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SubjectRe: [PATCH 13/15] sched: Use a static_key for sched_clock_stable
On Wed, Jan 22, 2014 at 11:45:32AM +0100, Peter Zijlstra wrote:
> Ho humm.

OK, so I had me a ponder; does the below fix things for you and David?
I've only done a boot test on real proper hardware :-)

---
kernel/sched/clock.c | 42 +++++++++++++++++++++++++++++++++---------
1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 6bd6a6731b21..6bbcd97f4532 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -77,35 +77,45 @@ __read_mostly int sched_clock_running;

#ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
static struct static_key __sched_clock_stable = STATIC_KEY_INIT;
+static int __sched_clock_stable_early;

int sched_clock_stable(void)
{
- if (static_key_false(&__sched_clock_stable))
- return false;
- return true;
+ return static_key_false(&__sched_clock_stable);
}

void set_sched_clock_stable(void)
{
+ __sched_clock_stable_early = 1;
+
+ smp_mb(); /* matches sched_clock_init() */
+
+ if (!sched_clock_running)
+ return;
+
if (!sched_clock_stable())
- static_key_slow_dec(&__sched_clock_stable);
+ static_key_slow_inc(&__sched_clock_stable);
}

static void __clear_sched_clock_stable(struct work_struct *work)
{
/* XXX worry about clock continuity */
if (sched_clock_stable())
- static_key_slow_inc(&__sched_clock_stable);
+ static_key_slow_dec(&__sched_clock_stable);
}

static DECLARE_WORK(sched_clock_work, __clear_sched_clock_stable);

void clear_sched_clock_stable(void)
{
- if (keventd_up())
- schedule_work(&sched_clock_work);
- else
- __clear_sched_clock_stable(&sched_clock_work);
+ __sched_clock_stable_early = 0;
+
+ smp_mb(); /* matches sched_clock_init() */
+
+ if (!sched_clock_running)
+ return;
+
+ schedule_work(&sched_clock_work);
}

struct sched_clock_data {
@@ -140,6 +150,20 @@ void sched_clock_init(void)
}

sched_clock_running = 1;
+
+ /*
+ * Ensure that it is impossible to not do a static_key update.
+ *
+ * Either {set,clear}_sched_clock_stable() must see sched_clock_running
+ * and do the update, or we must see their __sched_clock_stable_early
+ * and do the update, or both.
+ */
+ smp_mb(); /* matches {set,clear}_sched_clock_stable() */
+
+ if (__sched_clock_stable_early)
+ set_sched_clock_stable();
+ else
+ clear_sched_clock_stable();
}

/*

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