linux-kernel - Re: [RFC, PATCH 1/2] sched: change the fairness model of the CFS group scheduler

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Date:	Fri, 29 Feb 2008 11:37:55 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
Cc:	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Sudhir Kumar <skumar@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ibm.com>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, vgoyal@...hat.com,
	serue@...ibm.com, menage@...gle.com
Subject: Re: [RFC, PATCH 1/2] sched: change the fairness model of the CFS
	group scheduler


On Fri, 2008-02-29 at 14:34 +0530, Dhaval Giani wrote:

> > > +#elif defined CONFIG_USER_SCHED
> > > +		/*
> > > +		 * In case of task-groups formed thr' the user id of tasks,
> > > +		 * init_task_group represents tasks belonging to root user.
> > > +		 * Hence it forms a sibling of all subsequent groups formed.
> > > +		 * In this case, init_task_group gets only a fraction of overall
> > > +		 * system cpu resource, based on the weight assigned to root
> > > +		 * user's cpu share (INIT_TASK_GROUP_LOAD). This is accomplished
> > > +		 * by letting tasks of init_task_group sit in a separate cfs_rq
> > > +		 * (init_cfs_rq) and having one entity represent this group of
> > > +		 * tasks in rq->cfs (i.e init_task_group->se[] != NULL).
> > > +		 */
> > >  		init_tg_cfs_entry(rq, &init_task_group,
> > >  				&per_cpu(init_cfs_rq, i),
> > >  				&per_cpu(init_sched_entity, i), i, 1);
> > 
> > But I fail to parse this lengthy comment. What does it do:
> > 
> >     init_group
> >    /     |    \
> > uid-0 uid-1000 uid-n
> > 
> > or does it blend uid-0 into the init_group?
> > 
> 
> It blends uid-0 (root) into init_group.

Any particular reason why? It seems to me uid-0 should be treated like
any other uid.

> > > @@ -1100,6 +1127,27 @@ static void check_preempt_wakeup(struct 
> > >  	if (!sched_feat(WAKEUP_PREEMPT))
> > >  		return;
> > >  
> > > +	/*
> > > +	 * preemption test can be made between sibling entities who are in the
> > > +	 * same cfs_rq i.e who have a common parent. Walk up the hierarchy of
> > > +	 * both tasks untill we find their ancestors who are siblings of common
> > > +	 * parent.
> > > +	 */
> > > +
> > > +	/* First walk up until both entities are at same depth */
> > > +	se_depth = depth_se(se);
> > > +	pse_depth = depth_se(pse);
> > > +
> > > +	while (se_depth > pse_depth) {
> > > +		se_depth--;
> > > +		se = parent_entity(se);
> > > +	}
> > > +
> > > +	while (pse_depth > se_depth) {
> > > +		pse_depth--;
> > > +		pse = parent_entity(pse);
> > > +	}
> > 
> > Sad, but needed.. for now..
> > 
> 
> better ideas if any are welcome! Cannot think of anything right now :(

Single rq can do without :-) If only I could get that latency isolation
going...

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