Messages in this thread | | | Date | Thu, 8 Mar 2012 12:35:33 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [RFC PATCH 5/5 single-thread-version] implement per-domain single-thread state machine call_srcu() |
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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@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,
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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