From: Quentin Perret <quentin.perret@arm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>,
linux-kernel@vger.kernel.org,
Thara Gopinath <thara.gopinath@linaro.org>,
linux-pm@vger.kernel.org,
Morten Rasmussen <morten.rasmussen@arm.com>,
Chris Redpath <chris.redpath@arm.com>,
Patrick Bellasi <patrick.bellasi@arm.com>,
Valentin Schneider <valentin.schneider@arm.com>,
"Rafael J . Wysocki" <rjw@rjwysocki.net>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Vincent Guittot <vincent.guittot@linaro.org>,
Viresh Kumar <viresh.kumar@linaro.org>,
Todd Kjos <tkjos@google.com>, Joel Fernandes <joelaf@google.com>,
Juri Lelli <juri.lelli@redhat.com>,
Steve Muckle <smuckle@google.com>,
Eduardo Valentin <edubezval@gmail.com>
Subject: Re: [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up
Date: Tue, 10 Apr 2018 19:14:42 +0100 [thread overview]
Message-ID: <20180410181442.GA9565@e108498-lin.cambridge.arm.com> (raw)
In-Reply-To: <20180410172932.GD4043@hirez.programming.kicks-ass.net>
On Tuesday 10 Apr 2018 at 19:29:32 (+0200), Peter Zijlstra wrote:
> On Fri, Apr 06, 2018 at 04:36:06PM +0100, Dietmar Eggemann wrote:
> > + for_each_freq_domain(fd) {
> > + unsigned long spare_cap, max_spare_cap = 0;
> > + int max_spare_cap_cpu = -1;
> > + unsigned long util;
> > +
> > + /* Find the CPU with the max spare cap in the freq. dom. */
> > + for_each_cpu_and(cpu, freq_domain_span(fd), sched_domain_span(sd)) {
> > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> > + continue;
> > +
> > + if (cpu == prev_cpu)
> > + continue;
> > +
> > + util = cpu_util_wake(cpu, p);
> > + cpu_cap = capacity_of(cpu);
> > + if (!util_fits_capacity(util + task_util, cpu_cap))
> > + continue;
> > +
> > + spare_cap = cpu_cap - util;
> > + if (spare_cap > max_spare_cap) {
> > + max_spare_cap = spare_cap;
> > + max_spare_cap_cpu = cpu;
> > + }
> > + }
> > +
> > + /* Evaluate the energy impact of using this CPU. */
> > + if (max_spare_cap_cpu >= 0) {
> > + cur_energy = compute_energy(p, max_spare_cap_cpu);
> > + if (cur_energy < best_energy) {
> > + best_energy = cur_energy;
> > + best_energy_cpu = max_spare_cap_cpu;
> > + }
> > + }
> > + }
>
> If each CPU has its own frequency domain, then the above loop ends up
> being O(n^2), no?
Hmmm, yes, that should be O(n^2) indeed.
> Is there really nothing we can do about that?
So, the only thing I see just now would be to make compute_energy()
smarter somehow. Today we compute the energy consumed by each frequency
domain and then sum them up to get the system energy. We re-compute the
energy of each frequency domain, even when it is not involved in the
migration. In the case you describe, we will end up re-computing the
energy of many frequency domains on which nothing happens every time
we re-call compute_energy(). So there is probably something we could do
by caching those values somehow.
Otherwise, on systems with 2 frequency domains (e.g. big.LITTLE), the
current code should behave relatively well. And I think that covers a
large portion of the real-world systems for which EAS is useful, as of
today at least ... :-)
> Also, I
> feel that warrants a comment warning about this.
>
> Someone, somewhere will try and build a 64+64 cpu system and get
> surprised it doesn't work :-)
next prev parent reply other threads:[~2018-04-10 18:14 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-06 15:36 [RFC PATCH v2 0/6] Energy Aware Scheduling Dietmar Eggemann
2018-04-06 15:36 ` [RFC PATCH v2 1/6] sched/fair: Create util_fits_capacity() Dietmar Eggemann
2018-04-12 7:02 ` Viresh Kumar
2018-04-12 8:20 ` Dietmar Eggemann
2018-04-06 15:36 ` [RFC PATCH v2 2/6] sched: Introduce energy models of CPUs Dietmar Eggemann
2018-04-10 11:54 ` Peter Zijlstra
2018-04-10 12:03 ` Dietmar Eggemann
2018-04-13 4:02 ` Viresh Kumar
2018-04-13 8:37 ` Quentin Perret
2018-04-06 15:36 ` [RFC PATCH v2 3/6] sched: Add over-utilization/tipping point indicator Dietmar Eggemann
2018-04-13 23:56 ` Joel Fernandes
2018-04-18 11:17 ` Quentin Perret
2018-04-20 8:13 ` Joel Fernandes
2018-04-20 8:14 ` Joel Fernandes
2018-04-20 8:31 ` Quentin Perret
2018-04-20 8:57 ` Juri Lelli
2018-04-17 14:25 ` Leo Yan
2018-04-17 17:39 ` Dietmar Eggemann
2018-04-18 0:18 ` Leo Yan
2018-04-06 15:36 ` [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function Dietmar Eggemann
2018-04-10 12:51 ` Peter Zijlstra
2018-04-10 13:56 ` Quentin Perret
2018-04-10 14:08 ` Peter Zijlstra
2018-04-13 6:27 ` Viresh Kumar
2018-04-17 15:22 ` Leo Yan
2018-04-18 8:13 ` Quentin Perret
2018-04-18 9:19 ` Leo Yan
2018-04-18 11:06 ` Quentin Perret
2018-04-18 9:23 ` Leo Yan
2018-04-20 14:51 ` Quentin Perret
2018-04-18 12:15 ` Leo Yan
2018-04-20 14:42 ` Quentin Perret
2018-04-20 16:27 ` Leo Yan
2018-04-25 8:23 ` Quentin Perret
2018-04-06 15:36 ` [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up Dietmar Eggemann
2018-04-09 16:30 ` Peter Zijlstra
2018-04-09 16:43 ` Quentin Perret
2018-04-10 17:29 ` Peter Zijlstra
2018-04-10 18:14 ` Quentin Perret [this message]
2018-04-17 15:39 ` Leo Yan
2018-04-18 7:57 ` Quentin Perret
2018-04-06 15:36 ` [RFC PATCH v2 6/6] drivers: base: arch_topology.c: Enable EAS for arm/arm64 platforms Dietmar Eggemann
2018-04-17 12:50 ` [RFC PATCH v2 0/6] Energy Aware Scheduling Leo Yan
2018-04-17 17:22 ` Dietmar Eggemann
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