Messages in this thread Patch in this message | | | From | Jon Hunter <> | Subject | [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle | Date | Tue, 18 Aug 2009 12:45:10 -0500 |
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From: Jon Hunter <jon-hunter@ti.com>
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 9ea40ff..09ed7f1 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -151,6 +151,7 @@ extern u64 timecounter_cyc2time(struct timecounter *tc, * subtraction of non 64 bit counters * @mult: cycle to nanosecond multiplier * @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 @@ -168,6 +169,7 @@ struct clocksource { cycle_t mask; u32 mult; 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 f505988..e68a480 100644 --- a/include/linux/time.h +++ b/include/linux/time.h @@ -146,6 +146,7 @@ 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); extern void timekeeping_leap_insert(int leapsecond); diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 02dc22d..7fffe54 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -369,6 +369,50 @@ void clocksource_touch_watchdog(void) clocksource_resume_watchdog(); } +/** + * 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 = clocksource_cyc2ns(max_cycles, cs->mult, cs->shift); + + /* + * 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; +} + #ifdef CONFIG_GENERIC_TIME static int finished_booting; @@ -461,6 +505,9 @@ static void clocksource_enqueue(struct clocksource *cs) */ int clocksource_register(struct clocksource *cs) { + /* calculate max idle time permitted for this clocksource */ + cs->max_idle_ns = clocksource_max_deferment(cs); + mutex_lock(&clocksource_mutex); clocksource_enqueue(cs); clocksource_select(); diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index e0f59a2..7a98e90 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); @@ -270,6 +271,18 @@ 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 */ @@ -289,11 +302,30 @@ 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 @@ -337,22 +369,19 @@ 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_PINNED); diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c index 15e06de..2e57251 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -487,6 +487,17 @@ 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 timekeeper.clock->max_idle_ns; +} + +/** * read_persistent_clock - Return time from the persistent clock. * * Weak dummy function for arches that do not yet support it. -- 1.6.0.4
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