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

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Date:   Tue, 17 Oct 2017 01:58:13 +0200
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>
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 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?

> 
> 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?

> >> ---
> >>  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. :-)

Thanks,
Rafael