[PATCH v3 4/5] sched/deadline: Account for normal deadline tasks in GRUB

Vineeth Pillai
2023-05-15T02:57:55Z

GRUB algorithm assumes that all tasks participate in the reclaim. So
when there is a mix of normal deadline and SCHED_FLAG_RECLAIM tasks,
reclaiming the unused bandwidth is not accurate.

Running two deadline tasks on a cpu where one is SCHED_FLAG_RECLAIM
and other is a normal deadline task, we can see the utilization as
follows:

Task 1(Normal DL): (5, 10), Task 2(SCHED_FLAG_RECLAIM): (1, 10)
TID[673]: RECLAIM=0, (r=5ms, d=10ms, p=10ms), Util: 50.11
TID[672]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 15.93
TID[673]: RECLAIM=0, (r=5ms, d=10ms, p=10ms), Util: 50.01
TID[672]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 15.83

GRUB rule says, runtime is dpreciated as:
"dq = -(max{u, (Umax - Uinact - Uextra)} / Umax) dt"
Where Umax is the maximum allowed bandwidth for DL tasks
Uinact is the inactive utilization for the runqueue
Uextra is the free bandwidth available for reclaim

To account for a mix of normal deadline and SCHED_RECLAIM_FLAG tasks
running together, we do not consider the bandwidth of normal tasks in
the equation. So the equation becomes:
"dq = -(max{u, (Umax_reclaim - Uinact - Uextra)} / Umax_reclaim) dt"

"Umax_reclaim" is the maximum allowed bandwidth for SCHED_FLAG_RECLAIM
tasks. When only SCHED_FLAG_RECLAIM tasks are running,
"Umax_reclaim = Umax". Otherwise:
"Umax_reclaim = Umax - running_bw + Ureclaim"
Where Ureclaim is the total bandwidth of SCHED_FLAG_RECLAIM tasks in
active state for this runqueue.

With this fix, the results of above test is as follows:
Task 1(Normal DL): (5, 10), Task 2(SCHED_FLAG_RECLAIM): (1, 10)
TID[591]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 45.11
TID[592]: RECLAIM=0, (r=5ms, d=10ms, p=10ms), Util: 50.18
TID[591]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 44.99
TID[592]: RECLAIM=0, (r=5ms, d=10ms, p=10ms), Util: 49.88

Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
---
kernel/sched/deadline.c | 53 ++++++++++++++++++++++++++++++++---------
kernel/sched/sched.h | 11 +++++++++
2 files changed, 53 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 67c1138df43a..66a1b9365429 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -206,11 +206,13 @@ __dl_overflow(struct dl_bw *dl_b, unsigned long cap, u64 old_bw, u64 new_bw)
}

static inline
-void __add_running_bw(u64 dl_bw, struct dl_rq *dl_rq)
+void __add_running_bw(u64 dl_bw, struct dl_rq *dl_rq, bool reclaim_bw_se)
{
u64 old = dl_rq->running_bw;

lockdep_assert_rq_held(rq_of_dl_rq(dl_rq));
+ if (reclaim_bw_se)
+ dl_rq->reclaim_bw += dl_bw;
dl_rq->running_bw += dl_bw;
SCHED_WARN_ON(dl_rq->running_bw < old); /* overflow */
SCHED_WARN_ON(dl_rq->running_bw > dl_rq->this_bw);
@@ -219,15 +221,19 @@ void __add_running_bw(u64 dl_bw, struct dl_rq *dl_rq)
}

static inline
-void __sub_running_bw(u64 dl_bw, struct dl_rq *dl_rq)
+void __sub_running_bw(u64 dl_bw, struct dl_rq *dl_rq, bool reclaim_bw_se)
{
u64 old = dl_rq->running_bw;

lockdep_assert_rq_held(rq_of_dl_rq(dl_rq));
+ if (reclaim_bw_se)
+ dl_rq->reclaim_bw -= dl_bw;
dl_rq->running_bw -= dl_bw;
SCHED_WARN_ON(dl_rq->running_bw > old); /* underflow */
- if (dl_rq->running_bw > old)
+ if (dl_rq->running_bw > old) {
+ dl_rq->reclaim_bw = 0;
dl_rq->running_bw = 0;
+ }
/* kick cpufreq (see the comment in kernel/sched/sched.h). */
cpufreq_update_util(rq_of_dl_rq(dl_rq), 0);
}
@@ -273,14 +279,14 @@ static inline
void add_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
{
if (!dl_entity_is_special(dl_se))
- __add_running_bw(dl_se->dl_bw, dl_rq);
+ __add_running_bw(dl_se->dl_bw, dl_rq, dl_entity_is_reclaim(dl_se));
}

static inline
void sub_running_bw(struct sched_dl_entity *dl_se, struct dl_rq *dl_rq)
{
if (!dl_entity_is_special(dl_se))
- __sub_running_bw(dl_se->dl_bw, dl_rq);
+ __sub_running_bw(dl_se->dl_bw, dl_rq, dl_entity_is_reclaim(dl_se));
}

static void dl_change_utilization(struct task_struct *p, u64 new_bw)
@@ -499,6 +505,7 @@ void init_dl_rq(struct dl_rq *dl_rq)
#endif

dl_rq->running_bw = 0;
+ dl_rq->reclaim_bw = 0;
dl_rq->this_bw = 0;
init_dl_rq_bw(dl_rq);
}
@@ -1257,20 +1264,44 @@ int dl_runtime_exceeded(struct sched_dl_entity *dl_se)
* but only a portion of it denoted by "Umax". So the equation becomes:
* "dq = -(max{u, (Umax - Uinact - Uextra)} / Umax) dt",
*
+ * To account for the fact that we have a mix of normal deadline tasks and
+ * SCHED_RECLAIM_FLAG tasks running together, we do not consider the bandwidth
+ * of normal tasks in the equation. So the equation becomes:
+ * "dq = -(max{u, (Umax_reclaim - Uinact - Uextra)} / Umax_reclaim) dt",
+ * where
+ * Umax_reclaim: Maximum reclaimable bandwidth for this rq.
+ *
+ * We can calculate Umax_reclaim as:
+ * "Umax_reclaim = Uextra + Uinact + Ureclaim"
+ * where:
+ * Ureclaim: Total bandwidth of SCHED_FLAG_RECLAIM tasks in active
+ * state for this rq.
+ *
* 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.
*/
static u64 grub_reclaim(u64 delta, struct rq *rq, struct sched_dl_entity *dl_se)
{
+ u64 u_max_reclaim;
+
+ /*
+ * In SMP, max_bw can be less than running_bw without violating the global
+ * bandwidth limits. If thats the case, we should not reclaim.
+ */
+ if (rq->dl.max_bw < rq->dl.running_bw)
+ return delta;
+
+ u_max_reclaim = rq->dl.max_bw - rq->dl.running_bw + rq->dl.reclaim_bw;
+
/*
- * max{u, Umax - Uinact - Uextra}
- * = max{u, max_bw - (this_bw - running_bw) + (this_bw - running_bw)}
- * = max{u, running_bw} = running_bw
- * So dq = -(max{u, Umax - Uinact - Uextra} / Umax) dt
- * = -(running_bw / max_bw) dt
+ * max{u, Umax_reclaim - Uinact - Uextra}
+ * = max{u, Uextra + Uinact + Ureclaim - Uinact - Uextra}
+ * = max{u, Ureclaim} = Ureclaim = reclaim_bw
+ * So dq = -(max{u, Umax_reclaim - Uinact - Uextra} / Umax_reclaim) dt
+ * = -(reclaim_bw / Umax_reclaim) dt
*/
- return div64_u64(delta * rq->dl.running_bw, rq->dl.max_bw);
+ return div64_u64(delta * rq->dl.reclaim_bw, u_max_reclaim);
}

/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 33db99756624..a6cb891835da 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -257,6 +257,11 @@ static inline bool dl_entity_is_special(const struct sched_dl_entity *dl_se)
#endif
}

+static inline bool dl_entity_is_reclaim(const struct sched_dl_entity *dl_se)
+{
+ return dl_se->flags & SCHED_FLAG_RECLAIM;
+}
+
/*
* Tells if entity @a should preempt entity @b.
*/
@@ -741,6 +746,12 @@ struct dl_rq {
*/
u64 running_bw;

+ /*
+ * Active bandwidth of SCHED_FLAG_RECLAIM tasks on this rq.
+ * This will be a subset of running_bw.
+ */
+ u64 reclaim_bw;
+
/*
* Utilization of the tasks "assigned" to this runqueue (including
* the tasks that are in runqueue and the tasks that executed on this
--
2.40.1