Re: [RFC PATCH 2/6] sched: Introduce energy models of CPUs

[Quentin Perret <quentin.perret@arm.com>]

On Tuesday 20 Mar 2018 at 10:52:15 (+0100), Greg Kroah-Hartman wrote:
> On Tue, Mar 20, 2018 at 09:43:08AM +0000, Dietmar Eggemann wrote:
> > From: Quentin Perret <quentin.perret@arm.com>
> > 
> > The energy consumption of each CPU in the system is modeled with a list
> > of values representing its dissipated power and compute capacity at each
> > available Operating Performance Point (OPP). These values are derived
> > from existing information in the kernel (currently used by the thermal
> > subsystem) and don't require the introduction of new platform-specific
> > tunables. The energy model is also provided with a simple representation
> > of all frequency domains as cpumasks, hence enabling the scheduler to be
> > aware of dependencies between CPUs. The data required to build the energy
> > model is provided by the OPP library which enables an abstract view of
> > the platform from the scheduler. The new data structures holding these
> > models and the routines to populate them are stored in
> > kernel/sched/energy.c.
> > 
> > For the sake of simplicity, it is assumed in the energy model that all
> > CPUs in a frequency domain share the same micro-architecture. As long as
> > this assumption is correct, the energy models of different CPUs belonging
> > to the same frequency domain are equal. Hence, this commit builds only one
> > energy model per frequency domain, and links all relevant CPUs to it in
> > order to save time and memory. If needed for future hardware platforms,
> > relaxing this assumption should imply relatively simple modifications in
> > the code but a significantly higher algorithmic complexity.
> > 
> > As it appears that energy-aware scheduling really makes a difference on
> > heterogeneous systems (e.g. big.LITTLE platforms), it is restricted to
> > systems having:
> > 
> >    1. SD_ASYM_CPUCAPACITY flag set
> >    2. Dynamic Voltage and Frequency Scaling (DVFS) is enabled
> >    3. Available power estimates for the OPPs of all possible CPUs
> > 
> > Moreover, the scheduler is notified of the energy model availability
> > using a static key in order to minimize the overhead on non-energy-aware
> > systems.
> > 
> > Cc: Ingo Molnar <mingo@redhat.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Signed-off-by: Quentin Perret <quentin.perret@arm.com>
> > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> > 
> > ---
> > This patch depends on additional infrastructure being merged in the OPP
> > core. As this infrastructure can also be useful for other clients, the
> > related patches have been posted separately [1].
> > 
> > [1] https://marc.info/?l=linux-pm&m=151635516419249&w=2
> > ---
> >  include/linux/sched/energy.h |  31 +++++++
> >  kernel/sched/Makefile        |   2 +-
> >  kernel/sched/energy.c        | 190 +++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 222 insertions(+), 1 deletion(-)
> >  create mode 100644 include/linux/sched/energy.h
> >  create mode 100644 kernel/sched/energy.c
> > 
> > diff --git a/include/linux/sched/energy.h b/include/linux/sched/energy.h
> > new file mode 100644
> > index 000000000000..b4f43564ffe4
> > --- /dev/null
> > +++ b/include/linux/sched/energy.h
> > @@ -0,0 +1,31 @@
> > +#ifndef _LINUX_SCHED_ENERGY_H
> > +#define _LINUX_SCHED_ENERGY_H
> 
> No copyright or license info?  Not good :(
> 
> > --- /dev/null
> > +++ b/kernel/sched/energy.c
> > @@ -0,0 +1,190 @@
> > +/*
> > + * Released under the GPLv2 only.
> > + * SPDX-License-Identifier: GPL-2.0
> 
> Please read the documentation for the SPDX lines on how to do them
> correctly.  Newer versions of checkpatch.pl will catch this, but that is
> in linux-next for the moment.
> 
> And once you have the SPDX line, the "Released under..." line is not
> needed.
> 
> 
> > + *
> > + * Energy-aware scheduling models
> > + *
> > + * Copyright (C) 2018, Arm Ltd.
> > + * Written by: Quentin Perret, Arm Ltd.
> > + *
> > + * This file is subject to the terms and conditions of the GNU General Public
> > + * License.  See the file "COPYING" in the main directory of this archive
> > + * for more details.
> 
> This paragraph is not needed at all.

Right, I will fix all the licence issues and add one to the new header
file. I took example on existing files a while ago when I first wrote
the patches and forgot to update them later on. Sorry about that.

> 
> > + */
> > +
> > +#define pr_fmt(fmt) "sched-energy: " fmt
> > +
> > +#include <linux/sched/topology.h>
> > +#include <linux/sched/energy.h>
> > +#include <linux/pm_opp.h>
> > +
> > +#include "sched.h"
> > +
> > +DEFINE_STATIC_KEY_FALSE(sched_energy_present);
> > +struct sched_energy_model ** __percpu energy_model;
> > +
> > +/*
> > + * A copy of the cpumasks representing the frequency domains is kept private
> > + * to the scheduler. They are stacked in a dynamically allocated linked list
> > + * as we don't know how many frequency domains the system has.
> > + */
> > +LIST_HEAD(freq_domains);
> 
> global variable?  If so, please prefix it with something more unique
> than "freq_".

Will do.

> 
> > +#ifdef CONFIG_PM_OPP
> 
> #ifdefs go in .h files, not .c files, right?

Yes good point. Actually, I might be able to tweak only kernel/sched/Makefile
to ensure we have CONFIG_PM_OPP. I will look into it.

> 
> thanks,
> 
> greg k-h

Thanks,
Quentin