Re: [PATCH] tick: make sleep length calculation more accurate

[Daniel Lezcano <daniel.lezcano@linaro.org>]

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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