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SubjectRe: [PATCH] tick: make sleep length calculation more accurate
On 10/01/2013 11:31 PM, Stephen Boyd wrote:
> On 09/27/13 03:52, Daniel Lezcano wrote:
>> The sleep_length is computed in the tick_nohz_stop_sched_tick function but it
>> is used later in the code with in between the local irq enabled.
>>
>> cpu_idle_loop
>> tick_nohz_idle_enter [ exits with local irq enabled ]
>> __tick_nohz_idle_enter
>> tick_nohz_stop_sched_tick
>> ...
>>
>> arch_cpu_idle
>> menu_select [ uses here 'sleep_length' ]
>> ...
>>
>> Between the computation of the sleep length and its usage, some interrupts
>> can occur, making the sleep length shorter than actually it is.
>>
>> This patch fixes that by moving the sleep_length computation in the
>> tick_nohz_get_sleep_length function and store the next_event for the device
>> instead of the sleep_length.
>>
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
>> ---
>> include/linux/tick.h | 2 +-
>> kernel/time/tick-sched.c | 5 +++--
>> 2 files changed, 4 insertions(+), 3 deletions(-)
>>
>> diff --git a/include/linux/tick.h b/include/linux/tick.h
>> index 5128d33..4932004 100644
>> --- a/include/linux/tick.h
>> +++ b/include/linux/tick.h
>> @@ -67,7 +67,7 @@ struct tick_sched {
>> ktime_t idle_exittime;
>> ktime_t idle_sleeptime;
>> ktime_t iowait_sleeptime;
>> - ktime_t sleep_length;
>> + ktime_t next_event;
>> unsigned long last_jiffies;
>> unsigned long next_jiffies;
>> ktime_t idle_expires;
>
> Documentation update?
>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index 3612fc7..2007a7f 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -673,7 +673,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
>> out:
>> ts->next_jiffies = next_jiffies;
>> ts->last_jiffies = last_jiffies;
>> - ts->sleep_length = ktime_sub(dev->next_event, now);
>> + ts->next_event = dev->next_event;
>>
>> return ret;
>> }
>> @@ -837,8 +837,9 @@ void tick_nohz_irq_exit(void)
>> ktime_t tick_nohz_get_sleep_length(void)
>> {
>> struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
>> + ktime_t now = ktime_get();
>>
>> - return ts->sleep_length;
>> + return ktime_sub(ts->next_event, now);
>> }
>>
>> static void tick_nohz_restart(struct tick_sched *ts, ktime_t now)
>
> What happens if the idling CPU's next_event is updated via that
> interrupt? Say if the interrupt handler schedules a timer to fire before
> the next timer on the CPU? It looks like we won't notice that.

Yes, or after.

It sounds like this issue also occurs with the current code, no ?

> Perhaps it's better to do this instead?
>
> ktime_t tick_nohz_get_sleep_length(void)
> {
> struct tick_sched *ts = &__get_cpu_var(tick_cpu_sched);
> + ktime_t now = ktime_get();
> + struct clock_event_device *dev = __get_cpu_var(tick_cpu_device).evtdev;
>
> - return ts->sleep_length;
> + return ktime_sub(dev->next_event, now);
> }

Yes, I agree.

Thanks for the review.

-- Daniel


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