linux-kernel - Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

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Date:   Wed, 8 May 2019 10:17:57 +0200
From:   luca abeni <luca.abeni@...tannapisa.it>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Quentin Perret <quentin.perret@....com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [RFC PATCH 2/6] sched/dl: Capacity-aware migrations

On Wed, 8 May 2019 10:04:36 +0200
Juri Lelli <juri.lelli@...hat.com> wrote:

> Hi Luca,
> 
> On 06/05/19 06:48, Luca Abeni wrote:
> > From: luca abeni <luca.abeni@...tannapisa.it>
> > 
> > Currently, the SCHED_DEADLINE scheduler uses a global EDF scheduling
> > algorithm, migrating tasks to CPU cores without considering the core
> > capacity and the task utilization. This works well on homogeneous
> > systems (SCHED_DEADLINE tasks are guaranteed to have a bounded
> > tardiness), but presents some issues on heterogeneous systems. For
> > example, a SCHED_DEADLINE task might be migrated on a core that has
> > not enough processing capacity to correctly serve the task (think
> > about a task with runtime 70ms and period 100ms migrated to a core
> > with processing capacity 0.5)
> > 
> > This commit is a first step to address the issue: When a task wakes
> > up or migrates away from a CPU core, the scheduler tries to find an
> > idle core having enough processing capacity to serve the task.
> > 
> > Signed-off-by: luca abeni <luca.abeni@...tannapisa.it>
> > ---
> >  kernel/sched/cpudeadline.c | 31 +++++++++++++++++++++++++++++--
> >  kernel/sched/deadline.c    |  8 ++++++--
> >  kernel/sched/sched.h       |  7 ++++++-
> >  3 files changed, 41 insertions(+), 5 deletions(-)
> > 
> > diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> > index 50316455ea66..d21f7905b9c1 100644
> > --- a/kernel/sched/cpudeadline.c
> > +++ b/kernel/sched/cpudeadline.c
> > @@ -110,6 +110,22 @@ static inline int cpudl_maximum(struct cpudl
> > *cp) return cp->elements[0].cpu;
> >  }
> >  
> > +static inline int dl_task_fit(const struct sched_dl_entity *dl_se,
> > +			      int cpu, u64 *c)
> > +{
> > +	u64 cap = (arch_scale_cpu_capacity(NULL, cpu) *
> > arch_scale_freq_capacity(cpu)) >> SCHED_CAPACITY_SHIFT;
> > +	s64 rel_deadline = dl_se->dl_deadline;
> > +	u64 rem_runtime  = dl_se->dl_runtime;  
> 
> This is not the dynamic remaining one, is it?

Right; I preferred to split this in two patches so that if we decide to
use only the static task parameters (dl_deadline and dl_runtime) I can
simply drop a patch ;-)


				Luca

> 
> I see however 4/6.. lemme better look at that.
> 
> Best,
> 
> - Juri