linux-kernel - Re: [PATCH v2 3/6] sched/deadline: Add dl_bw

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Date:   Wed, 6 May 2020 14:37:38 +0200
From:   Juri Lelli <juri.lelli@...hat.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Luca Abeni <luca.abeni@...tannapisa.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Wei Wang <wvw@...gle.com>, Quentin Perret <qperret@...gle.com>,
        Alessio Balsini <balsini@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Qais Yousef <qais.yousef@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/6] sched/deadline: Add dl_bw_capacity()

On 06/05/20 12:54, Dietmar Eggemann wrote:
> On 27/04/2020 10:37, Dietmar Eggemann wrote:
> 
> [...]
> 
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 4ae22bfc37ae..eb23e6921d94 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -69,6 +69,25 @@ static inline int dl_bw_cpus(int i)
> >  
> >  	return cpus;
> >  }
> > +
> > +static inline unsigned long dl_bw_capacity(int i)
> > +{
> > +	struct root_domain *rd = cpu_rq(i)->rd;
> > +	unsigned long cap;
> > +
> > +	RCU_LOCKDEP_WARN(!rcu_read_lock_sched_held(),
> > +			 "sched RCU must be held");
> > +
> > +	if (cpumask_subset(rd->span, cpu_active_mask))
> > +		return rd->sum_cpu_capacity;
> > +
> > +	cap = 0;
> > +
> > +	for_each_cpu_and(i, rd->span, cpu_active_mask)
> > +		cap += capacity_orig_of(i);
> > +
> > +	return cap;
> > +}
> 
> There is an issue w/ excl. cpusets and cpuset.sched_load_balance=0. The
> latter is needed to demonstrate the problem since DL task affinity can't
> be altered.
> 
> A CPU in such a cpuset has its rq attached to def_root_domain which does
> not have its 'sum_cpu_capacity' properly set.

Hummm, but if sched_load_balance is disabled it means that we've now got
a subset of CPUs which (from a DL AC pow) are partitioned. So, I'd tend
to say that we actually want to check new tasks bw requirement against
the available bandwidth of the particular CPU they happen to be running
(and will continue to run) when setscheduler is called.

If then load balance is enabled again, AC check we did above should
still be valid for all tasks admitted in the meantime, no?