From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754238AbZFDT3v (ORCPT ); Thu, 4 Jun 2009 15:29:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752703AbZFDT3o (ORCPT ); Thu, 4 Jun 2009 15:29:44 -0400 Received: from arroyo.ext.ti.com ([192.94.94.40]:44399 "EHLO arroyo.ext.ti.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752457AbZFDT3n (ORCPT ); Thu, 4 Jun 2009 15:29:43 -0400 Message-ID: <4A282089.5020103@ti.com> Date: Thu, 4 Jun 2009 14:29:13 -0500 From: Jon Hunter User-Agent: Thunderbird 2.0.0.21 (X11/20090318) MIME-Version: 1.0 To: Thomas Gleixner CC: john stultz , "linux-kernel@vger.kernel.org" , Ingo Molnar Subject: Re: [PATCH 1/2] Dynamic Tick: Prevent clocksource wrapping during idle References: <4A1D52E3.3040204@ti.com> <1243455615.7440.17.camel@localhost.localdomain> <4A1EF244.5040108@ti.com> <4A1EFDBD.8040007@ti.com> <4A208538.3040406@ti.com> In-Reply-To: <4A208538.3040406@ti.com> Content-Type: text/plain; charset="iso-8859-1"; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- 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