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL);
+}
+
+/*
+ * This is the work-queue function that handles SRCU grace periods.
+ */
+static void process_srcu(struct work_struct *work)
+{
+ struct srcu_struct *sp;
+
+ sp = container_of(work, struct srcu_struct, work.work);
+
+ srcu_collect_new(sp);
+ srcu_advance_batches(sp, 1);
+ srcu_invoke_callbacks(sp);
+ srcu_reschedule(sp);
+}
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