urcu-defer: make call_rcu() energy efficient using futex()

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

diff --git a/urcu-defer-static.h b/urcu-defer-static.h
index a00ced033b6cc44fcf9b9e02d66cfbc54081e705..dd24c61d85d7885d685323757f83c52c2df2f15f 100644 (file)
--- a/urcu-defer-static.h
+++ b/urcu-defer-static.h
@@ -125,6 +125,7 @@ struct defer_queue {
 extern struct defer_queue __thread defer_queue;
 
 extern void rcu_defer_barrier_thread(void);
+extern void wake_up_defer(void);
 
 /*
  * not signal-safe.
@@ -183,6 +184,10 @@ static inline void _rcu_defer_queue(void (*fct)(void *p), void *p)
        smp_wmb();      /* Publish new pointer before head */
                        /* Write q[] before head. */
        STORE_SHARED(defer_queue.head, head);
+       /*
+        * Wake-up any waiting defer thread.
+        */
+       wake_up_defer();
 }
 
 #endif /* _URCU_DEFER_STATIC_H */

diff --git a/urcu-defer.c b/urcu-defer.c
index 412eb30477908b2a60acff9dde794d9ecfc793f8..5b904f8321a23c2ef3b84752c64c84cc9f9c865a 100644 (file)
--- a/urcu-defer.c
+++ b/urcu-defer.c
@@ -28,11 +28,17 @@
 #include <string.h>
 #include <errno.h>
 #include <poll.h>
+#include <linux/futex.h>
+#include <sys/time.h>
+#include <syscall.h>
+#include <unistd.h>
 
 #include "urcu-defer-static.h"
 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
 #include "urcu-defer.h"
 
+#define futex(...)     syscall(__NR_futex, __VA_ARGS__)
+
 void __attribute__((destructor)) urcu_defer_exit(void);
 
 extern void synchronize_rcu(void);
@@ -43,6 +49,8 @@ extern void synchronize_rcu(void);
 static pthread_mutex_t urcu_defer_mutex = PTHREAD_MUTEX_INITIALIZER;
 static pthread_mutex_t defer_thread_mutex = PTHREAD_MUTEX_INITIALIZER;
 
+int defer_thread_futex;
+
 /*
  * Written to only by each individual deferer. Read by both the deferer and
  * the reclamation tread.
@@ -62,7 +70,28 @@ static struct deferer_registry *registry;
 static int num_deferers, alloc_deferers;
 
 static pthread_t tid_defer;
-static int exit_defer;
+
+/*
+ * Wake-up any waiting defer thread. Called from many concurrent threads.
+ */
+void wake_up_defer(void)
+{
+       if (unlikely(atomic_read(&defer_thread_futex) == -1))
+               atomic_set(&defer_thread_futex, 0);
+               futex(&defer_thread_futex, FUTEX_WAKE,
+                     0, NULL, NULL, 0);
+}
+
+/*
+ * Defer thread waiting. Single thread.
+ */
+static void wait_defer(void)
+{
+       atomic_dec(&defer_thread_futex);
+       if (atomic_read(&defer_thread_futex) == -1)
+               futex(&defer_thread_futex, FUTEX_WAIT, -1,
+                     NULL, NULL, 0);
+}
 
 static void internal_urcu_lock(pthread_mutex_t *mutex)
 {
@@ -81,6 +110,7 @@ static void internal_urcu_lock(pthread_mutex_t *mutex)
                        perror("Error in pthread mutex lock");
                        exit(-1);
                }
+               pthread_testcancel();
                poll(NULL,0,10);
        }
 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
@@ -195,10 +225,22 @@ end:
 void *thr_defer(void *args)
 {
        for (;;) {
-               if (LOAD_SHARED(exit_defer))
-                       break;
-               poll(NULL,0,100);       /* wait for 100ms */
+               pthread_testcancel();
+               printf("a\n");
+               /*
+                * "Be green". Don't wake up the CPU if there is no RCU work
+                * to perform whatsoever. Aims at saving laptop battery life by
+                * leaving the processor in sleep state when idle.
+                */
+               printf("b\n");
+               wait_defer();
+               printf("e\n");
+               /* Sleeping after wait_defer to let many callbacks enqueue */
+               //TEST poll(NULL,0,100);        /* wait for 100ms */
+               printf("f\n");
                rcu_defer_barrier();
+               printf("perform deferred call_rcu() from worker thread %lu.\n",
+                       time(NULL));
        }
 
        return NULL;
@@ -277,7 +319,8 @@ static void stop_defer_thread(void)
        int ret;
        void *tret;
 
-       STORE_SHARED(exit_defer, 1);
+       pthread_cancel(tid_defer);
+       wake_up_defer();
        ret = pthread_join(tid_defer, &tret);
        assert(!ret);
 }