Messages in this thread Patch in this message | | | Date | Wed, 13 May 2009 10:14:43 -0500 | From | Jon Hunter <> | Subject | Re: [RFC][PATCH] Dynamic Tick: Allow 32-bit machines to sleep formorethan2.15 seconds |
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john stultz wrote: > Well, the mult adjustments should be quite small, especially compared to > the NSEC_PER_SEC/HZ adjustment. > > Hmm... Although, I guess we could get bitten if the max_deferment was > like an hour, and the adjustment was enough that it scaled out to and we > ended up being a second late or so. So you have a point. > > But since the clockevent driver is not scaled, we probably can get away > with using the orig_mult value instead of mult, and be ok. > > Alternatively instead of NSEC_PER_SEC/HZ, we could always drop the > larger of NSEC_PER_SEC/HZ or max_deferment/10? That way we should scale > up without a problem.
Yes, may be this would be a safer option. Thinking about this I was wondering if we should always use max_deferement/10, because I did not think that there would ever be a case where NSEC_PER_SEC/HZ would be greater. If NSEC_PER_SEC/HZ was greater than max_deferement/10 this would imply that the clocksource would wrap after only 10 jiffies, if I have the math right...
> I suspect it would be tough to hit this issue though.
Agree.
> Two patches should be fine.
Ok, I will re-post as two once we have the final version.
> Looks good overall. We may want to add the -10% (or -5%) to be totally > safe, but that's likely just me being paranoid.
I am paranoid too! Do you care if we use 6.25% or 12.5% margin instead of 10% or 5%? This way we can avoid a 64-bit division by using a simple shift. See below. I have implemented a 6.25% margin for now. Let me know your thoughts.
One final question, I noticed in clocksource.h that the definition of function cyc2ns returns a type of s64, however, in the function itself a variable of type u64 is used and returned. Should this function be modified as follows?
static inline s64 cyc2ns(struct clocksource *cs, cycle_t cycles) { - u64 ret = (u64)cycles; + s64 ret = (s64)cycles; ret = (ret * cs->mult) >> cs->shift; return ret; }
Cheers Jon
Signed-off-by: Jon Hunter <jon-hunter@ti.com> --- include/linux/time.h | 1 + kernel/time/tick-sched.c | 36 +++++++++++++++++++++++++----------- kernel/time/timekeeping.c | 28 ++++++++++++++++++++++++++++ 3 files changed, 54 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..e764ac8 100644 --- a/kernel/time/timekeeping.c +++ b/kernel/time/timekeeping.c @@ -271,6 +271,34 @@ 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, margin; + + max_nsecs = cyc2ns(clock, clock->mask); + + /* + * To ensure that the clocksource does not wrap whilst we are idle, + * let's limit the time the clocksource can be deferred by 6.25% of + * the total time the clocksource can count. Please note a margin + * of 6.25% is used because this can be computed with a shift, + * versus say 5% which would require division. + */ + margin = max_nsecs >> 4; + + max_nsecs = max_nsecs - margin; + + 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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