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Subject: Re: [patch 08/18] sched: add support for throttling group entities
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Lin Ming <minggr@gmail.com>
Cc: Paul Turner <pjt@google.com>, linux-kernel@vger.kernel.org,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Dhaval Giani <dhaval.giani@gmail.com>,
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        Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
        Srivatsa Vaddagiri <vatsa@in.ibm.com>,
        Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
        Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>,
        Ingo Molnar <mingo@elte.hu>, Pavel Emelyanov <xemul@openvz.org>,
        Jason Baron <jbaron@redhat.com>
Date: Mon, 08 Aug 2011 18:00:28 +0200
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On Mon, 2011-08-08 at 23:46 +0800, Lin Ming wrote:
> On Fri, Jul 22, 2011 at 12:43 AM, Paul Turner <pjt@google.com> wrote:
> 
> > +static __used void throttle_cfs_rq(struct cfs_rq *cfs_rq)
> > +{
> > +       struct rq *rq = rq_of(cfs_rq);
> > +       struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
> > +       struct sched_entity *se;
> > +       long task_delta, dequeue = 1;
> > +
> > +       se = cfs_rq->tg->se[cpu_of(rq_of(cfs_rq))];
> > +
> > +       /* account load preceding throttle */
> > +       update_cfs_load(cfs_rq, 0);
> > +
> > +       task_delta = cfs_rq->h_nr_running;
> > +       for_each_sched_entity(se) {
> > +               struct cfs_rq *qcfs_rq = cfs_rq_of(se);
> > +               /* throttled entity or throttle-on-deactivate */
> > +               if (!se->on_rq)
> > +                       break;
> 
> Does it mean it's possible that child se is unthrottled but parent se
> is throttled?

Yep..

> I thought if parent group was throttled then its children should be
> throttled too.
> I may misunderstood the code, please correct me then.

That would be costly, as throttling a parent would require throttling
all its children (of which there can be arbitrary many).