linux-kernel - Re: [PATCH v5 2/2] sched: make update_cpu_load_nohz() consider missed ticks in NOHZ

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Message-ID: <20151123023214.GA3514@byungchulpark-X58A-UD3R>
Date:	Mon, 23 Nov 2015 11:32:14 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org, efault@....de,
	tglx@...utronix.de, fweisbec@...il.com
Subject: Re: [PATCH v5 2/2] sched: make update_cpu_load_nohz() consider
 missed ticks in NOHZ_FULL

On Fri, Nov 20, 2015 at 02:19:55PM +0100, Peter Zijlstra wrote:
> On Tue, Nov 10, 2015 at 09:36:02AM +0900, byungchul.park@....com wrote:
> > +++ b/kernel/sched/fair.c
> > @@ -4419,10 +4419,11 @@ static void update_idle_cpu_load(struct rq *this_rq)
> >  /*
> >   * 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)
> >  {
> >  	struct rq *this_rq = this_rq();
> >  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> > +	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
> >  	unsigned long pending_updates;
> >  
> >  	if (curr_jiffies == this_rq->last_load_update_tick)
> > @@ -4433,10 +4434,11 @@ void update_cpu_load_nohz(void)
> >  	if (pending_updates) {
> >  		this_rq->last_load_update_tick = curr_jiffies;
> >  		/*
> > -		 * We were idle, this means load 0, the current load might be
> > -		 * !0 due to remote wakeups and the sort.
> > +		 * In the regular NOHZ case, we were idle, this means load 0.
> > +		 * In the NOHZ_FULL case, we were non-idle, we should consider
> > +		 * its weighted load.
> >  		 */
> > -		__update_cpu_load(this_rq, 0, pending_updates, 0);
> > +		__update_cpu_load(this_rq, load, pending_updates, active);
> >  	}
> >  	raw_spin_unlock(&this_rq->lock);
> >  }
> 
> Bah, so I did all the work to get the actual number of lost ticks in
> there, only to _then_ find out that's mostly pointless :-)
> 
> The problem is update_idle_cpu_load() is called while idle (from another
> CPU), so it still needs the whole jiffy based thing.
> 
> So I'll take this patch for now. Thanks.

Thank you.

Byungchul

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