linux-kernel - Re: [PATCH v3 1/7] sched: introduce primitives to account for CFS bandwidth tracking

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Date:	Mon, 6 Dec 2010 14:32:57 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Balbir Singh <balbir@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval.giani@...il.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>,
	Paul Turner <pjt@...gle.com>, Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 1/7] sched: introduce primitives to account for CFS
	bandwidth tracking

On Thu, Oct 14, 2010 at 06:08:34PM +0530, Balbir Singh wrote:
> * Peter Zijlstra <peterz@...radead.org> [2010-10-14 09:52:17]:
> 
> > On Wed, 2010-10-13 at 18:30 +0530, Balbir Singh wrote:
> > > > +static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
> > > > +{
> > > > +	int i;
> > > > +	static DEFINE_MUTEX(mutex);
> > > > +
> > > > +	if (tg == &init_task_group)
> > > > +		return -EINVAL;
> > > > +
> > > > +	if (!period)
> > > > +		return -EINVAL;
> > > > +
> > > > +	/*
> > > > +	 * Ensure we have at least one tick of bandwidth every period.  This is
> > > > +	 * to prevent reaching a state of large arrears when throttled via
> > > > +	 * entity_tick() resulting in prolonged exit starvation.
> > > > +	 */
> > > > +	if (NS_TO_JIFFIES(quota) < 1)
> > > > +		return -EINVAL;
> > > 
> > > I hope we document this in the Documentation :)
> > 
> > /me went and looked up arrears in a dictionary and wonders why 'debt'
> > wasn't good enough.
> > > > +
> > > > +	mutex_lock(&mutex);
> > > > +	raw_spin_lock_irq(&tg->cfs_bandwidth.lock);
> > > > +	tg->cfs_bandwidth.period = ns_to_ktime(period);
> > > > +	tg->cfs_bandwidth.runtime = tg->cfs_bandwidth.quota = quota;
> > > > +	raw_spin_unlock_irq(&tg->cfs_bandwidth.lock);
> > > > +
> > > > +	for_each_possible_cpu(i) {
> > > 
> > > Why not for_each_online_cpu()?
> > 
> > Probably could be cured with a hotplug handler, but then you need to
> > track more state iirc.
> >
> 
> What more state? If a CPU is offline, we never get to it, do we? I
> think we need to do just an init and destroy - no?

Here we essentially initialize tg->cfs_rq[cpu]->quota_used{assigned}
for all CPUs. Given that we don't destroy tg->cfs_rq[cpu] and tg->se->[cpu]
when a CPU goes offline, is it really worth to have a notifier to just
initialize quota_used and quota_assigned when a CPU comes online ?

Regards,
Bharata.

> 
> > > > +		struct cfs_rq *cfs_rq = tg->cfs_rq[i];
> > > > +		struct rq *rq = rq_of(cfs_rq);
> > > > +
> > > > +		raw_spin_lock_irq(&rq->lock);
> > > > +		init_cfs_rq_quota(cfs_rq);
> > > > +		raw_spin_unlock_irq(&rq->lock);
> > > > +	}
> > > > +	mutex_unlock(&mutex);
> > > > +
> > > > +	return 0;
> > > > +}
> > 
> 
> -- 
> 	Three Cheers,
> 	Balbir
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