[PATCH 09/10] sched/deadline: also reclaim bandwidth not used by dl tasks

[luca abeni]

From: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>

This commit introduces a per-runqueue "extra utilization" that can be
reclaimed by deadline tasks. In this way, the maximum fraction of CPU
time that can reclaimed by deadline tasks is fixed (and configurable)
and does not depend on the total deadline utilization.
The GRUB accounting rule is modified to add this "extra utilization"
to the inactive utilization of the runqueue, and to avoid reclaiming
more than a maximum fraction of the CPU time.

Signed-off-by: Luca Abeni <luca.abeni@xxxxxxxxxxxxxxx>
Tested-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
---
 kernel/sched/core.c     | 17 ++++++++++-------
 kernel/sched/deadline.c | 42 +++++++++++++++++++++++++++---------------
 kernel/sched/sched.h    | 37 +++++++++++++++++++++++++++++++++++--
 3 files changed, 72 insertions(+), 24 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7fcc3bd..0628c50 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2456,7 +2456,7 @@ inline struct dl_bw *dl_bw_of(int i)
 	return &cpu_rq(i)->rd->dl_bw;
 }
 
-static inline int dl_bw_cpus(int i)
+inline int dl_bw_cpus(int i)
 {
 	struct root_domain *rd = cpu_rq(i)->rd;
 	int cpus = 0;
@@ -2474,7 +2474,7 @@ inline struct dl_bw *dl_bw_of(int i)
 	return &cpu_rq(i)->dl.dl_bw;
 }
 
-static inline int dl_bw_cpus(int i)
+inline int dl_bw_cpus(int i)
 {
 	return 1;
 }
@@ -2512,8 +2512,8 @@ static int dl_overflow(struct task_struct *p, int policy,
 	if (dl_policy(policy) && !task_has_dl_policy(p) &&
 	    !__dl_overflow(dl_b, cpus, 0, new_bw)) {
 		if (hrtimer_active(&p->dl.inactive_timer))
-			__dl_clear(dl_b, p->dl.dl_bw);
-		__dl_add(dl_b, new_bw);
+			__dl_clear(dl_b, p->dl.dl_bw, cpus);
+		__dl_add(dl_b, new_bw, cpus);
 		err = 0;
 	} else if (dl_policy(policy) && task_has_dl_policy(p) &&
 		   !__dl_overflow(dl_b, cpus, p->dl.dl_bw, new_bw)) {
@@ -2524,8 +2524,8 @@ static int dl_overflow(struct task_struct *p, int policy,
 		 * But this would require to set the task's "inactive
 		 * timer" when the task is not inactive.
 		 */
-		__dl_clear(dl_b, p->dl.dl_bw);
-		__dl_add(dl_b, new_bw);
+		__dl_clear(dl_b, p->dl.dl_bw, cpus);
+		__dl_add(dl_b, new_bw, cpus);
 		dl_change_utilization(p, new_bw);
 		err = 0;
 	} else if (!dl_policy(policy) && task_has_dl_policy(p)) {
@@ -5527,7 +5527,7 @@ int task_can_attach(struct task_struct *p,
 			 * We will free resources in the source root_domain
 			 * later on (see set_cpus_allowed_dl()).
 			 */
-			__dl_add(dl_b, p->dl.dl_bw);
+			__dl_add(dl_b, p->dl.dl_bw, cpus);
 		}
 		raw_spin_unlock_irqrestore(&dl_b->lock, flags);
 		rcu_read_unlock_sched();
@@ -6759,9 +6759,12 @@ void init_dl_rq_bw_ratio(struct dl_rq *dl_rq)
 {
 	if (global_rt_runtime() == RUNTIME_INF) {
 		dl_rq->bw_ratio = 1 << RATIO_SHIFT;
+		dl_rq->extra_bw = 1 << BW_SHIFT;
 	} else {
 		dl_rq->bw_ratio = to_ratio(global_rt_runtime(),
 			  global_rt_period()) >> (BW_SHIFT - RATIO_SHIFT);
+		dl_rq->extra_bw = to_ratio(global_rt_period(),
+						    global_rt_runtime());
 	}
 }
 
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 4286bb8..1da44b3 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -209,7 +209,7 @@ static void task_non_contending(struct task_struct *p)
 			if (p->state == TASK_DEAD)
 				sub_rq_bw(p->dl.dl_bw, &rq->dl);
 			raw_spin_lock(&dl_b->lock);
-			__dl_clear(dl_b, p->dl.dl_bw);
+			__dl_clear(dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
 			__dl_clear_params(p);
 			raw_spin_unlock(&dl_b->lock);
 		}
@@ -955,28 +955,40 @@ extern bool sched_rt_bandwidth_account(struct rt_rq *rt_rq);
 /*
  * This function implements the GRUB accounting rule:
  * according to the GRUB reclaiming algorithm, the runtime is
- * not decreased as "dq = -dt", but as "dq = -max{u, (1 - Uinact)} dt",
- * where u is the utilization of the task and Uinact is the
- * (per-runqueue) inactive utilization, computed as the difference
- * between the "total runqueue utilization" and the runqueue
- * active utilization.
+ * not decreased as "dq = -dt", but as
+ * "dq = -max{u / Umax, (1 - Uinact - Uextra)} dt",
+ * where u is the utilization of the task, Umax is the maximum reclaimable
+ * utilization, Uinact is the (per-runqueue) inactive utilization, computed
+ * as the difference between the "total runqueue utilization" and the
+ * runqueue active utilization, and Uextra is the (per runqueue) extra
+ * reclaimable utilization.
  * Since rq->dl.running_bw and rq->dl.this_bw contain utilizations
- * multiplied by 2^BW_SHIFT, the result has to be shifted right by BW_SHIFT.
+ * multiplied by 2^BW_SHIFT, the result has to be shifted right by
+ * BW_SHIFT.
+ * Since rq->dl.bw_ratio contains 1 / Umax multipled by 2^RATIO_SHIFT,
+ * dl_bw is multiped by rq->dl.bw_ratio and shifted right by RATIO_SHIFT.
+ * Since delta is a 64 bit variable, to have an overflow its value
+ * should be larger than 2^(64 - 20 - 8), which is more than 64 seconds.
+ * So, overflow is not an issue here.
  */
 u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity *dl_se)
 {
 	u64 u_inact = rq->dl.this_bw - rq->dl.running_bw; /* Utot - Uact */
 	u64 u_act;
+	u64 u_act_min = (dl_se->dl_bw * rq->dl.bw_ratio) >> RATIO_SHIFT;
 
 	/*
-	 * Instead of computing max{u, (1 - u_inact)}, we compare
-	 * u_inact with 1 - u, because u_inact can be larger than 1
-	 * (so, 1 - u_inact would be negative leading to wrong results)
+	 * Instead of computing max{u * bw_ratio, (1 - u_inact - u_extra)},
+	 * we compare u_inact + rq->dl.extra_bw with
+	 * 1 - (u * rq->dl.bw_ratio >> RATIO_SHIFT), because
+	 * u_inact + rq->dl.extra_bw can be larger than
+	 * 1 * (so, 1 - u_inact - rq->dl.extra_bw would be negative
+	 * leading to wrong results)
 	 */
-	if (u_inact > BW_UNIT - dl_se->dl_bw)
-		u_act = dl_se->dl_bw;
+	if (u_inact + rq->dl.extra_bw > BW_UNIT - u_act_min)
+		u_act = u_act_min;
 	else
-		u_act = BW_UNIT - u_inact;
+		u_act = BW_UNIT - u_inact - rq->dl.extra_bw;
 
 	return (delta * u_act) >> BW_SHIFT;
 }
@@ -1085,7 +1097,7 @@ static enum hrtimer_restart inactive_task_timer(struct hrtimer *timer)
 		}
 
 		raw_spin_lock(&dl_b->lock);
-		__dl_clear(dl_b, p->dl.dl_bw);
+		__dl_clear(dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
 		raw_spin_unlock(&dl_b->lock);
 		__dl_clear_params(p);
 
@@ -2054,7 +2066,7 @@ static void set_cpus_allowed_dl(struct task_struct *p,
 		 * until we complete the update.
 		 */
 		raw_spin_lock(&src_dl_b->lock);
-		__dl_clear(src_dl_b, p->dl.dl_bw);
+		__dl_clear(src_dl_b, p->dl.dl_bw, dl_bw_cpus(task_cpu(p)));
 		raw_spin_unlock(&src_dl_b->lock);
 	}
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 0939ac4..fdbc30d 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -219,22 +219,27 @@ static inline int dl_bandwidth_enabled(void)
 }
 
 extern struct dl_bw *dl_bw_of(int i);
+extern int dl_bw_cpus(int i);
 
 struct dl_bw {
 	raw_spinlock_t lock;
 	u64 bw, total_bw;
 };
 
+static inline void __dl_update(struct dl_bw *dl_b, s64 bw);
+
 static inline
-void __dl_clear(struct dl_bw *dl_b, u64 tsk_bw)
+void __dl_clear(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
 {
 	dl_b->total_bw -= tsk_bw;
+	__dl_update(dl_b, (s32)tsk_bw / cpus);
 }
 
 static inline
-void __dl_add(struct dl_bw *dl_b, u64 tsk_bw)
+void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
 {
 	dl_b->total_bw += tsk_bw;
+	__dl_update(dl_b, -((s32)tsk_bw / cpus));
 }
 
 static inline
@@ -576,6 +581,7 @@ struct dl_rq {
 	 * runqueue (inactive utilization = this_bw - running_bw).
 	 */
 	u64 this_bw;
+	u64 extra_bw;
 
 	/*
 	 * Inverse of the fraction of CPU utilization that can be reclaimed
@@ -1958,6 +1964,33 @@ extern void nohz_balance_exit_idle(unsigned int cpu);
 static inline void nohz_balance_exit_idle(unsigned int cpu) { }
 #endif
 
+
+#ifdef CONFIG_SMP
+static inline
+void __dl_update(struct dl_bw *dl_b, s64 bw)
+{
+	struct root_domain *rd = container_of(dl_b, struct root_domain, dl_bw);
+	int i;
+
+	RCU_LOCKDEP_WARN(!rcu_read_lock_sched_held(),
+			 "sched RCU must be held");
+	for_each_cpu_and(i, rd->span, cpu_active_mask) {
+		struct rq *rq = cpu_rq(i);
+
+		rq->dl.extra_bw += bw;
+	}
+}
+#else
+static inline
+void __dl_update(struct dl_bw *dl_b, s64 bw)
+{
+	struct dl_rq *dl = container_of(dl_b, struct dl_rq, dl_bw);
+
+	dl->extra_bw += bw;
+}
+#endif
+
+
 #ifdef CONFIG_IRQ_TIME_ACCOUNTING
 struct irqtime {
 	u64			total;
-- 
2.7.4