linux-kernel - Re: [RFC PATCH v4 3/7] sched: nominate preferred wakeup cpu

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Date:	Mon, 24 Nov 2008 11:44:31 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	David Collier-Brown <davecb@....com>,
	Tim Connors <tconnors@...ro.swin.edu.au>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Gregory Haskins <gregory.haskins@...il.com>
Subject: Re: [RFC PATCH v4 3/7] sched: nominate preferred wakeup cpu

* Peter Zijlstra <a.p.zijlstra@...llo.nl> [2008-11-23 03:03:58]:

> On Fri, 2008-11-21 at 14:01 +0530, Vaidyanathan Srinivasan wrote:
> > When the system utilisation is low and more cpus are idle,
> > then the process waking up from sleep should prefer to
> > wakeup an idle cpu from semi-idle cpu package (multi core
> > package) rather than a completely idle cpu package which
> > would waste power.
> > 
> > Use the sched_mc balance logic in find_busiest_group() to
> > nominate a preferred wakeup cpu.
> > 
> > This info can be sored in appropriate sched_domain, but
> > updating this info in all copies of sched_domain is not
> > practical.  Hence this information is stored in root_domain
> > struct which is one copy per partitioned sched domain.
> > The root_domain can be accessed from each cpu's runqueue
> > and there is one copy per partitioned sched domain.
> > 
> > Signed-off-by: Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
> > ---
> > 
> >  kernel/sched.c |   15 +++++++++++++++
> >  1 files changed, 15 insertions(+), 0 deletions(-)
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 79b71f3..d28cd98 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -493,6 +493,17 @@ struct root_domain {
> >  #ifdef CONFIG_SMP
> >  	struct cpupri cpupri;
> >  #endif
> > +#if defined(CONFIG_SCHED_MC) || defined(CONFIG_SCHED_SMT)
> > +
> > +	/*
> > +	 * Preferred wake up cpu nominated by sched_mc balance that will be
> > +	 * used when most cpus are idle in the system indicating overall very
> > +	 * low system utilisation. Triggered at POWERSAVINGS_BALANCE_WAKEUP(2)
> > +	 */
> > +	unsigned int sched_mc_preferred_wakeup_cpu;
> > +
> > +#endif
> > +
> >  };
> >  
> >  /*
> 
> Do we really need that extra whitespace?
> 
> > @@ -3090,6 +3101,7 @@ static int move_one_task(struct rq *this_rq, int this_cpu, struct rq *busiest,
> >  	return 0;
> >  }
> >  
> > +
> >  /*
> >   * find_busiest_group finds and returns the busiest CPU group within the
> >   * domain. It calculates and returns the amount of weighted load which
> 
> Ditto?
> 
> > @@ -3406,6 +3418,9 @@ out_balanced:
> >  
> >  	if (this == group_leader && group_leader != group_min) {
> >  		*imbalance = min_load_per_task;
> > +		if (sched_mc_power_savings >= POWERSAVINGS_BALANCE_WAKEUP)
> > +			cpu_rq(this_cpu)->rd->sched_mc_preferred_wakeup_cpu =
> > +					first_cpu(group_leader->cpumask);
> 
> While not strictly needed, I prefer braces around multi-line statments.
> It's easier on the eyes.

Hi Peter,

Thanks for the detailed review.  I will fix these issues in the next
iteration.

--Vaidy
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