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

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Date:	Fri, 2 Oct 2015 09:43:17 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [RESEND PATCH] sched: consider missed ticks when updating global
 cpu load

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@....com wrote:
> > From: Byungchul Park <byungchul.park@....com>
> > 
> > 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@....com>
> > 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@....com>
> > ---
> >  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.

thanks,
byungchul

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