linux-kernel - Re: sched: Improve load balancing in the presence of idle CPUs

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Date:	Mon, 13 Apr 2015 11:55:47 -0700
From:	Jason Low <jason.low2@...com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	"riel@...hat.com" <riel@...hat.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"benh@...nel.crashing.org" <benh@...nel.crashing.org>,
	"efault@....de" <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"iamjoonsoo.kim@....com" <iamjoonsoo.kim@....com>,
	"svaidy@...ux.vnet.ibm.com" <svaidy@...ux.vnet.ibm.com>,
	"tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
	jason.low2@...com
Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Fri, 2015-04-10 at 14:07 +0530, Srikar Dronamraju wrote:
> > > >
> > > >  #ifdef CONFIG_NO_HZ_COMMON
> > > > +static inline bool nohz_kick_needed(struct rq *rq);
> > > > +
> > > > +static inline void pass_nohz_balance(struct rq *this_rq, int this_cpu)
> > > > +{
> > > > +	clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> > > > +	nohz.next_balance = jiffies;
> > >
> > > Why are we updating nohz.next_balance here?
> >
> > This was just to make sure that since we're continuing the balancing on
> > another CPU that the nohz next_balance is guaranteed to be "now".
> >
> 
> Since we are in nohz_idle_balance(), nohz.next_balance is guaranteed be
> less than now. We do check for time_before(now, nohz.next_balance) in
> nohz_kick_needed(). So in effect we are incrementing the nohz.next_balance.

Hi Srikar,

If now is equal to nohz.next_balance, we may attempt
nohz_balancer_kick().

After it does nohz.next_balance++ in nohz_balancer_kick(), now can be 1
less than the new nohz.next_balance value by the time
nohz_idle_balance() is attempted(). Without updating nohz.next_balance,
the time_before(now, nohz.next_balance) check in nohz_kick_needed() may
cause it to return false.

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