cpu: Provide vcpu throttling interface

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 <jjherne@linux.vnet.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>

cpus.c

@@ -69,6 +69,14 @@ 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_NS 10000000
+
 bool cpu_is_stopped(CPUState *cpu)
 {
     return cpu->stopped || !runstate_is_running();
@@ -505,10 +513,80 @@ static const VMStateDescription vmstate_timers = {
     }
 };
 
+static void cpu_throttle_thread(void *opaque)
+{
+    CPUState *cpu = opaque;
+    double pct;
+    double throttle_ratio;
+    long sleeptime_ns;
+
+    if (!cpu_throttle_get_percentage()) {
+        return;
+    }
+
+    pct = (double)cpu_throttle_get_percentage()/100;
+    throttle_ratio = pct / (1 - pct);
+    sleeptime_ns = (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS);
+
+    qemu_mutex_unlock_iothread();
+    atomic_set(&cpu->throttle_thread_scheduled, 0);
+    g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */
+    qemu_mutex_lock_iothread();
+}
+
+static void cpu_throttle_timer_tick(void *opaque)
+{
+    CPUState *cpu;
+    double pct;
+
+    /* Stop the timer if needed */
+    if (!cpu_throttle_get_percentage()) {
+        return;
+    }
+    CPU_FOREACH(cpu) {
+        if (!atomic_xchg(&cpu->throttle_thread_scheduled, 1)) {
+            async_run_on_cpu(cpu, cpu_throttle_thread, cpu);
+        }
+    }
+
+    pct = (double)cpu_throttle_get_percentage()/100;
+    timer_mod(throttle_timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT) +
+                                   CPU_THROTTLE_TIMESLICE_NS / (1-pct));
+}
+
+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);
+    new_throttle_pct = MAX(new_throttle_pct, CPU_THROTTLE_PCT_MIN);
+
+    atomic_set(&throttle_percentage, new_throttle_pct);
+
+    timer_mod(throttle_timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT) +
+                                       CPU_THROTTLE_TIMESLICE_NS);
+}
+
+void cpu_throttle_stop(void)
+{
+    atomic_set(&throttle_percentage, 0);
+}
+
+bool cpu_throttle_active(void)
+{
+    return (cpu_throttle_get_percentage() != 0);
+}
+
+int cpu_throttle_get_percentage(void)
+{
+    return atomic_read(&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_ns(QEMU_CLOCK_VIRTUAL_RT,
+                                           cpu_throttle_timer_tick, NULL);
 }
 
 void configure_icount(QemuOpts *opts, Error **errp)

include/qom/cpu.h

@@ -321,6 +321,11 @@ struct CPUState {
     uint32_t can_do_io;
     int32_t exception_index; /* used by m68k TCG */
 
+    /* Used to keep track of an outstanding cpu throttle thread for migration
+     * autoconverge
+     */
+    bool throttle_thread_scheduled;
+
     /* Note that this is accessed at the start of every TB via a negative
        offset from AREG0.  Leave this field at the end so as to make the
        (absolute value) offset as small as possible.  This reduces code
@@ -565,6 +570,43 @@ CPUState *qemu_get_cpu(int index);
  */
 bool cpu_exists(int64_t id);
 
+/**
+ * cpu_throttle_set:
+ * @new_throttle_pct: Percent of sleep 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 25 corresponds to a 75% duty cycle roughly.
+ * (example: 10ms sleep for every 30ms 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);