[Qemu-devel] [PATCH v4 1/5] cpu: Provide vcpu throttling interface

[Jason J. Herne]

Provide a method to throttle guest cpu execution. CPUState is augmented with
timeout controls and throttle start/stop functions. To throttle the guest cpu
the caller simply has to call the throttle set function and provide a percentage
of throttle time.

Signed-off-by: Jason J. Herne <address@hidden>
Reviewed-by: Matthew Rosato <address@hidden>
---
 cpus.c            | 66 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 include/qom/cpu.h | 38 ++++++++++++++++++++++++++++++++
 2 files changed, 104 insertions(+)

diff --git a/cpus.c b/cpus.c
index de6469f..6f86da0 100644
--- a/cpus.c
+++ b/cpus.c
@@ -68,6 +68,14 @@ static CPUState *next_cpu;
 int64_t max_delay;
 int64_t max_advance;
 
+/* vcpu throttling controls */
+static QEMUTimer *throttle_timer;
+static unsigned int throttle_percentage;
+
+#define CPU_THROTTLE_PCT_MIN 1
+#define CPU_THROTTLE_PCT_MAX 99
+#define CPU_THROTTLE_TIMESLICE 10
+
 bool cpu_is_stopped(CPUState *cpu)
 {
     return cpu->stopped || !runstate_is_running();
@@ -486,10 +494,68 @@ static const VMStateDescription vmstate_timers = {
     }
 };
 
+static void cpu_throttle_thread(void *opaque)
+{
+    double pct = (double)throttle_percentage/100;
+    double throttle_ratio = pct / (1 - pct);
+    long sleeptime_ms = (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE);
+
+    if (!throttle_percentage) {
+        return;
+    }
+
+    qemu_mutex_unlock_iothread();
+    g_usleep(sleeptime_ms * 1000); /* Convert ms to us for usleep call */
+    qemu_mutex_lock_iothread();
+}
+
+static void cpu_throttle_timer_tick(void *opaque)
+{
+    CPUState *cpu;
+
+    /* Stop the timer if needed */
+    if (!throttle_percentage) {
+        return;
+    }
+    CPU_FOREACH(cpu) {
+        async_run_on_cpu(cpu, cpu_throttle_thread, NULL);
+    }
+
+    timer_mod(throttle_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+                                   CPU_THROTTLE_TIMESLICE);
+}
+
+void cpu_throttle_set(int new_throttle_pct)
+{
+    /* Ensure throttle percentage is within valid range */
+    new_throttle_pct = MIN(new_throttle_pct, CPU_THROTTLE_PCT_MAX);
+    throttle_percentage = MAX(new_throttle_pct, CPU_THROTTLE_PCT_MIN);
+
+    timer_mod(throttle_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+                                       CPU_THROTTLE_TIMESLICE);
+}
+
+void cpu_throttle_stop(void)
+{
+    throttle_percentage = 0;
+}
+
+bool cpu_throttle_active(void)
+{
+    return (throttle_percentage != 0);
+}
+
+int cpu_throttle_get_percentage(void)
+{
+    return throttle_percentage;
+}
+
 void cpu_ticks_init(void)
 {
     seqlock_init(&timers_state.vm_clock_seqlock, NULL);
     vmstate_register(NULL, 0, &vmstate_timers, &timers_state);
+    throttle_timer = timer_new_ms(QEMU_CLOCK_VIRTUAL_RT,
+                                           cpu_throttle_timer_tick, NULL);
 }
 
 void configure_icount(QemuOpts *opts, Error **errp)
diff --git a/include/qom/cpu.h b/include/qom/cpu.h
index 39f0f19..56eb964 100644
--- a/include/qom/cpu.h
+++ b/include/qom/cpu.h
@@ -553,6 +553,44 @@ CPUState *qemu_get_cpu(int index);
  */
 bool cpu_exists(int64_t id);
 
+/**
+ * cpu_throttle_set:
+ * @new_throttle_pct: Percent of sleep time to running time.
+ *                    Valid range is 1 to 99.
+ *
+ * Throttles all vcpus by forcing them to sleep for the given percentage of
+ * time. A throttle_percentage of 50 corresponds to a 50% duty cycle roughly.
+ * (example: 10ms sleep for every 10ms awake).
+ *
+ * cpu_throttle_set can be called as needed to adjust new_throttle_pct.
+ * Once the throttling starts, it will remain in effect until cpu_throttle_stop
+ * is called.
+ */
+void cpu_throttle_set(int new_throttle_pct);
+
+/**
+ * cpu_throttle_stop:
+ *
+ * Stops the vcpu throttling started by cpu_throttle_set.
+ */
+void cpu_throttle_stop(void);
+
+/**
+ * cpu_throttle_active:
+ *
+ * Returns %true if the vcpus are currently being throttled, %false otherwise.
+ */
+bool cpu_throttle_active(void);
+
+/**
+ * cpu_throttle_get_percentage:
+ *
+ * Returns the vcpu throttle percentage. See cpu_throttle_set for details.
+ *
+ * Returns The throttle percentage in range 1 to 99.
+ */
+int cpu_throttle_get_percentage(void);
+
 #ifndef CONFIG_USER_ONLY
 
 typedef void (*CPUInterruptHandler)(CPUState *, int);
-- 
1.9.1