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Message-ID: <35f686220905271354l5f372d39t28700b1cf29ce6b2@mail.gmail.com>
Date: Wed, 27 May 2009 13:54:24 -0700
From: Alok Kataria <alokkataria1@...il.com>
To: Jon Hunter <jon-hunter@...com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
john stultz <johnstul@...ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...e.hu>, akataria@...are.com
Subject: Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during
idle
On Wed, May 27, 2009 at 7:49 AM, Jon Hunter <jon-hunter@...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@...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
> --
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