[PATCH v4] sched/deadline: Fix the precision problem in the microsecond range

[xiaofeng.yan]

The overrun could happen in function start_hrtick_dl()
when a task with SCHED_DEADLINE runs in the microsecond range.
For example, a task with SCHED_DEADLINE has the following parameters
Task  runtime  deadline  period
 P1   200us     500us    500us

The deadline and period from task P1 are less than 1ms.
In order to achieve microsecond precision, we need to enable HRTICK feature
by the next command.

PC#echo "HRTICK" > /sys/kernel/debug/sched_features
PC#trace-cmd record -e sched_switch &
PC#./schedtool -E -t 200000:500000:500000 -e ./test
The binary test is in an endless while(1) loop here.
Some pieces of trace.dat are as follows:
(Remove some irrelevant information)
<idle>-0   157.603157: sched_switch: :R ==> 2481:4294967295: test
test-2481  157.603203: sched_switch:  2481:R ==> 0:120: swapper/2
<idle>-0   157.605657: sched_switch:  :R ==> 2481:4294967295: test
test-2481  157.608183: sched_switch:  2481:R ==> 2483:120: trace-cmd
trace-cmd-2483 157.609656: sched_switch:2483:R==>2481:4294967295: test

We can get a runtime of P1 from the information above.
runtime = 157.608183 - 157.605657
runtime = 0.002526(2.526ms)
The correct runtime should be less than or equal to 200us at some point.

The problem is caused by a conditional judgment "delta > 10000"
in function start_hrtick_dl().
Because no hrtimer start up to control the rest of runtime
when the reset of runtime is less than 10us.
So the process will continue to run until tick-period coming.

Move the code with the limit of the least time slice
from hrtick_start_fair() to hrtick_start() because
EDF schedule class also need this function in start_hrtick_dl().

To fix this problem, we call hrtimer_start() unconditionally in start_hrtick_dl(),
and make sure schedule slice won't be smaller than 10us in hrtimer_start().

Signed-off-by: Xiaofeng Yan <xiaofeng.yan@xxxxxxxxxx>
Reviewed-by:   Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Reviewed-by:   Li Zefan <lizefan@xxxxxxxxxx>
Acked-by:      Juri Lelli <juri.lelli@xxxxxxx>
---
 kernel/sched/core.c     |    8 +++++++-
 kernel/sched/deadline.c |    5 +----
 kernel/sched/fair.c     |    8 --------
 3 files changed, 8 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ec1a286..da2c6f3 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -449,8 +449,14 @@ static void __hrtick_start(void *arg)
 void hrtick_start(struct rq *rq, u64 delay)
 {
 	struct hrtimer *timer = &rq->hrtick_timer;
-	ktime_t time = ktime_add_ns(timer->base->get_time(), delay);
+	ktime_t time;
 
+	/*
+	 * Don't schedule slices shorter than 10000ns, that just
+	 * doesn't make sense and can cause timer DoS.
+	 */
+	s64 delta = max_t(s64, delay, 10000LL);
+	time = ktime_add_ns(timer->base->get_time(), delta);
 	hrtimer_set_expires(timer, time);
 
 	if (rq == this_rq()) {
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 255ce13..ce52d07 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -997,10 +997,7 @@ static void check_preempt_curr_dl(struct rq *rq, struct task_struct *p,
 #ifdef CONFIG_SCHED_HRTICK
 static void start_hrtick_dl(struct rq *rq, struct task_struct *p)
 {
-	s64 delta = p->dl.dl_runtime - p->dl.runtime;
-
-	if (delta > 10000)
-		hrtick_start(rq, p->dl.runtime);
+	hrtick_start(rq, p->dl.runtime);
 }
 #endif
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bfa3c86..0d6b3e6 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3892,14 +3892,6 @@ static void hrtick_start_fair(struct rq *rq, struct task_struct *p)
 				resched_curr(rq);
 			return;
 		}
-
-		/*
-		 * Don't schedule slices shorter than 10000ns, that just
-		 * doesn't make sense. Rely on vruntime for fairness.
-		 */
-		if (rq->curr != p)
-			delta = max_t(s64, 10000LL, delta);
-
 		hrtick_start(rq, delta);
 	}
 }
-- 
1.7.9.5

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