urcu-qsbr: move offline threads to separate list

Accelerate 2-phase grace period by not having to iterate on offline
threads twice.

CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu-qsbr.c b/urcu-qsbr.c
index eb167d128be77125ce450b1bac808870fb994975..5b341b5cfd6d97034b08ca5ea6df913a0c8a7736 100644 (file)
--- a/urcu-qsbr.c
+++ b/urcu-qsbr.c
@@ -116,9 +116,10 @@ static void wait_gp(void)
                      NULL, NULL, 0);
 }
 
-static void wait_for_readers(void)
+static void wait_for_readers(struct cds_list_head *input_readers,
+                       struct cds_list_head *cur_snap_readers,
+                       struct cds_list_head *qsreaders)
 {
-       CDS_LIST_HEAD(qsreaders);
        int wait_loops = 0;
        struct rcu_reader *index, *tmp;
 
@@ -136,18 +137,36 @@ static void wait_for_readers(void)
                         * reads them in the opposite order).
                         */
                        cmm_smp_wmb();
-                       cds_list_for_each_entry(index, &registry, node) {
+                       cds_list_for_each_entry(index, input_readers, node) {
                                _CMM_STORE_SHARED(index->waiting, 1);
                        }
                        /* Write futex before read reader_gp */
                        cmm_smp_mb();
                }
-               cds_list_for_each_entry_safe(index, tmp, &registry, node) {
-                       if (!rcu_gp_ongoing(&index->ctr))
-                               cds_list_move(&index->node, &qsreaders);
+               cds_list_for_each_entry_safe(index, tmp, input_readers, node) {
+                       switch (rcu_reader_state(&index->ctr)) {
+                       case RCU_READER_ACTIVE_CURRENT:
+                               if (cur_snap_readers) {
+                                       cds_list_move(&index->node,
+                                               cur_snap_readers);
+                                       break;
+                               }
+                               /* Fall-through */
+                       case RCU_READER_INACTIVE:
+                               cds_list_move(&index->node, qsreaders);
+                               break;
+                       case RCU_READER_ACTIVE_OLD:
+                               /*
+                                * Old snapshot. Leaving node in
+                                * input_readers will make us busy-loop
+                                * until the snapshot becomes current or
+                                * the reader becomes inactive.
+                                */
+                               break;
+                       }
                }
 
-               if (cds_list_empty(&registry)) {
+               if (cds_list_empty(input_readers)) {
                        if (wait_loops >= RCU_QS_ACTIVE_ATTEMPTS) {
                                /* Read reader_gp before write futex */
                                cmm_smp_mb();
@@ -166,8 +185,6 @@ static void wait_for_readers(void)
                        }
                }
        }
-       /* put back the reader list in the registry */
-       cds_list_splice(&qsreaders, &registry);
 }
 
 /*
@@ -178,6 +195,8 @@ static void wait_for_readers(void)
 #if (CAA_BITS_PER_LONG < 64)
 void synchronize_rcu(void)
 {
+       CDS_LIST_HEAD(cur_snap_readers);
+       CDS_LIST_HEAD(qsreaders);
        unsigned long was_online;
 
        was_online = URCU_TLS(rcu_reader).ctr;
@@ -203,7 +222,7 @@ void synchronize_rcu(void)
        /*
         * Wait for readers to observe original parity or be quiescent.
         */
-       wait_for_readers();
+       wait_for_readers(&registry, &cur_snap_readers, &qsreaders);
 
        /*
         * Must finish waiting for quiescent state for original parity
@@ -244,7 +263,12 @@ void synchronize_rcu(void)
        /*
         * Wait for readers to observe new parity or be quiescent.
         */
-       wait_for_readers();
+       wait_for_readers(&cur_snap_readers, NULL, &qsreaders);
+
+       /*
+        * Put quiescent reader list back into registry.
+        */
+       cds_list_splice(&qsreaders, &registry);
 out:
        mutex_unlock(&rcu_gp_lock);
 
@@ -260,6 +284,7 @@ out:
 #else /* !(CAA_BITS_PER_LONG < 64) */
 void synchronize_rcu(void)
 {
+       CDS_LIST_HEAD(qsreaders);
        unsigned long was_online;
 
        was_online = URCU_TLS(rcu_reader).ctr;
@@ -300,7 +325,12 @@ void synchronize_rcu(void)
        /*
         * Wait for readers to observe new count of be quiescent.
         */
-       wait_for_readers();
+       wait_for_readers(&registry, NULL, &qsreaders);
+
+       /*
+        * Put quiescent reader list back into registry.
+        */
+       cds_list_splice(&qsreaders, &registry);
 out:
        mutex_unlock(&rcu_gp_lock);
 

diff --git a/urcu/static/urcu-qsbr.h b/urcu/static/urcu-qsbr.h
index f3149569b2a2e2573df57936673d82eaadf0e1d7..f6e55807db267ba66227f1d7046511f56589aa76 100644 (file)
--- a/urcu/static/urcu-qsbr.h
+++ b/urcu/static/urcu-qsbr.h
@@ -62,6 +62,12 @@ extern "C" {
 #define rcu_assert(args...)
 #endif
 
+enum rcu_state {
+       RCU_READER_ACTIVE_CURRENT,
+       RCU_READER_ACTIVE_OLD,
+       RCU_READER_INACTIVE,
+};
+
 #ifdef DEBUG_YIELD
 #include <sched.h>
 #include <time.h>
@@ -149,12 +155,16 @@ static inline void wake_up_gp(void)
        }
 }
 
-static inline int rcu_gp_ongoing(unsigned long *ctr)
+static inline enum rcu_state rcu_reader_state(unsigned long *ctr)
 {
        unsigned long v;
 
        v = CMM_LOAD_SHARED(*ctr);
-       return v && (v != rcu_gp_ctr);
+       if (!v)
+               return RCU_READER_INACTIVE;
+       if (v == rcu_gp_ctr)
+               return RCU_READER_ACTIVE_CURRENT;
+       return RCU_READER_ACTIVE_OLD;
 }
 
 /*