Re: [PATCH v4 2/2] sched: consider missed ticks in full NOHZ

[Byungchul Park]

On Mon, Nov 02, 2015 at 05:10:16PM +0100, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 06:47:36PM +0900, byungchul.park@xxxxxxx wrote:
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4428,7 +4428,7 @@ static void update_idle_cpu_load(struct rq *this_rq)
> 
> So if one were to read the comment above update_idle_cpu_load() one
> would find there's a problem with jiffy based accounting.
> 
> >  /*
> >   * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
> >   */
> > -void update_cpu_load_nohz(void)
> > +void update_cpu_load_nohz(int active)
> 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 7c7ec45..515edf3 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> 
> > -static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now)
> > +static void tick_nohz_restart_sched_tick(struct tick_sched *ts, ktime_t now, int active)
> >  {
> >  	/* Update jiffies first */
> >  	tick_do_update_jiffies64(now);
> > -	update_cpu_load_nohz();
> > +	update_cpu_load_nohz(active);
> >  
> >  	calc_load_exit_idle();
> >  	touch_softlockup_watchdog();
> 
> And we could solve all that nicely if we pull up the hrtimer_forward()
> result from tick_nohz_restart(), that way we have the actual number of
> ticks lost on this cpu, and no need to start guessing about it.

hello,

are you talking about the lag between writer and reader for jiffies?
i think your proposal can solve the problem of update_cpu_load_nohz().
but it's still hard to care the cases of update_idle_cpu_load()
and update_cpu_load_active() even by the way you proposed.

do you think it would be ok even if it solves only one case?
update_idle_cpu_load() still need to guess about it. is there something
i missed? or did i mis-understand what you intend?

> 
> Hmm?
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