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Subject[RFC PATCH 5/5 single-thread-version] implement per-domain single-thread state machine call_srcu()
This patch is on the top of the 4 previous patches(1/6, 2/6, 3/6, 4/6).

o state machine is light way and single-threaded, it is preemptible when checking.

o state machine is a work_struct. So, there is no thread occupied
by SRCU when the srcu is not actived(no callback). And it does
not sleep(avoid to occupy a thread when sleep).

o state machine is the only thread can flip/check/write(*) the srcu_struct,
so we don't need any mutex.
(write(*): except ->per_cpu_ref, ->running, ->batch_queue)

o synchronize_srcu() is always call call_srcu().
synchronize_srcu_expedited() is also.
It is OK for mb()-based srcu are extremely fast.

o In current kernel, we can expect that there are only 1 callback per gp.
so callback is probably called in the same CPU when it is queued.

The trip of a callback:
1) ->batch_queue when call_srcu()

2) ->batch_check0 when try to do check_zero

3) ->batch_check1 after finish its first check_zero and the flip

4) ->batch_done after finish its second check_zero

The current requirement of the callbacks:
The callback will be called inside process context.
The callback should be fast without any sleeping path.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
include/linux/rcupdate.h | 2 +-
include/linux/srcu.h | 28 +++++-
kernel/rcupdate.c | 24 ++++-
kernel/rcutorture.c | 44 ++++++++-
kernel/srcu.c | 238 ++++++++++++++++++++++++++++++++-------------
5 files changed, 259 insertions(+), 77 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9372174..d98eab2 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -222,7 +222,7 @@ extern void rcu_irq_exit(void);
* TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
*/

-typedef void call_rcu_func_t(struct rcu_head *head,
+typedef void (*call_rcu_func_t)(struct rcu_head *head,
void (*func)(struct rcu_head *head));
void wait_rcu_gp(call_rcu_func_t crf);

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index df8f5f7..56cb774 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -29,6 +29,7 @@

#include <linux/mutex.h>
#include <linux/rcupdate.h>
+#include <linux/workqueue.h>

struct srcu_struct_array {
unsigned long c[2];
@@ -39,10 +40,23 @@ struct srcu_struct_array {
#define SRCU_REF_MASK (ULONG_MAX >> SRCU_USAGE_BITS)
#define SRCU_USAGE_COUNT (SRCU_REF_MASK + 1)

+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;
unsigned long snap[NR_CPUS];
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
@@ -67,12 +81,24 @@ int init_srcu_struct(struct srcu_struct *sp);

#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

+/* draft
+ * queue callbacks which will be invoked after grace period.
+ * The callback will be called inside process context.
+ * The callback should be fast without any sleeping path.
+ */
+void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
+ void (*func)(struct rcu_head *head));
+
+typedef void (*call_srcu_func_t)(struct srcu_struct *sp, struct rcu_head *head,
+ void (*func)(struct rcu_head *head));
+void __wait_srcu_gp(struct srcu_struct *sp, call_srcu_func_t crf);
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/rcupdate.c b/kernel/rcupdate.c
index a86f174..f9b551f 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -45,6 +45,7 @@
#include <linux/mutex.h>
#include <linux/export.h>
#include <linux/hardirq.h>
+#include <linux/srcu.h>

#define CREATE_TRACE_POINTS
#include <trace/events/rcu.h>
@@ -123,20 +124,39 @@ static void wakeme_after_rcu(struct rcu_head *head)
complete(&rcu->completion);
}

-void wait_rcu_gp(call_rcu_func_t crf)
+static void __wait_rcu_gp(void *domain, void *func)
{
struct rcu_synchronize rcu;

init_rcu_head_on_stack(&rcu.head);
init_completion(&rcu.completion);
+
/* Will wake me after RCU finished. */
- crf(&rcu.head, wakeme_after_rcu);
+ if (!domain) {
+ call_rcu_func_t crf = func;
+ crf(&rcu.head, wakeme_after_rcu);
+ } else {
+ call_srcu_func_t crf = func;
+ crf(domain, &rcu.head, wakeme_after_rcu);
+ }
+
/* Wait for it. */
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
}
+
+void wait_rcu_gp(call_rcu_func_t crf)
+{
+ __wait_rcu_gp(NULL, crf);
+}
EXPORT_SYMBOL_GPL(wait_rcu_gp);

+/* srcu.c internel */
+void __wait_srcu_gp(struct srcu_struct *sp, call_srcu_func_t crf)
+{
+ __wait_rcu_gp(sp, crf);
+}
+
#ifdef CONFIG_PROVE_RCU
/*
* wrapper function to avoid #include problems.
diff --git a/kernel/rcutorture.c b/kernel/rcutorture.c
index 54e5724..40d24d0 100644
--- a/kernel/rcutorture.c
+++ b/kernel/rcutorture.c
@@ -623,6 +623,11 @@ static int srcu_torture_completed(void)
return srcu_batches_completed(&srcu_ctl);
}

+static void srcu_torture_deferred_free(struct rcu_torture *rp)
+{
+ call_srcu(&srcu_ctl, &rp->rtort_rcu, rcu_torture_cb);
+}
+
static void srcu_torture_synchronize(void)
{
synchronize_srcu(&srcu_ctl);
@@ -652,7 +657,7 @@ static struct rcu_torture_ops srcu_ops = {
.read_delay = srcu_read_delay,
.readunlock = srcu_torture_read_unlock,
.completed = srcu_torture_completed,
- .deferred_free = rcu_sync_torture_deferred_free,
+ .deferred_free = srcu_torture_deferred_free,
.sync = srcu_torture_synchronize,
.call = NULL,
.cb_barrier = NULL,
@@ -660,6 +665,21 @@ static struct rcu_torture_ops srcu_ops = {
.name = "srcu"
};

+static struct rcu_torture_ops srcu_sync_ops = {
+ .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,
+ .call = NULL,
+ .cb_barrier = NULL,
+ .stats = srcu_torture_stats,
+ .name = "srcu_sync"
+};
+
static int srcu_torture_read_lock_raw(void) __acquires(&srcu_ctl)
{
return srcu_read_lock_raw(&srcu_ctl);
@@ -677,7 +697,7 @@ static struct rcu_torture_ops srcu_raw_ops = {
.read_delay = srcu_read_delay,
.readunlock = srcu_torture_read_unlock_raw,
.completed = srcu_torture_completed,
- .deferred_free = rcu_sync_torture_deferred_free,
+ .deferred_free = srcu_torture_deferred_free,
.sync = srcu_torture_synchronize,
.call = NULL,
.cb_barrier = NULL,
@@ -685,6 +705,21 @@ static struct rcu_torture_ops srcu_raw_ops = {
.name = "srcu_raw"
};

+static struct rcu_torture_ops srcu_raw_sync_ops = {
+ .init = srcu_torture_init,
+ .cleanup = srcu_torture_cleanup,
+ .readlock = srcu_torture_read_lock_raw,
+ .read_delay = srcu_read_delay,
+ .readunlock = srcu_torture_read_unlock_raw,
+ .completed = srcu_torture_completed,
+ .deferred_free = rcu_sync_torture_deferred_free,
+ .sync = srcu_torture_synchronize,
+ .call = NULL,
+ .cb_barrier = NULL,
+ .stats = srcu_torture_stats,
+ .name = "srcu_raw_sync"
+};
+
static void srcu_torture_synchronize_expedited(void)
{
synchronize_srcu_expedited(&srcu_ctl);
@@ -1673,7 +1708,7 @@ static int rcu_torture_barrier_init(void)
for (i = 0; i < n_barrier_cbs; i++) {
init_waitqueue_head(&barrier_cbs_wq[i]);
barrier_cbs_tasks[i] = kthread_run(rcu_torture_barrier_cbs,
- (void *)i,
+ (void *)(long)i,
"rcu_torture_barrier_cbs");
if (IS_ERR(barrier_cbs_tasks[i])) {
ret = PTR_ERR(barrier_cbs_tasks[i]);
@@ -1857,7 +1892,8 @@ rcu_torture_init(void)
static struct rcu_torture_ops *torture_ops[] =
{ &rcu_ops, &rcu_sync_ops, &rcu_expedited_ops,
&rcu_bh_ops, &rcu_bh_sync_ops, &rcu_bh_expedited_ops,
- &srcu_ops, &srcu_raw_ops, &srcu_expedited_ops,
+ &srcu_ops, &srcu_sync_ops, &srcu_raw_ops,
+ &srcu_raw_sync_ops, &srcu_expedited_ops,
&sched_ops, &sched_sync_ops, &sched_expedited_ops, };

mutex_lock(&fullstop_mutex);
diff --git a/kernel/srcu.c b/kernel/srcu.c
index d101ed5..532f890 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -34,10 +34,60 @@
#include <linux/delay.h>
#include <linux/srcu.h>

+static inline void rcu_batch_init(struct rcu_batch *b)
+{
+ b->head = NULL;
+ b->tail = &b->head;
+}
+
+static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
+{
+ *b->tail = head;
+ b->tail = &head->next;
+}
+
+static inline bool rcu_batch_empty(struct rcu_batch *b)
+{
+ return b->tail == &b->head;
+}
+
+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;
+}
+
+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;
}
@@ -254,11 +304,9 @@ 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 SYNCHRONIZE_SRCU_TRYCOUNT 2
-#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
+#define SRCU_RETRY_CHECK_DELAY 5

-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)
{
/*
* If a reader fetches the index before the ->completed increment,
@@ -271,19 +319,12 @@ static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
*/
smp_mb(); /* D */

- /*
- * 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))
+ break;
+ if (--trycount <= 0)
+ return false;
+ udelay(SRCU_RETRY_CHECK_DELAY);
}

/*
@@ -297,6 +338,8 @@ static void wait_idx(struct srcu_struct *sp, int idx, int trycount)
* the next flipping.
*/
smp_mb(); /* E */
+
+ return true;
}

/*
@@ -308,10 +351,27 @@ static void srcu_flip(struct srcu_struct *sp)
ACCESS_ONCE(sp->completed)++;
}

+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);
+
/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
-static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
+static void __synchronize_srcu(struct srcu_struct *sp)
{
rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
!lock_is_held(&rcu_bh_lock_map) &&
@@ -319,54 +379,7 @@ 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);
-
- /*
- * 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, the current SRCU grace period does have to wait for
- * that reader. This is handled by invoking wait_idx() on the
- * non-active set of counters (hence sp->completed - 1). Once
- * wait_idx() returns, we know that all readers that picked up
- * the old value of ->completed and that already incremented their
- * counter will have completed.
- *
- * But what about readers that picked up the old value of
- * ->completed, but -still- have not managed to increment their
- * counter? We do not need to wait for those readers, because
- * they will have started their SRCU read-side critical section
- * after the current grace period starts.
- *
- * Because it is unlikely that readers will be preempted between
- * fetching ->completed and incrementing their counter, wait_idx()
- * will normally not need to wait.
- */
- wait_idx(sp, (sp->completed - 1) & 0x1, trycount);
-
- /*
- * Now that wait_idx() has waited for the really old readers,
- *
- * Flip the readers' index by incrementing ->completed, then wait
- * until there are no more readers using the counters referenced by
- * the old index value. (Recall that the index is the bottom bit
- * of ->completed.)
- *
- * Of course, it is possible that a reader might be delayed for the
- * full duration of flip_idx_and_wait() between fetching the
- * index and incrementing its counter. This possibility is handled
- * by the next __synchronize_srcu() invoking wait_idx() for such
- * readers before starting a new grace period.
- */
- srcu_flip(sp);
- wait_idx(sp, (sp->completed - 1) & 0x1, trycount);
-
- mutex_unlock(&sp->mutex);
+ __wait_srcu_gp(sp, call_srcu);
}

/**
@@ -385,7 +398,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
*/
void synchronize_srcu(struct srcu_struct *sp)
{
- __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
+ __synchronize_srcu(sp);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

@@ -406,10 +419,16 @@ EXPORT_SYMBOL_GPL(synchronize_srcu);
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)
{
- __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
+ __synchronize_srcu(sp);
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);

+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.
@@ -423,3 +442,84 @@ 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
+
+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);
+ }
+}
+
+static void srcu_advance_batches(struct srcu_struct *sp)
+{
+ int idx = 1 - (sp->completed & 0x1UL);
+
+ /*
+ * SRCU read-side critical sections are normally short, so check
+ * twice after a flip.
+ */
+ if (!rcu_batch_empty(&sp->batch_check1) ||
+ !rcu_batch_empty(&sp->batch_check0)) {
+ if (try_check_zero(sp, idx, 1)) {
+ rcu_batch_move(&sp->batch_done, &sp->batch_check1);
+ rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
+ if (!rcu_batch_empty(&sp->batch_check1)) {
+ srcu_flip(sp);
+ if (try_check_zero(sp, 1 - idx, 2)) {
+ rcu_batch_move(&sp->batch_done,
+ &sp->batch_check1);
+ }
+ }
+ }
+ }
+}
+
+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;
+ head->func(head);
+ }
+}
+
+static void srcu_reschedule(struct srcu_struct *sp)
+{
+ bool running = 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_queue)) {
+ sp->running = false;
+ running = false;
+ }
+ spin_unlock_irq(&sp->queue_lock);
+ }
+
+ if (running)
+ queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL);
+}
+
+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);
+ srcu_invoke_callbacks(sp);
+ srcu_reschedule(sp);
+}




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