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SubjectRe: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle

Jon Hunter wrote:
> Jon Hunter wrote:
>> + * 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;
>
> Thinking about this more, although it is very unlikely, for 64-bit
> machines there is a chance that the above multiply could overflow if
> delta_jiffies is very large.
>
> tick_period.tv64 should always be less than NSEC_PER_SEC and so you
> would need delta_jiffies to be greater than 2^32 to cause overflow. On a
> 32-bit machine an unsigned long will not be greater than 2^32 as it is
> only 32-bits but this would be possible on a 64-bit machines.
>
> So to be safe we should make sure that delta_jiffies is not greater than
> NEXT_TIMER_MAX_DELTA (2^30 - 1) before doing the multiply. If you
> think that this is a valid concern, then I can re-work and re-post.
> Sorry for not catching this before.

With regard to the above, to ensure that there are no overflows with the
above calculation, I re-worked this patch a little. The below should be
equivalent to the current code, just re-organised a little. Let me know
if this would be acceptable or not.

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@ti.com>
---
include/linux/clocksource.h | 2 +
include/linux/time.h | 1 +
kernel/time/clocksource.c | 47 +++++++++++++++++++++++++++++++++++
kernel/time/tick-sched.c | 57
++++++++++++++++++++++++++++++++----------
kernel/time/timekeeping.c | 11 ++++++++
5 files changed, 104 insertions(+), 14 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..9988e5e 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 +296,30 @@ 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 expiry time for the next timer wheel
+ * timer. delta_jiffies >= 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 set the expiry to the end of time.
+ */
+ if (likely(delta_jiffies < NEXT_TIMER_MAX_DELTA)) {
+
+ /*
+ * 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;
+
+ expires = ktime_add_ns(last_update, time_delta);
+ } else {
+ expires.tv64 = KTIME_MAX;
+ }

/*
* If this cpu is the one which updates jiffies, then
@@ -331,22 +363,19 @@ void tick_nohz_stop_sched_tick(int inidle)

ts->idle_sleeps++;

+ /* Mark expires */
+ ts->idle_expires = expires;
+
/*
- * 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:
+ * If the expiration time == KTIME_MAX, then
+ * in this case we simply stop the tick timer.
*/
- if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
- ts->idle_expires.tv64 = KTIME_MAX;
+ if (unlikely(expires.tv64 == KTIME_MAX)) {
if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
hrtimer_cancel(&ts->sched_timer);
goto out;
}

- /* Mark expiries */
- ts->idle_expires = expires;
-
if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
hrtimer_start(&ts->sched_timer, expires,
HRTIMER_MODE_ABS);
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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