linux-kernel - [34-longterm 091/260] sched: Fix rq->clock synchronization when migrating tasks

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Message-Id: <1293952756-15010-92-git-send-email-paul.gortmaker@windriver.com>
Date:	Sun,  2 Jan 2011 02:16:27 -0500
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	stable@...nel.org, linux-kernel@...r.kernel.org
Cc:	stable-review@...nel.org, Peter Zijlstra <peterz@...radead.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: [34-longterm 091/260] sched: Fix rq->clock synchronization when migrating tasks

From: Peter Zijlstra <peterz@...radead.org>

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@...il.com>
Tested-by: Philby John <pjohn@...sta.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
LKML-Reference: <1281002322.1923.1708.camel@...top>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
---
 kernel/sched_fair.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 684e173..461d312 100644
--- 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);
 
-- 
1.7.3.3

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