urcu: make the code of urcu-qsbr as normal urcu

urcu-qsbr's read site's quiescence is much longer than normal urcu ==>
synchronize_rcu() is much slower ==>
rcu_gp_ctr is updated much less ==>
the whole urcu-qsbr will not be slowed down by false sharing of rcu_gp_ctr.

But this patch makes sense to keep the code of urcu-qsbr like normal urcu,
better readability and maintenance.

Test: (4*6 CPUs)
Before patch:
[root@localhost userspace-rcu]# ./tests/test_urcu_qsbr 20 1 20
SUMMARY ./tests/test_urcu_qsbr    testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads  65498297587 nr_writes      2000665 nr_ops  65500298252
[root@localhost userspace-rcu]# ./tests/test_urcu_qsbr 20 1 20
SUMMARY ./tests/test_urcu_qsbr    testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads  67218079467 nr_writes      1981593 nr_ops  67220061060

After patch
./tests/test_urcu_qsbr 20 1 20
SUMMARY ./tests/test_urcu_qsbr    testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads  67473798999 nr_writes      1999151 nr_ops  67475798150
[root@localhost userspace-rcu]# ./tests/test_urcu_qsbr 20 1 20
SUMMARY ./tests/test_urcu_qsbr    testdur   20 nr_readers  20 rdur      0 wdur      0 nr_writers   1 wdelay      0 nr_reads  67065521397 nr_writes      1993956 nr_ops  67067515353

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index 786a80d6b16b1936dc62b71b43db18850854dd41..5bdf259492156681e23e873d79286b5ca55093fb 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -53,13 +53,7 @@
 void __attribute__((destructor)) rcu_exit(void);
 
 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
-
-int32_t rcu_gp_futex;
-
-/*
- * Global grace period counter.
- */
-unsigned long rcu_gp_ctr = RCU_GP_ONLINE;
+struct urcu_gp rcu_gp = { .ctr = RCU_GP_ONLINE };
 
 /*
  * Active attempts to check for reader Q.S. before calling futex().
@@ -118,8 +112,8 @@ static void wait_gp(void)
 {
        /* Read reader_gp before read futex */
        cmm_smp_rmb();
-       if (uatomic_read(&rcu_gp_futex) == -1)
-               futex_noasync(&rcu_gp_futex, FUTEX_WAIT, -1,
+       if (uatomic_read(&rcu_gp.futex) == -1)
+               futex_noasync(&rcu_gp.futex, FUTEX_WAIT, -1,
                      NULL, NULL, 0);
 }
 
@@ -133,12 +127,12 @@ static void wait_for_readers(struct cds_list_head *input_readers,
        /*
         * Wait for each thread URCU_TLS(rcu_reader).ctr to either
         * indicate quiescence (offline), or for them to observe the
-        * current rcu_gp_ctr value.
+        * current rcu_gp.ctr value.
         */
        for (;;) {
                wait_loops++;
                if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
-                       uatomic_set(&rcu_gp_futex, -1);
+                       uatomic_set(&rcu_gp.futex, -1);
                        /*
                         * Write futex before write waiting (the other side
                         * reads them in the opposite order).
@@ -177,7 +171,7 @@ static void wait_for_readers(struct cds_list_head *input_readers,
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                cmm_smp_mb();
-                               uatomic_set(&rcu_gp_futex, 0);
+                               uatomic_set(&rcu_gp.futex, 0);
                        }
                        break;
                } else {
@@ -253,11 +247,11 @@ void synchronize_rcu(void)
 
        /*
         * Must finish waiting for quiescent state for original parity
-        * before committing next rcu_gp_ctr update to memory. Failure
+        * before committing next rcu_gp.ctr update to memory. Failure
         * to do so could result in the writer waiting forever while new
         * readers are always accessing data (no progress).  Enforce
         * compiler-order of load URCU_TLS(rcu_reader).ctr before store
-        * to rcu_gp_ctr.
+        * to rcu_gp.ctr.
         */
        cmm_barrier();
 
@@ -269,13 +263,13 @@ void synchronize_rcu(void)
        cmm_smp_mb();
 
        /* Switch parity: 0 -> 1, 1 -> 0 */
-       CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR);
+       CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr ^ RCU_GP_CTR);
 
        /*
-        * Must commit rcu_gp_ctr update to memory before waiting for
+        * Must commit rcu_gp.ctr update to memory before waiting for
         * quiescent state. Failure to do so could result in the writer
         * waiting forever while new readers are always accessing data
-        * (no progress). Enforce compiler-order of store to rcu_gp_ctr
+        * (no progress). Enforce compiler-order of store to rcu_gp.ctr
         * before load URCU_TLS(rcu_reader).ctr.
         */
        cmm_barrier();
@@ -353,13 +347,13 @@ void synchronize_rcu(void)
                goto out;
 
        /* Increment current G.P. */
-       CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr + RCU_GP_CTR);
+       CMM_STORE_SHARED(rcu_gp.ctr, rcu_gp.ctr + RCU_GP_CTR);
 
        /*
-        * Must commit rcu_gp_ctr update to memory before waiting for
+        * Must commit rcu_gp.ctr update to memory before waiting for
         * quiescent state. Failure to do so could result in the writer
         * waiting forever while new readers are always accessing data
-        * (no progress). Enforce compiler-order of store to rcu_gp_ctr
+        * (no progress). Enforce compiler-order of store to rcu_gp.ctr
         * before load URCU_TLS(rcu_reader).ctr.
         */
        cmm_barrier();

diff --git a/urcu/static/urcu-qsbr.h b/urcu/static/urcu-qsbr.h
index ea5e13adebd5fae0374a5ad81693dd6cf46436c2..ef1f600d6c2248d3b1320b2056ef49b661660873 100644 (file)
--- a/urcu/static/urcu-qsbr.h
+++ b/urcu/static/urcu-qsbr.h
@@ -119,12 +119,18 @@ static inline void rcu_debug_yield_init(void)
 #define RCU_GP_ONLINE          (1UL << 0)
 #define RCU_GP_CTR             (1UL << 1)
 
-/*
- * Global quiescent period counter with low-order bits unused.
- * Using a int rather than a char to eliminate false register dependencies
- * causing stalls on some architectures.
- */
-extern unsigned long rcu_gp_ctr;
+struct urcu_gp {
+       /*
+        * Global quiescent period counter with low-order bits unused.
+        * Using a int rather than a char to eliminate false register
+        * dependencies causing stalls on some architectures.
+        */
+       unsigned long ctr;
+
+       int32_t futex;
+} __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
+
+extern struct urcu_gp rcu_gp;
 
 struct rcu_reader {
        /* Data used by both reader and synchronize_rcu() */
@@ -137,8 +143,6 @@ struct rcu_reader {
 
 extern DECLARE_URCU_TLS(struct rcu_reader, rcu_reader);
 
-extern int32_t rcu_gp_futex;
-
 /*
  * Wake-up waiting synchronize_rcu(). Called from many concurrent threads.
  */
@@ -147,10 +151,10 @@ static inline void wake_up_gp(void)
        if (caa_unlikely(_CMM_LOAD_SHARED(URCU_TLS(rcu_reader).waiting))) {
                _CMM_STORE_SHARED(URCU_TLS(rcu_reader).waiting, 0);
                cmm_smp_mb();
-               if (uatomic_read(&rcu_gp_futex) != -1)
+               if (uatomic_read(&rcu_gp.futex) != -1)
                        return;
-               uatomic_set(&rcu_gp_futex, 0);
-               futex_noasync(&rcu_gp_futex, FUTEX_WAKE, 1,
+               uatomic_set(&rcu_gp.futex, 0);
+               futex_noasync(&rcu_gp.futex, FUTEX_WAKE, 1,
                      NULL, NULL, 0);
        }
 }
@@ -162,7 +166,7 @@ static inline enum rcu_state rcu_reader_state(unsigned long *ctr)
        v = CMM_LOAD_SHARED(*ctr);
        if (!v)
                return RCU_READER_INACTIVE;
-       if (v == rcu_gp_ctr)
+       if (v == rcu_gp.ctr)
                return RCU_READER_ACTIVE_CURRENT;
        return RCU_READER_ACTIVE_OLD;
 }
@@ -228,7 +232,7 @@ static inline void _rcu_quiescent_state_update_and_wakeup(unsigned long gp_ctr)
  * to be invoked directly from non-LGPL code.
  *
  * We skip the memory barriers and gp store if our local ctr already
- * matches the global rcu_gp_ctr value: this is OK because a prior
+ * matches the global rcu_gp.ctr value: this is OK because a prior
  * _rcu_quiescent_state() or _rcu_thread_online() already updated it
  * within our thread, so we have no quiescent state to report.
  */
@@ -236,7 +240,7 @@ static inline void _rcu_quiescent_state(void)
 {
        unsigned long gp_ctr;
 
-       if ((gp_ctr = CMM_LOAD_SHARED(rcu_gp_ctr)) == URCU_TLS(rcu_reader).ctr)
+       if ((gp_ctr = CMM_LOAD_SHARED(rcu_gp.ctr)) == URCU_TLS(rcu_reader).ctr)
                return;
        _rcu_quiescent_state_update_and_wakeup(gp_ctr);
 }
@@ -269,7 +273,7 @@ static inline void _rcu_thread_offline(void)
 static inline void _rcu_thread_online(void)
 {
        cmm_barrier();  /* Ensure the compiler does not reorder us with mutex */
-       _CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, CMM_LOAD_SHARED(rcu_gp_ctr));
+       _CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, CMM_LOAD_SHARED(rcu_gp.ctr));
        cmm_smp_mb();
 }