Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping duringidle

From: Jon Hunter
Date: Fri May 29 2009 - 15:44:30 EST


Thomas Gleixner wrote:
On Thu, 28 May 2009, Jon Hunter wrote:
/**
+ * timekeeping_max_deferment - Returns max time the clocksource can be
deferred
+ *
+ * IMPORTANT: Caller must observe xtime_lock via read_seqbegin/read_seqretry
+ * to ensure that the clocksource does not change!
+ */

Just nitpicking here. For the intended use case this is irrelevant.
On UP this is called from an irq disabled section, so nothing is
going to change the clock source.

On SMP it does not matter if CPU A goes to sleep with the old clock
source and CPU B changes the clock source while A is idle. When B
goes idle it will take the change into account.

Ok, understood. Let me know if you would like me to remove the comment above. I wanted to make sure that if someone was to use this function else where (can't think of why right now) that they would not over look this.

But that leads me to an interesting observation:

On SMP we really should only care for the CPU which has the do_timer
duty assigned. All other CPUs can sleep as long as they want. When
that CPU goes idle and drops the do_timer duty it needs to look at
max_deferement, but the others can sleep as long as they want.

So the rule would be:

if (cpu == tick_do_timer_cpu || tick_do_timer_cpu == TICK_DO_TIMER_NONE)
check_max_deferment();
else
sleep_as_long_as_you_want;

Could you add that perhaps ?

Absolutely. Please see below and let me know if this is ok.

Thanks for your patience,

No problem. Thanks for the feedback.

Cheers
Jon

The dynamic tick allows the kernel to sleep for periods longer
than a single tick. This patch prevents that the kernel from
sleeping for a period longer than the maximum time that the
current clocksource can count. This ensures that the kernel will
not lose track of time. This patch adds a function called
"clocksource_max_deferment()" that calculates the maximum time the
kernel can sleep for a given clocksource and function called
"timekeeping_max_deferment()" that returns maximum time the kernel
can sleep for the current clocksource.

Signed-off-by: Jon Hunter <jon-hunter@xxxxxx>
---
include/linux/clocksource.h | 2 +
include/linux/time.h | 1 +
kernel/time/clocksource.c | 47 +++++++++++++++++++++++++++++++++++++++++++
kernel/time/tick-sched.c | 47 ++++++++++++++++++++++++++++++++----------
kernel/time/timekeeping.c | 11 ++++++++++
5 files changed, 97 insertions(+), 11 deletions(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index 5a40d14..465af22 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -151,6 +151,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc,
* @mult: cycle to nanosecond multiplier (adjusted by NTP)
* @mult_orig: cycle to nanosecond multiplier (unadjusted by NTP)
* @shift: cycle to nanosecond divisor (power of two)
+ * @max_idle_ns: max idle time permitted by the clocksource (nsecs)
* @flags: flags describing special properties
* @vread: vsyscall based read
* @resume: resume function for the clocksource, if necessary
@@ -171,6 +172,7 @@ struct clocksource {
u32 mult;
u32 mult_orig;
u32 shift;
+ s64 max_idle_ns;
unsigned long flags;
cycle_t (*vread)(void);
void (*resume)(void);
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/clocksource.c b/kernel/time/clocksource.c
index ecfd7b5..18d2b9f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -321,6 +321,50 @@ void clocksource_touch_watchdog(void)
}

/**
+ * clocksource_max_deferment - Returns max time the clocksource can be deferred
+ * @cs: Pointer to clocksource
+ *
+ */
+static s64 clocksource_max_deferment(struct clocksource *cs)
+{
+ 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/cs->mult which
+ * is equivalent to the below.
+ * max_cycles < (2^63)/cs->mult
+ * max_cycles < 2^(log2((2^63)/cs->mult))
+ * max_cycles < 2^(log2(2^63) - log2(cs->mult))
+ * max_cycles < 2^(63 - log2(cs->mult))
+ * max_cycles < 1 << (63 - log2(cs->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(cs->mult) + 1));
+
+ /*
+ * The actual maximum number of cycles we can defer the clocksource is
+ * determined by the minimum of max_cycles and cs->mask.
+ */
+ max_cycles = min(max_cycles, cs->mask);
+ max_nsecs = cyc2ns(cs, max_cycles);
+
+ /*
+ * To ensure that the clocksource does not wrap whilst we are idle,
+ * limit the time the clocksource can be deferred by 12.5%. Please
+ * note a margin of 12.5% is used because this can be computed with
+ * a shift, versus say 10% which would require division.
+ */
+ max_nsecs = max_nsecs - (max_nsecs >> 5);
+
+ return max_nsecs;
+}
+
+/**
* clocksource_get_next - Returns the selected clocksource
*
*/
@@ -405,6 +449,9 @@ int clocksource_register(struct clocksource *c)
/* save mult_orig on registration */
c->mult_orig = c->mult;

+ /* calculate max idle time permitted for this clocksource */
+ c->max_idle_ns = clocksource_max_deferment(c);
+
spin_lock_irqsave(&clocksource_lock, flags);
ret = clocksource_enqueue(c);
if (!ret)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index d3f1ef4..318cf8a 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 +265,18 @@ void tick_nohz_stop_sched_tick(int inidle)
seq = read_seqbegin(&xtime_lock);
last_update = last_jiffies_update;
last_jiffies = jiffies;
+
+ /*
+ * On SMP we really should only care for the CPU which
+ * has the do_timer duty assigned. All other CPUs can
+ * sleep as long as they want.
+ */
+ if (cpu == tick_do_timer_cpu ||
+ tick_do_timer_cpu == TICK_DO_TIMER_NONE)
+ max_time_delta = timekeeping_max_deferment();
+ else
+ max_time_delta = KTIME_MAX;
+
} while (read_seqretry(&xtime_lock, seq));

/* Get the next timer wheel timer */
@@ -283,11 +296,22 @@ 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 +324,7 @@ 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 +356,13 @@ 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..659cae3 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -271,6 +271,17 @@ int timekeeping_valid_for_hres(void)
}

/**
+ * timekeeping_max_deferment - Returns max time the clocksource can be deferred
+ *
+ * IMPORTANT: Caller must observe xtime_lock via read_seqbegin/read_seqretry
+ * to ensure that the clocksource does not change!
+ */
+s64 timekeeping_max_deferment(void)
+{
+ return clock->max_idle_ns;
+}
+
+/**
* 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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