Re: [RFC PATCH v2 5/8] timers: keep sleep length updated as needed

Li, Aubrey
2017-10-16T06:46:46Z

On 2017/10/14 8:56, Rafael J. Wysocki wrote:
> On Saturday, September 30, 2017 9:20:31 AM CEST Aubrey Li wrote:
>> sleep length indicates how long we'll be idle. Currently, it's updated
>> only when tick nohz enters. These patch series make a new requirement
>> with tick, so we should keep sleep length updated as needed
>
> So what exactly would be the problem with leaving things as they are?

Previously ts->sleep_length is only updated when tick is stopped.

As follows, in

__tick_nohz_idle_enter()
{
if (can_stop_idle_tick() /* return true */) {
tick_nohz_stop_sched_tick()
|
|-----> update sleep_length
}
}

Now ts->sleep_length is required out of tick_nohz_idle_enter(), so we want
to update sleep_length every time we read it

If we leave it unchanged, the prediction could read a sleep_length long time
ago if the system keep ticking.
>
>> ---
>> kernel/time/tick-sched.c | 3 +++
>> 1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
>> index d663fab..94fb9b8 100644
>> --- a/kernel/time/tick-sched.c
>> +++ b/kernel/time/tick-sched.c
>> @@ -1008,8 +1008,11 @@ void tick_nohz_irq_exit(void)
>> */
>> ktime_t tick_nohz_get_sleep_length(void)
>> {
>> + struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
>> struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
>>
>> + ts->sleep_length = ktime_sub(dev->next_event, ktime_get());
>> +
>> return ts->sleep_length;
>> }
>>
>>
>
> I probably wouldn't do it this way ...
>
>

May I know the detailed thoughts?

Thanks,
-Aubrey