Re: [RFC PATCH 5/5 single-thread-version] implement per-domainsingle-thread state machine call_srcu()
From: Paul E. McKenney
Date: Fri Mar 09 2012 - 10:59:20 EST
On Wed, Mar 07, 2012 at 11:54:02AM +0800, Lai Jiangshan wrote:
> 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@xxxxxxxxxxxxxx>
> ---
> 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,
I don't see what this applies against. The old patch 5/6 created
a "(*call_rcu_func_t)(struct rcu_head *head," and I don't see what
created the "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;
Why not use your multiple-tail-pointer trick here? (The one that is
used in treercu.)
> 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);
> }
Mightn't it be simpler and faster to just have a separate wait_srcu_gp()
that doesn't share code with wait_rcu_gp()? I am all for sharing code,
but this might be hrting more than helping.
> +
> +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
OK, so your original patch #6 is folded into this? I don't have a strong
view either way, just need to know.
> --- 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);
> + }
> +}
And perhaps this is why you don't want the multi-tailed queue?
> +
> +/* 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);
> }
OK, I'll bite... Why aren't synchronize_srcu_expedited() and
synchronize_srcu() different?
Thanx, Paul
> 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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