Gilad reported at http://lkml.kernel.org/r/1336056962-10465-2-git-send-email-gilad@benyossef.com "Current timer code fails to correctly return a value meaning that there is no future timer event, with the result that the timer keeps getting re-armed in HZ one shot mode even when we could turn it off, generating unneeded interrupts. What is happening is that when __next_timer_interrupt() wishes to return a value that signifies "there is no future timer event", it returns (base->timer_jiffies + NEXT_TIMER_MAX_DELTA). However, the code in tick_nohz_stop_sched_tick(), which called __next_timer_interrupt() via get_next_timer_interrupt(), compares the return value to (last_jiffies + NEXT_TIMER_MAX_DELTA) to see if the timer needs to be re-armed. base->timer_jiffies != last_jiffies and so tick_nohz_stop_sched_tick() interperts the return value as indication that there is a distant future event 12 days from now and programs the timer to fire next after KTIME_MAX nsecs instead of avoiding to arm it. This ends up causing a needless interrupt once every KTIME_MAX nsecs." Fix this by using the new active timer accounting. This avoids scans when no active timer is enqueued completely, so we don't have to rely on base->timer_next and base->timer_jiffies anymore.
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| @@ -1326,18 +1326,21 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now, · 1325 unmodified lines | |||
| 1326 | unsigned long get_next_timer_interrupt(unsigned long now) | 1326 | unsigned long get_next_timer_interrupt(unsigned long now) |
| 1327 | { | 1327 | { |
| 1328 | struct tvec_base *base = __this_cpu_read(tvec_bases); | 1328 | struct tvec_base *base = __this_cpu_read(tvec_bases); |
| 1329 | unsigned long expires; | 1329 | unsigned long expires = now + NEXT_TIMER_MAX_DELTA; |
| 1330 | 1330 | ||
| 1331 | /* | 1331 | /* |
| 1332 | * Pretend that there is no timer pending if the cpu is offline. | 1332 | * Pretend that there is no timer pending if the cpu is offline. |
| 1333 | * Possible pending timers will be migrated later to an active cpu. | 1333 | * Possible pending timers will be migrated later to an active cpu. |
| 1334 | */ | 1334 | */ |
| 1335 | if (cpu_is_offline(smp_processor_id())) | 1335 | if (cpu_is_offline(smp_processor_id())) |
| 1336 | return now + NEXT_TIMER_MAX_DELTA; | 1336 | return expires; |
| 1337 | |||
| 1337 | spin_lock(&base->lock); | 1338 | spin_lock(&base->lock); |
| 1338 | if (time_before_eq(base->next_timer, base->timer_jiffies)) | 1339 | if (base->active_timers) { |
| 1339 | base->next_timer = __next_timer_interrupt(base); | 1340 | if (time_before_eq(base->next_timer, base->timer_jiffies)) |
| 1340 | expires = base->next_timer; | 1341 | base->next_timer = __next_timer_interrupt(base); |
| 1342 | expires = base->next_timer; | ||
| 1343 | } | ||
| 1341 | spin_unlock(&base->lock); | 1344 | spin_unlock(&base->lock); |
| 1342 | 1345 | ||
| 1343 | if (time_before_eq(expires, now)) | 1346 | if (time_before_eq(expires, now)) |