Messages in this thread | | | Date | Wed, 27 May 2009 13:54:24 -0700 | Subject | Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle | From | Alok Kataria <> |
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On Wed, May 27, 2009 at 7:49 AM, Jon Hunter <jon-hunter@ti.com> wrote: > > 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 new function > called "timekeeping_max_deferment()" that calculates the maximum time the > kernel can sleep for a given clocksource. >
From the patch description I understand that this will avoid wrapping around for only the *current* clocksource. What happens if, say, TSC is the clocksource and ACPI_PM is being used as the watchdog_clocksource, in that case the timekeeping_max_deferement will give TSC' max allowed sleep value (which is greater than ACPI_PMs). i.e. We could still sleep beyond ACPI_PM's wrap around threshold which may result in us marking TSC as unsuable as a clocksource. That could still result in incorrect timekeeping right ?
Thanks, Alok
> 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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