Litmus core: add plugin-independent precise budget enforcement infrastructure

Simple logic: if a task requires precise enforcement, then program a
hr-timer to fire when the task must be descheduled. When the timer
fires, simply activate the scheduler. When we switch to a different
task, either reprogram the timer or cancel it.

2 files changed, 110 insertions, 0 deletions

diff --git a/litmus/Makefile b/litmus/Makefile
index 30369787ece2..f301d2842e43 100644
--- a/litmus/Makefile
+++ b/litmus/Makefile
@@ -3,6 +3,7 @@
 #
 
 obj-y     = sched_plugin.o litmus.o \
+            budget.o \
             jobs.o \
             sync.o \
             rt_domain.o \
diff --git a/litmus/budget.c b/litmus/budget.c
new file mode 100644
index 000000000000..b99177a26313
--- /dev/null
+++ b/litmus/budget.c
@@ -0,0 +1,109 @@
+#include <linux/percpu.h>
+#include <linux/hrtimer.h>
+
+#include <litmus/litmus.h>
+
+struct enforcement_timer {
+        /* The enforcement timer is used to accurately police
+         * slice budgets. */
+        struct hrtimer          timer;
+        int                     armed;
+};
+
+DEFINE_PER_CPU(struct enforcement_timer, budget_timer);
+
+static enum hrtimer_restart on_enforcement_timeout(struct hrtimer *timer)
+{
+        struct enforcement_timer* et = container_of(timer,
+                                                    struct enforcement_timer,
+                                                    timer);
+        unsigned long flags;
+
+        local_irq_save(flags);
+        TRACE("enforcement timer fired.\n");
+        et->armed = 0;
+        /* activate scheduler */
+        set_tsk_need_resched(current);
+        local_irq_restore(flags);
+
+        return  HRTIMER_NORESTART;
+}
+
+/* assumes called with IRQs off */
+static void cancel_enforcement_timer(struct enforcement_timer* et)
+{
+        int ret;
+
+        TRACE("cancelling enforcement timer.\n");
+
+        /* Since interrupts are disabled and et->armed is only
+         * modified locally, we do not need any locks.
+         */
+
+        if (et->armed) {
+                ret = hrtimer_try_to_cancel(&et->timer);
+                /* Should never be inactive. */
+                BUG_ON(ret == 0);
+                /* Should never be running concurrently. */
+                BUG_ON(ret == -1);
+
+                et->armed = 0;
+        }
+}
+
+/* assumes called with IRQs off */
+static void arm_enforcement_timer(struct enforcement_timer* et,
+                                  struct task_struct* t)
+{
+        lt_t when_to_fire;
+        TRACE_TASK(t, "arming enforcement timer.\n");
+
+        /* Calling this when there is no budget left for the task
+         * makes no sense, unless the task is non-preemptive. */
+        BUG_ON(budget_exhausted(t) && (!is_np(t)));
+
+        /* __hrtimer_start_range_ns() cancels the timer
+         * anyway, so we don't have to check whether it is still armed */
+
+        if (likely(!is_np(t))) {
+                when_to_fire = litmus_clock() + budget_remaining(t);
+                __hrtimer_start_range_ns(&et->timer,
+                                         ns_to_ktime(when_to_fire),
+                                         0 /* delta */,
+                                         HRTIMER_MODE_ABS_PINNED,
+                                         0 /* no wakeup */);
+                et->armed = 1;
+        }
+}
+
+
+/* expects to be called with IRQs off */
+void update_enforcement_timer(struct task_struct* t)
+{
+        struct enforcement_timer* et = &__get_cpu_var(budget_timer);
+
+        if (t && budget_precisely_enforced(t)) {
+                /* Make sure we call into the scheduler when this budget
+                 * expires. */
+                arm_enforcement_timer(et, t);
+        } else if (et->armed) {
+                /* Make sure we don't cause unnecessary interrupts. */
+                cancel_enforcement_timer(et);
+        }
+}
+
+
+static int __init init_budget_enforcement(void)
+{
+        int cpu;
+        struct enforcement_timer* et;
+
+        for (cpu = 0; cpu < NR_CPUS; cpu++)  {
+                et = &per_cpu(budget_timer, cpu);
+                hrtimer_init(&et->timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS);
+                et->timer.function = on_enforcement_timeout;
+        }
+        return 0;
+}
+
+module_init(init_budget_enforcement);