[PATCH 15/33] sched: Update rq clock on tickless CPUs before calling check_preempt_curr()

From: Frederic Weisbecker
Date: Mon Jan 07 2013 - 21:14:40 EST

check_preempt_wakeup() of fair class needs an uptodate sched clock
value to update runtime stats of the current task.

When a task is woken up, activate_task() is usually called right before
ttwu_do_wakeup() unless the task is already in the runqueue. In this
case we need to update the rq clock manually in case the CPU runs
tickless because ttwu_do_wakeup() calls check_preempt_wakeup().

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Alessio Igor Bogani <abogani@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Geoff Levand <geoff@xxxxxxxxxxxxx>
Cc: Gilad Ben Yossef <gilad@xxxxxxxxxxxxx>
Cc: Hakan Akkan <hakanakkan@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
Cc: Namhyung Kim <namhyung.kim@xxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
kernel/sched/core.c | 17 ++++++++++++++++-
1 files changed, 16 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2fcbb03..3c1a806 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1346,6 +1346,12 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)

rq = __task_rq_lock(p);
if (p->on_rq) {
+ /*
+ * Ensure check_preempt_curr() won't deal with a stale value
+ * of rq clock if the CPU is tickless. BTW do we actually need
+ * check_preempt_curr() to be called here?
+ */
+ update_nohz_rq_clock(rq);
ttwu_do_wakeup(rq, p, wake_flags);
ret = 1;
@@ -1523,8 +1529,17 @@ static void try_to_wake_up_local(struct task_struct *p)
if (!(p->state & TASK_NORMAL))
goto out;

- if (!p->on_rq)
+ if (!p->on_rq) {
ttwu_activate(rq, p, ENQUEUE_WAKEUP);
+ } else {
+ /*
+ * Even if the task is on the runqueue we still
+ * need to ensure check_preempt_curr() won't
+ * deal with a stale rq clock value on a tickless
+ * CPU
+ */
+ update_nohz_rq_clock(rq);
+ }

ttwu_do_wakeup(rq, p, 0);
ttwu_stat(p, smp_processor_id(), 0);

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