linux-kernel - Re: [PATCH 2/4] sched: Consolidate nohz CPU load update code

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Message-ID: <20160120014104.GD9882@X58A-UD3R>
Date:	Wed, 20 Jan 2016 10:41:04 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/4] sched: Consolidate nohz CPU load update code

On Tue, Jan 19, 2016 at 05:49:34PM +0100, Frederic Weisbecker wrote:
> On Thu, Jan 14, 2016 at 02:18:40PM +0900, Byungchul Park wrote:
> > On Wed, Jan 13, 2016 at 05:01:29PM +0100, Frederic Weisbecker wrote:
> > >  #ifdef CONFIG_NO_HZ_COMMON
> > > +static void __update_cpu_load_nohz(struct rq *this_rq,
> > > +				   unsigned long curr_jiffies,
> > 
> > Do we need to pass current jiffies as a function parameter?
> 
> I guess we don't, I just wasn't much sure of the possible overhead of READ_ONCE()

Ah. But I think passing an additional argument can cause additional
overhead, too, e.g. additional store/load on stack. But I am not sure
which one is larger, and it depends on architecture and abi.

> 
> > 
> > > +				   unsigned long load,
> > > +				   int active)
> > > +{
> > > +	unsigned long pending_updates;
> > > +
> > > +	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> > > +	if (pending_updates) {
> > > +		this_rq->last_load_update_tick = curr_jiffies;
> > > +		/*
> > > +		 * 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, load, pending_updates, active);
> > > +	}
> > > +}