releases/3.18.3/sched-deadline-fix-migration-of-sched_deadline-tasks.patch

   1 From 6a503c3be937d275113b702e0421e5b0720abe8a Mon Sep 17 00:00:00 2001
   2 From: Luca Abeni <luca.abeni@unitn.it>
   3 Date: Wed, 17 Dec 2014 11:50:31 +0100
   4 Subject: sched/deadline: Fix migration of SCHED_DEADLINE tasks
   5
   6 From: Luca Abeni <luca.abeni@unitn.it>
   7
   8 commit 6a503c3be937d275113b702e0421e5b0720abe8a upstream.
   9
  10 According to global EDF, tasks should be migrated between runqueues
  11 without checking if their scheduling deadlines and runtimes are valid.
  12 However, SCHED_DEADLINE currently performs such a check:
  13 a migration happens doing:
  14
  15         deactivate_task(rq, next_task, 0);
  16         set_task_cpu(next_task, later_rq->cpu);
  17         activate_task(later_rq, next_task, 0);
  18
  19 which ends up calling dequeue_task_dl(), setting the new CPU, and then
  20 calling enqueue_task_dl().
  21
  22 enqueue_task_dl() then calls enqueue_dl_entity(), which calls
  23 update_dl_entity(), which can modify scheduling deadline and runtime,
  24 breaking global EDF scheduling.
  25
  26 As a result, some of the properties of global EDF are not respected:
  27 for example, a taskset {(30, 80), (40, 80), (120, 170)} scheduled on
  28 two cores can have unbounded response times for the third task even
  29 if 30/80+40/80+120/170 = 1.5809 < 2
  30
  31 This can be fixed by invoking update_dl_entity() only in case of
  32 wakeup, or if this is a new SCHED_DEADLINE task.
  33
  34 Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
  35 Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
  36 Acked-by: Juri Lelli <juri.lelli@gmail.com>
  37 Cc: Dario Faggioli <raistlin@linux.it>
  38 Cc: Linus Torvalds <torvalds@linux-foundation.org>
  39 Link: http://lkml.kernel.org/r/1418813432-20797-2-git-send-email-luca.abeni@unitn.it
  40 Signed-off-by: Ingo Molnar <mingo@kernel.org>
  41 Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
  42
  43 ---
  44  kernel/sched/deadline.c |    6 +++---
  45  1 file changed, 3 insertions(+), 3 deletions(-)
  46
  47 --- a/kernel/sched/deadline.c
  48 +++ b/kernel/sched/deadline.c
  49 @@ -831,10 +831,10 @@ enqueue_dl_entity(struct sched_dl_entity
  50          * parameters of the task might need updating. Otherwise,
  51          * we want a replenishment of its runtime.
  52          */
  53 -       if (!dl_se->dl_new && flags & ENQUEUE_REPLENISH)
  54 -               replenish_dl_entity(dl_se, pi_se);
  55 -       else
  56 +       if (dl_se->dl_new || flags & ENQUEUE_WAKEUP)
  57                 update_dl_entity(dl_se, pi_se);
  58 +       else if (flags & ENQUEUE_REPLENISH)
  59 +               replenish_dl_entity(dl_se, pi_se);
  60
  61         __enqueue_dl_entity(dl_se);
  62  }