sched: Fix rq->clock synchronization when migrating tasks

commit 861d034ee814917a83bd5de4b26e3b8336ddeeb8 upstream.

sched_fork() -- we do task placement in ->task_fork_fair() ensure we
  update_rq_clock() so we work with current time. We leave the vruntime
  in relative state, so the time delay until wake_up_new_task() doesn't
  matter.

wake_up_new_task() -- Since task_fork_fair() left p->vruntime in
  relative state we can safely migrate, the activate_task() on the
  remote rq will call update_rq_clock() and causes the clock to be
  synced (enough).

Tested-by: Jack Daniel <wanders.thirst@gmail.com>
Tested-by: Philby John <pjohn@mvista.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <1281002322.1923.1708.camel@laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 684e173c00f4796b9866f3b8c3c06013d9168638..461d312d54db453e85b5097cb667fdbf5d0b3ceb 100644 (file)
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -3588,6 +3588,8 @@ static void task_fork_fair(struct task_struct *p)
 
        raw_spin_lock_irqsave(&rq->lock, flags);
 
+       update_rq_clock(rq);
+
        if (unlikely(task_cpu(p) != this_cpu))
                __set_task_cpu(p, this_cpu);