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Date: Tue, 12 May 2009 18:35:50 -0500
From: Jon Hunter <jon-hunter@...com>
To: john stultz <johnstul@...ibm.com>
CC: Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep formorethan
2.15 seconds
john stultz wrote:
> Yea. NSEC_PER_SEC/HZ would probably be safe. I was initially thinking
> being more paranoid and just dividing it in half, but that's probably a
> bit silly.
Thanks, I have added the code to subtract NSEC_PER_SEC/HZ. Should we
have any concerns about the adjustment of the mult value? This is the
only thing that could impact the value returned from
timekeeping_max_deferment(). I am not familiar with exactly how this is
working so just wanted to ask.
> As far the decision to defer if the next even is greater then one jiffy
> away, that seems reasonable, but I'd not embed that into the
> timekeeping_max_deferrment().
>
> I'm suggesting we drop timekeeping_max_deferrment() down since that's
> the absolute maximum and we're sure to break if we actually wait that
> long (since the time between clocksource reads would certainly be longer
> due to execution delay). 1HZ seems reasonable, since we should easily be
> able to run the tick code twice in that time, as well as it should be
> easily within the interrupt programming granularity.
>
> Any additional decisions as to how far out we should be before we start
> skipping ticks would be up to the tick resched code, and shouldn't be in
> the timekeeping function.
>
> Sound sane? If so add that in and I'll ack it.
Yes, agree. See below. By the way I have kept the below patch separate
from the original I posted here:
http://marc.info/?l=linux-kernel&m=124026224019895&w=2
I was not sure if you would prefer to keep these as two patch series or
make it one single patch. Let me know if you would like me to combine or
re-post as a two patch series.
Please note that the environment I have been running some basic tests on
is a single core ARM device. I just wanted to let you know in case you
have any concerns with this.
> This looks *much* better to me. Thanks for reworking it!
Great! No problem. Thanks for your help and feedback.
Cheers
Jon
Signed-off-by: Jon Hunter <jon-hunter@...com>
---
include/linux/time.h | 1 +
kernel/time/tick-sched.c | 36 +++++++++++++++++++++++++-----------
kernel/time/timekeeping.c | 19 +++++++++++++++++++
3 files changed, 45 insertions(+), 11 deletions(-)
diff --git a/include/linux/time.h b/include/linux/time.h
index 242f624..090be07 100644
--- a/include/linux/time.h
+++ b/include/linux/time.h
@@ -130,6 +130,7 @@ extern void monotonic_to_bootbased(struct timespec *ts);
extern struct timespec timespec_trunc(struct timespec t, unsigned gran);
extern int timekeeping_valid_for_hres(void);
+extern s64 timekeeping_max_deferment(void);
extern void update_wall_time(void);
extern void update_xtime_cache(u64 nsec);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d3f1ef4..f0155ae 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -217,6 +217,7 @@ void tick_nohz_stop_sched_tick(int inidle)
ktime_t last_update, expires, now;
struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
int cpu;
+ s64 time_delta, max_time_delta;
local_irq_save(flags);
@@ -264,6 +265,7 @@ void tick_nohz_stop_sched_tick(int inidle)
seq = read_seqbegin(&xtime_lock);
last_update = last_jiffies_update;
last_jiffies = jiffies;
+ max_time_delta = timekeeping_max_deferment();
} while (read_seqretry(&xtime_lock, seq));
/* Get the next timer wheel timer */
@@ -283,11 +285,22 @@ void tick_nohz_stop_sched_tick(int inidle)
if ((long)delta_jiffies >= 1) {
/*
- * calculate the expiry time for the next timer wheel
- * timer
- */
- expires = ktime_add_ns(last_update, tick_period.tv64 *
- delta_jiffies);
+ * Calculate the time delta for the next timer event.
+ * If the time delta exceeds the maximum time delta
+ * permitted by the current clocksource then adjust
+ * the time delta accordingly to ensure the
+ * clocksource does not wrap.
+ */
+ time_delta = tick_period.tv64 * delta_jiffies;
+
+ if (time_delta > max_time_delta)
+ time_delta = max_time_delta;
+
+ /*
+ * calculate the expiry time for the next timer wheel
+ * timer
+ */
+ expires = ktime_add_ns(last_update, time_delta);
/*
* If this cpu is the one which updates jiffies, then
@@ -300,7 +313,7 @@ void tick_nohz_stop_sched_tick(int inidle)
if (cpu == tick_do_timer_cpu)
tick_do_timer_cpu = TICK_DO_TIMER_NONE;
- if (delta_jiffies > 1)
+ if (time_delta > tick_period.tv64)
cpumask_set_cpu(cpu, nohz_cpu_mask);
/* Skip reprogram of event if its not changed */
@@ -332,12 +345,13 @@ void tick_nohz_stop_sched_tick(int inidle)
ts->idle_sleeps++;
/*
- * delta_jiffies >= NEXT_TIMER_MAX_DELTA signals that
- * there is no timer pending or at least extremly far
- * into the future (12 days for HZ=1000). In this case
- * we simply stop the tick timer:
+ * time_delta >= (tick_period.tv64 * NEXT_TIMER_MAX_DELTA)
+ * signals that there is no timer pending or at least
+ * extremely far into the future (12 days for HZ=1000).
+ * In this case we simply stop the tick timer:
*/
- if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
+ if (unlikely(time_delta >=
+ (tick_period.tv64 * NEXT_TIMER_MAX_DELTA))) {
ts->idle_expires.tv64 = KTIME_MAX;
if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
hrtimer_cancel(&ts->sched_timer);
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 687dff4..7617fbe 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -271,6 +271,25 @@ int timekeeping_valid_for_hres(void)
}
/**
+ * timekeeping_max_deferment - Returns max time the clocksource can be
deferred
+ *
+ * IMPORTANT: Must be called with xtime_lock held!
+ */
+s64 timekeeping_max_deferment(void)
+{
+ s64 max_nsecs;
+
+ /*
+ * Limit the time the clocksource can be
+ * deferred by one jiffie period to ensure
+ * that the clocksource will not wrap.
+ */
+ max_nsecs = cyc2ns(clock, clock->mask) - (NSEC_PER_SEC/HZ);
+
+ return max_nsecs;
+}
+
+/**
* read_persistent_clock - Return time in seconds from the persistent
clock.
*
* Weak dummy function for arches that do not yet support it.
--
1.6.1
--
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