urcu-bp: move quiescent threads to separate list

Accelerate 2-phase grace period by not having to iterate twice on
threads not within RCU read-side critical section.

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-bp.c b/urcu-bp.c
index 4b3cf01801764c9c132c525876e2cf40ec5895d8..f99c0e502ee20f197b5f225a2b6a49088d3b138a 100644 (file)
--- a/urcu-bp.c
+++ b/urcu-bp.c
@@ -115,7 +115,7 @@ DEFINE_URCU_TLS(unsigned int, rcu_rand_yield);
  * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
  * Written to only by writer with mutex taken. Read by both writer and readers.
  */
-long rcu_gp_ctr = RCU_GP_COUNT;
+unsigned long rcu_gp_ctr = RCU_GP_COUNT;
 
 /*
  * Pointer to registry elements. Written to only by each individual reader. Read
@@ -164,9 +164,10 @@ static void mutex_unlock(pthread_mutex_t *mutex)
                urcu_die(ret);
 }
 
-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;
 
@@ -177,12 +178,30 @@ static void wait_for_readers(void)
         */
        for (;;) {
                wait_loops++;
-               cds_list_for_each_entry_safe(index, tmp, &registry, node) {
-                       if (!rcu_old_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)) {
                        break;
                } else {
                        if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
@@ -191,12 +210,12 @@ static void wait_for_readers(void)
                                caa_cpu_relax();
                }
        }
-       /* put back the reader list in the registry */
-       cds_list_splice(&qsreaders, &registry);
 }
 
 void synchronize_rcu(void)
 {
+       CDS_LIST_HEAD(cur_snap_readers);
+       CDS_LIST_HEAD(qsreaders);
        sigset_t newmask, oldmask;
        int ret;
 
@@ -221,7 +240,7 @@ 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);
 
        /*
         * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
@@ -250,7 +269,12 @@ 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);
 
        /*
         * Finish waiting for reader threads before letting the old ptr being

diff --git a/urcu/static/urcu-bp.h b/urcu/static/urcu-bp.h
index c52a68870dbadfb9c45ca2e688a5e45d39d25f3c..c7f532640e3c0feab165cb5a699c4c3760df011d 100644 (file)
--- a/urcu/static/urcu-bp.h
+++ b/urcu/static/urcu-bp.h
@@ -58,6 +58,12 @@ 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>
@@ -129,11 +135,11 @@ extern void rcu_bp_register(void);
  * Using a int rather than a char to eliminate false register dependencies
  * causing stalls on some architectures.
  */
-extern long rcu_gp_ctr;
+extern unsigned long rcu_gp_ctr;
 
 struct rcu_reader {
        /* Data used by both reader and synchronize_rcu() */
-       long ctr;
+       unsigned long ctr;
        /* Data used for registry */
        struct cds_list_head node __attribute__((aligned(CAA_CACHE_LINE_SIZE)));
        pthread_t tid;
@@ -147,19 +153,22 @@ struct rcu_reader {
  */
 extern DECLARE_URCU_TLS(struct rcu_reader *, rcu_reader);
 
-static inline int rcu_old_gp_ongoing(long *value)
+static inline enum rcu_state rcu_reader_state(unsigned long *ctr)
 {
-       long v;
+       unsigned long v;
 
-       if (value == NULL)
-               return 0;
+       if (ctr == NULL)
+               return RCU_READER_INACTIVE;
        /*
         * Make sure both tests below are done on the same version of *value
         * to insure consistency.
         */
-       v = CMM_LOAD_SHARED(*value);
-       return (v & RCU_GP_CTR_NEST_MASK) &&
-                ((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE);
+       v = CMM_LOAD_SHARED(*ctr);
+       if (!(v & RCU_GP_CTR_NEST_MASK))
+               return RCU_READER_INACTIVE;
+       if (!((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE))
+               return RCU_READER_ACTIVE_CURRENT;
+       return RCU_READER_ACTIVE_OLD;
 }
 
 /*
@@ -190,7 +199,7 @@ static inline void _rcu_read_lock_update(unsigned long tmp)
  */
 static inline void _rcu_read_lock(void)
 {
-       long tmp;
+       unsigned long tmp;
 
        if (caa_unlikely(!URCU_TLS(rcu_reader)))
                rcu_bp_register(); /* If not yet registered. */