sched/deadline: Fix a precision problem in the microseconds range

An overrun could happen in function start_hrtick_dl()
when a task with SCHED_DEADLINE runs in the microseconds
range.

For example, if 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:

  <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 the 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 is coming.

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

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

Signed-off-by: Xiaofeng Yan <xiaofeng.yan@huawei.com>
Reviewed-by: Li Zefan <lizefan@huawei.com>
Acked-by: Juri Lelli <juri.lelli@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1409022941-5880-1-git-send-email-xiaofeng.yan@huawei.com
[ Massaged the changelog and the code. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a773c919d88d1a4257e2a35db7caa4468d38e088..8d00f4a8c126232193d8e8725b68599cbb42cada 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -455,7 +455,15 @@ 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;
+       s64 delta;
+
+       /*
+        * Don't schedule slices shorter than 10000ns, that just
+        * doesn't make sense and can cause timer DoS.
+        */
+       delta = max_t(s64, delay, 10000LL);
+       time = ktime_add_ns(timer->base->get_time(), delta);
 
        hrtimer_set_expires(timer, time);
 

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index d21a8e0259d2d16f69f1070f786955324ab60f6d..cc4eb89019c18d8dc14f1cba737a7728f8d4282b 100644 (file)
--- 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 02fc949eb34828c361055f1713e24d91886a45eb..50d2025c1777b4c4f1da655025ce19455eb65c39 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3897,14 +3897,6 @@ 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);
        }
 }