Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep formorethan2.15 seconds

[Jon Hunter <jon-hunter@ti.com>]

John Stultz wrote:
> So cyc2ns is a very very hot path, so I don't think we want to muck with
> that much.
> 
> Instead of catching the overflows, and then trying to handle them, we
> really want to prevent overflows from happening. That is the main point
> of the timekeeping_max_deferrment() interface after all ;)
> 
> So thinking about it a bit more, what we really want from
> timekeeping_max_deferrment() is roughly:
> 
> 	/* find the max cycle value that would overflow the mult */
> 	max_cycles = -1UUL/clocksource->mult;
> 
> 	/* pick the smaller of max_cycles or the mask value */
> 	max_cycles = min(max_cycles, clocksource->mask);
> 
> 	/* convert max_cycles into ns */
> 	max_time = cyc2ns(clocksource, max_cycles);
> 
> 	/* take off 5% of the max to make sure we don't show up late */
> 	max_time = max_time - max_time/20;
> 
> 
> We should be able to make this reasonably fast via:
> 	max_cycles = 1<<(64 - ilog2(clocksource->mult) - 1);
> 	max_cycles = min(max_cycles, clocksource->mask);
> 	max_time = cyc2ns(clocksource, max_cycles);
> 	max_time = max_time - max_time >> 4;
> 
> Does that seem reasonable?

Yes, good idea. Sorry for the delay in getting back to this. I have 
implemented the above and incorporated into the patch below.

Please note that the only modification I made to the above was to add 1 
to result of the log2. I found that the result of the log2 was rounded 
down and so this was still making max_cycles too big. Therefore to be on 
the safe side I add 1 to the result of log2 which will ensure max_cycles 
is not too big.

Thanks for the inputs. Let me know your thoughts.

Cheers
Jon


Signed-off-by: Jon Hunter <jon-hunter@ti.com>
---
  include/linux/time.h      |    1 +
  kernel/time/tick-sched.c  |   36 +++++++++++++++++++++++----------
  kernel/time/timekeeping.c |   47 
+++++++++++++++++++++++++++++++++++++++++++++
  3 files changed, 73 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..608fc6f 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -271,6 +271,53 @@ 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;
+	u64 max_cycles;
+
+	/*
+	 * Calculate the maximum number of cycles that we can pass to the
+	 * cyc2ns function without overflowing a 64-bit signed result. The
+	 * maximum number of cycles is equal to ULLONG_MAX/clock->mult which
+	 * is equivalent to the below.
+	 * max_cycles < (2^63)/clock->mult
+	 * max_cycles < 2^(log2((2^63)/clock->mult))
+	 * max_cycles < 2^(log2(2^63) - log2(clock->mult))
+	 * max_cycles < 2^(63 - log2(clock->mult))
+	 * max_cycles < 1 << (63 - log2(clock->mult))
+	 * Please note that we add 1 to the result of the log2 to account for
+	 * any rounding errors, ensure the above inequality is satisfied and
+	 * no overflow will occur.
+	 */
+	max_cycles = 1ULL << (63 - (ilog2(clock->mult) + 1));
+
+	/*
+	 * The actual maximum number of cycles we can defer the clocksource is
+	 * determined by the minimum of max_cycles and clock->mask.
+	 */
+	max_cycles = min(max_cycles, clock->mask);
+	max_nsecs = cyc2ns(clock, max_cycles);
+
+	/*
+	 * To ensure that the clocksource does not wrap whilst we are idle,
+	 * limit the time the clocksource can be deferred by 6.25%. Please
+	 * note a margin of 6.25% is used because this can be computed with
+	 * a shift, versus say 5% which would require division.
+	 */
+	max_nsecs = max_nsecs - (max_nsecs >> 4);
+
+	if (max_nsecs < 0)
+		max_nsecs = 0;
+
+	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