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

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Date:   Tue, 17 Oct 2017 14:10:08 +0800
From:   "Li, Aubrey" <aubrey.li@...ux.intel.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        peterz@...radead.org, len.brown@...el.com, ak@...ux.intel.com,
        tim.c.chen@...ux.intel.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v2 5/8] timers: keep sleep length updated as needed

On 2017/10/17 7:58, Rafael J. Wysocki wrote:
> On Monday, October 16, 2017 8:46:41 AM CEST Li, Aubrey wrote:
>> 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
>> 	}
>> }
> 
> Which is logical, because the tick will get in the way if we don't stop it,
> won't it?
> 
The scenario here is, tick(4ms) is too long for a short idle(e.g. 4us).
And there could be hundreds of short idle within one tick interval.

>>
>> 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.
> 
> Well, but does it make sense to estimate the sleep length without stopping
> the tick?

For example, for the first short idle, we just turned tick off last time, so
we may get the sleep length = 3900us. Then we keep tick on, and the 100th short
idle comes, then the original sleep length is still 3900us(not updated), but
actually it should be e.g. 100us.

> 
>>>> ---
>>>>  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?
> 
> That depends on the answer above. :-)

Does the above explanation address the concern?

Thanks,
-Aubrey