[PATCH/RFC 4/4] Add support for dynamic ticks.

[Luca Tettamanti <kronos.it-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

If DYNAMIC_TICKS is defined qemu does not attepmt to generate SIGALRM at a
constant rate. Rather, the system timer is set to generate SIGALRM only
when it is needed. DYNAMIC_TICKS reduces the number of SIGALRMs sent to
idle dynamic-ticked guests.
Original patch from Dan Kenigsberg <dank-atKUWr5tajBWk0Htik3J/w@public.gmane.org>

Signed-off-by: Luca Tettamanti <kronos.it-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
---
 qemu/configure |    5 ++
 qemu/vl.c      |  149 ++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 148 insertions(+), 6 deletions(-)

diff --git a/qemu/configure b/qemu/configure
index 365b7fb..38373db 100755
--- a/qemu/configure
+++ b/qemu/configure
@@ -262,6 +262,8 @@ for opt do
   ;;
   --enable-uname-release=*) uname_release="$optarg"
   ;;
+  --disable-dynamic-ticks) dynamic_ticks="no"
+  ;;
   esac
 done
 
@@ -788,6 +790,9 @@ echo "TARGET_DIRS=$target_list" >> $config_mak
 if [ "$build_docs" = "yes" ] ; then
   echo "BUILD_DOCS=yes" >> $config_mak
 fi
+if test "$dynamic_ticks" != "no" ; then
+  echo "#define DYNAMIC_TICKS 1" >> $config_h
+fi
 
 # XXX: suppress that
 if [ "$bsd" = "yes" ] ; then
diff --git a/qemu/vl.c b/qemu/vl.c
index 0373beb..096729d 100644
--- a/qemu/vl.c
+++ b/qemu/vl.c
@@ -748,12 +748,42 @@ struct QEMUTimer {
 
 struct qemu_alarm_timer {
     char const *name;
+    unsigned int flags;
 
     int (*start)(struct qemu_alarm_timer *t);
     void (*stop)(struct qemu_alarm_timer *t);
+    void (*rearm)(struct qemu_alarm_timer *t);
     void *priv;
 };
 
+#define ALARM_FLAG_DYNTICKS  0x1
+
+#ifdef DYNAMIC_TICKS
+
+static inline int alarm_has_dynticks(struct qemu_alarm_timer *t)
+{
+    return t->flags & ALARM_FLAG_DYNTICKS;
+}
+
+static void qemu_rearm_alarm_timer(struct qemu_alarm_timer *t) {
+    if (!alarm_has_dynticks(t))
+        return;
+
+    t->rearm(t);
+}
+
+#else /* DYNAMIC_TICKS */
+
+static inline int alarm_has_dynticks(struct qemu_alarm_timer *t)
+{
+    return 0;
+}
+
+static void qemu_rearm_alarm_timer(void) {
+}
+
+#endif /* DYNAMIC_TICKS */
+
 static struct qemu_alarm_timer *alarm_timer;
 
 #ifdef _WIN32
@@ -772,6 +802,14 @@ static void win32_stop_timer(struct qemu_alarm_timer *t);
 static int unix_start_timer(struct qemu_alarm_timer *t);
 static void unix_stop_timer(struct qemu_alarm_timer *t);
 
+#ifdef DYNAMIC_TICKS
+
+static int dynticks_start_timer(struct qemu_alarm_timer *t);
+static void dynticks_stop_timer(struct qemu_alarm_timer *t);
+static void dynticks_rearm_timer(struct qemu_alarm_timer *t);
+
+#endif
+
 #ifdef __linux__
 
 static int hpet_start_timer(struct qemu_alarm_timer *t);
@@ -785,16 +823,19 @@ static void rtc_stop_timer(struct qemu_alarm_timer *t);
 #endif /* _WIN32 */
 
 static struct qemu_alarm_timer alarm_timers[] = {
+#ifndef _WIN32
+#ifdef DYNAMIC_TICKS
+    {"dynticks", ALARM_FLAG_DYNTICKS, dynticks_start_timer, dynticks_stop_timer, dynticks_rearm_timer, NULL},
+#endif
 #ifdef __linux__
     /* HPET - if available - is preferred */
-    {"hpet", hpet_start_timer, hpet_stop_timer, NULL},
+    {"hpet", 0, hpet_start_timer, hpet_stop_timer, NULL, NULL},
     /* ...otherwise try RTC */
-    {"rtc", rtc_start_timer, rtc_stop_timer, NULL},
+    {"rtc", 0, rtc_start_timer, rtc_stop_timer, NULL, NULL},
 #endif
-#ifndef _WIN32
-    {"unix", unix_start_timer, unix_stop_timer, NULL},
+    {"unix", 0, unix_start_timer, unix_stop_timer, NULL, NULL},
 #else
-    {"win32", win32_start_timer, win32_stop_timer, &alarm_win32_data},
+    {"win32", 0, win32_start_timer, win32_stop_timer, NULL, &alarm_win32_data},
 #endif
     {NULL, }
 };
@@ -913,6 +954,8 @@ void qemu_del_timer(QEMUTimer *ts)
         }
         pt = &t->next;
     }
+
+    qemu_rearm_alarm_timer(alarm_timer);
 }
 
 /* modify the current timer so that it will be fired when current_time
@@ -972,6 +1015,7 @@ static void qemu_run_timers(QEMUTimer **ptimer_head, int64_t current_time)
         /* run the callback (the timer list can be modified) */
         ts->cb(ts->opaque);
     }
+    qemu_rearm_alarm_timer(alarm_timer);
 }
 
 int64_t qemu_get_clock(QEMUClock *clock)
@@ -1079,7 +1123,8 @@ static void host_alarm_handler(int host_signum)
         last_clock = ti;
     }
 #endif
-    if (qemu_timer_expired(active_timers[QEMU_TIMER_VIRTUAL],
+    if (alarm_has_dynticks(alarm_timer) ||
+        qemu_timer_expired(active_timers[QEMU_TIMER_VIRTUAL],
                            qemu_get_clock(vm_clock)) ||
         qemu_timer_expired(active_timers[QEMU_TIMER_REALTIME],
                            qemu_get_clock(rt_clock))) {
@@ -1207,6 +1252,97 @@ static void rtc_stop_timer(struct qemu_alarm_timer *t)
 
 #endif /* !defined(__linux__) */
 
+#ifdef DYNAMIC_TICKS
+static int dynticks_start_timer(struct qemu_alarm_timer *t)
+{
+    struct sigevent ev;
+    timer_t host_timer;
+    struct sigaction act;
+
+    sigfillset(&act.sa_mask);
+    act.sa_flags = 0;
+#if defined(TARGET_I386) && defined(USE_CODE_COPY)
+    act.sa_flags |= SA_ONSTACK;
+#endif
+    act.sa_handler = host_alarm_handler;
+
+    sigaction(SIGALRM, &act, NULL);
+
+    ev.sigev_value.sival_int = 0;
+    ev.sigev_notify = SIGEV_SIGNAL;
+    ev.sigev_signo = SIGALRM;
+
+    if (timer_create(CLOCK_REALTIME, &ev, &host_timer)) {
+        perror("timer_create");
+
+        /* disable dynticks */
+        fprintf(stderr, "Dynamic Ticks disabled\n");
+
+        return -1;
+    }
+
+    t->priv = (void *)host_timer;
+
+    return 0;
+}
+
+static void dynticks_stop_timer(struct qemu_alarm_timer *t)
+{
+    timer_t host_timer = (timer_t)t->priv;
+
+    timer_delete(host_timer);
+}
+
+static void dynticks_rearm_timer(struct qemu_alarm_timer *t)
+{
+    timer_t host_timer = (timer_t)t->priv;
+    struct itimerspec timeout;
+    int64_t nearest_delta_us = INT64_MAX;
+
+    if (active_timers[QEMU_TIMER_REALTIME] ||
+                    active_timers[QEMU_TIMER_VIRTUAL]) {
+        int64_t vmdelta_us, current_us;
+
+        if (active_timers[QEMU_TIMER_REALTIME])
+            nearest_delta_us = (active_timers[QEMU_TIMER_REALTIME]->expire_time - qemu_get_clock(rt_clock))*1000;
+
+        if (active_timers[QEMU_TIMER_VIRTUAL]) {
+            /* round up */
+            vmdelta_us = (active_timers[QEMU_TIMER_VIRTUAL]->expire_time - qemu_get_clock(vm_clock)+999)/1000;
+            if (vmdelta_us < nearest_delta_us)
+                nearest_delta_us = vmdelta_us;
+        }
+
+        /* Avoid arming the timer to negative, zero, or too low values */
+        /* TODO: MIN_TIMER_REARM_US should be optimized */
+        #define MIN_TIMER_REARM_US 250
+        if (nearest_delta_us <= MIN_TIMER_REARM_US)
+            nearest_delta_us = MIN_TIMER_REARM_US;
+
+        /* check whether a timer is already running */
+        if (timer_gettime(host_timer, &timeout)) {
+            perror("gettime");
+            fprintf(stderr, "Internal timer error: aborting\n");
+            exit(1);
+        }
+        current_us = timeout.it_value.tv_sec * 1000000 + timeout.it_value.tv_nsec/1000;
+        if (current_us && current_us <= nearest_delta_us)
+            return;
+
+        timeout.it_interval.tv_sec = 0;
+        timeout.it_interval.tv_nsec = 0; /* 0 for one-shot timer */
+        timeout.it_value.tv_sec =  nearest_delta_us / 1000000;
+        timeout.it_value.tv_nsec = (nearest_delta_us % 1000000) * 1000;
+        if (timer_settime(host_timer, 0 /* RELATIVE */, &timeout, NULL)) {
+            perror("settime");
+            fprintf(stderr, "Internal timer error: aborting\n");
+            exit(1);
+        }
+    }
+}
+
+#endif /* DYNAMIC_TICKS */
+
 static int unix_start_timer(struct qemu_alarm_timer *t)
 {
     struct sigaction act;
@@ -6259,6 +6395,7 @@ void vm_start(void)
         cpu_enable_ticks();
         vm_running = 1;
         vm_state_notify(1);
+        qemu_rearm_alarm_timer(alarm_timer);
     }
 }
 
-- 
1.5.2.4


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