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NOHZ: prevent multiplication overflow - stop timer for huge timeouts 

get_next_timer_interrupt() returns a delta of (LONG_MAX > 1) in case
there is no timer pending. On 64 bit machines this results in a
multiplication overflow in tick_nohz_stop_sched_tick().

Reported by: Dave Miller <davem@davemloft.net>

Make the return value a constant and limit the return value to a 32 bit
value.

When the max timeout value is returned, we can safely stop the tick
timer device. The max jiffies delta results in a 12 days timeout for
HZ=1000.

In the long term the get_next_timer_interrupt() code needs to be
reworked to return ktime instead of jiffies, but we have to wait until
the last users of the original NO_IDLE_HZ code are converted.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Thomas Gleixner 2007-05-29 23:47:39 +02:00 committed by Linus Torvalds
parent 6e98ee75c3
commit eaad084bb0
3 changed files with 30 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/linux/timer.h
View File

@ -68,6 +68,12 @@ extern int del_timer(struct timer_list * timer);
extern int __mod_timer(struct timer_list *timer, unsigned long expires);
extern int mod_timer(struct timer_list *timer, unsigned long expires);
/*
* The jiffies value which is added to now, when there is no timer
* in the timer wheel:
*/
#define NEXT_TIMER_MAX_DELTA ((1UL << 30) - 1)
/*
* Return when the next timer-wheel timeout occurs (in absolute jiffies),
* locks the timer base:

16
kernel/time/tick-sched.c
View File

@ -247,6 +247,21 @@ void tick_nohz_stop_sched_tick(void)
if (cpu == tick_do_timer_cpu)
tick_do_timer_cpu = -1;
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:
*/
if (unlikely(delta_jiffies >= NEXT_TIMER_MAX_DELTA)) {
ts->idle_expires.tv64 = KTIME_MAX;
if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
hrtimer_cancel(&ts->sched_timer);
goto out;
}
/*
* calculate the expiry time for the next timer wheel
* timer
@ -254,7 +269,6 @@ void tick_nohz_stop_sched_tick(void)
expires = ktime_add_ns(last_update, tick_period.tv64 *
delta_jiffies);
ts->idle_expires = expires;
ts->idle_sleeps++;
if (ts->nohz_mode == NOHZ_MODE_HIGHRES) {
hrtimer_start(&ts->sched_timer, expires,

10
kernel/timer.c
View File

@ -666,7 +666,7 @@ static inline void __run_timers(tvec_base_t *base)
static unsigned long __next_timer_interrupt(tvec_base_t *base)
{
unsigned long timer_jiffies = base->timer_jiffies;
unsigned long expires = timer_jiffies + (LONG_MAX >> 1);
unsigned long expires = timer_jiffies + NEXT_TIMER_MAX_DELTA;
int index, slot, array, found = 0;
struct timer_list *nte;
tvec_t *varray[4];
@ -752,6 +752,14 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
tsdelta = ktime_to_timespec(hr_delta);
delta = timespec_to_jiffies(&tsdelta);
/*
* Limit the delta to the max value, which is checked in
* tick_nohz_stop_sched_tick():
*/
if (delta > NEXT_TIMER_MAX_DELTA)
delta = NEXT_TIMER_MAX_DELTA;
/*
* Take rounding errors in to account and make sure, that it
* expires in the next tick. Otherwise we go into an endless