Re: [RESEND PATCH] sched: consider missed ticks when updating global cpu load

[Byungchul Park]

On Fri, Oct 02, 2015 at 09:43:17AM +0900, Byungchul Park wrote:
> On Sat, Sep 26, 2015 at 03:14:45PM +0200, Frederic Weisbecker wrote:
> > On Fri, Sep 25, 2015 at 05:52:37PM +0900, byungchul.park@xxxxxxx wrote:
> > > From: Byungchul Park <byungchul.park@xxxxxxx>
> > > 
> > > hello,
> > > 
> > > i have already sent this patch about 1 month ago.
> > > (see https://lkml.org/lkml/2015/8/13/160)
> > > 
> > > now, i am resending the same patch with adding some additional commit 
> > > message.
> > > 
> > > thank you,
> > > byungchul
> > > 
> > > ----->8-----
> > > From 8ece9a0482e74a39cd2e9165bf8eec1d04665fa9 Mon Sep 17 00:00:00 2001
> > > From: Byungchul Park <byungchul.park@xxxxxxx>
> > > Date: Fri, 25 Sep 2015 17:10:10 +0900
> > > Subject: [RESEND PATCH] sched: consider missed ticks when updating global cpu
> > >  load
> > > 
> > > in hrtimer_interrupt(), the first tick_program_event() can be failed
> > > because the next timer could be already expired due to,
> > > (see the comment in hrtimer_interrupt())
> > > 
> > > - tracing
> > > - long lasting callbacks
> > > - being scheduled away when running in a VM
> > > 
> > > in the case that the first tick_program_event() is failed, the second
> > > tick_program_event() set the expired time to more than one tick later.
> > > then next tick can happen after more than one tick, even though tick is
> > > not stopped by e.g. NOHZ.
> > > 
> > > when the next tick occurs, update_process_times() -> scheduler_tick()
> > > -> update_cpu_load_active() is performed, assuming the distance between
> > > last tick and current tick is 1 tick! it's wrong in this case. thus,
> > > this abnormal case should be considered in update_cpu_load_active().
> > > 
> > > Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> > > ---
> > >  kernel/sched/fair.c |    7 +++++--
> > >  1 file changed, 5 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 4d5f97b..829282f 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -4356,12 +4356,15 @@ void update_cpu_load_nohz(void)
> > >   */
> > >  void update_cpu_load_active(struct rq *this_rq)
> > >  {
> > > +	unsigned long curr_jiffies = READ_ONCE(jiffies);
> > > +	unsigned long pending_updates;
> > >  	unsigned long load = weighted_cpuload(cpu_of(this_rq));
> > >  	/*
> > >  	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
> > >  	 */
> > > -	this_rq->last_load_update_tick = jiffies;
> > > -	__update_cpu_load(this_rq, load, 1);
> > > +	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> > > +	this_rq->last_load_update_tick = curr_jiffies;
> > > +	__update_cpu_load(this_rq, load, pending_updates);
> > >  }
> > 
> > That's right but __update_cpu_load() doesn't handle correctly pending updates
> > with non-zero loads. Currently, pending updates are wheeled through decay_load_missed()
> > that assume it's all about idle load.
> 
> i see, i will check it.
> 
> > 
> > But in the cases you've enumerated, as well as in the nohz full case, missed pending
> > updates can be about buzy loads.
> 
> right. it can be about busy loads.
> 
> > 
> > I think we need to fix update_cpu_load() to handle that first, or your fix is
> > going to make things worse.
> 
> i will try to fix it at first if there's already that kind of bugs.

i checked it.. current code does not handle more than one active tick
at all, rather than bug..

> 
> thanks,
> byungchul
> 
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