From: Quentin Perret <quentin.perret@arm.com>
To: peterz@infradead.org, rjw@rjwysocki.net,
linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org
Cc: gregkh@linuxfoundation.org, mingo@redhat.com,
dietmar.eggemann@arm.com, morten.rasmussen@arm.com,
chris.redpath@arm.com, patrick.bellasi@arm.com,
valentin.schneider@arm.com, vincent.guittot@linaro.org,
thara.gopinath@linaro.org, viresh.kumar@linaro.org,
tkjos@google.com, joel@joelfernandes.org, smuckle@google.com,
adharmap@codeaurora.org, skannan@codeaurora.org,
pkondeti@codeaurora.org, juri.lelli@redhat.com,
edubezval@gmail.com, srinivas.pandruvada@linux.intel.com,
currojerez@riseup.net, javi.merino@kernel.org,
quentin.perret@arm.com
Subject: [PATCH v10 12/15] sched: Add over-utilization/tipping point indicator
Date: Mon, 3 Dec 2018 09:56:25 +0000 [thread overview]
Message-ID: <20181203095628.11858-13-quentin.perret@arm.com> (raw)
In-Reply-To: <20181203095628.11858-1-quentin.perret@arm.com>
From: Morten Rasmussen <morten.rasmussen@arm.com>
Energy-aware scheduling is only meant to be active while the system is
_not_ over-utilized. That is, there are spare cycles available to shift
tasks around based on their actual utilization to get a more
energy-efficient task distribution without depriving any tasks. When
above the tipping point task placement is done the traditional way based
on load_avg, spreading the tasks across as many cpus as possible based
on priority scaled load to preserve smp_nice. Below the tipping point we
want to use util_avg instead. We need to define a criteria for when we
make the switch.
The util_avg for each cpu converges towards 100% regardless of how many
additional tasks we may put on it. If we define over-utilized as:
sum_{cpus}(rq.cfs.avg.util_avg) + margin > sum_{cpus}(rq.capacity)
some individual cpus may be over-utilized running multiple tasks even
when the above condition is false. That should be okay as long as we try
to spread the tasks out to avoid per-cpu over-utilization as much as
possible and if all tasks have the _same_ priority. If the latter isn't
true, we have to consider priority to preserve smp_nice.
For example, we could have n_cpus nice=-10 util_avg=55% tasks and
n_cpus/2 nice=0 util_avg=60% tasks. Balancing based on util_avg we are
likely to end up with nice=-10 tasks sharing cpus and nice=0 tasks
getting their own as we 1.5*n_cpus tasks in total and 55%+55% is less
over-utilized than 55%+60% for those cpus that have to be shared. The
system utilization is only 85% of the system capacity, but we are
breaking smp_nice.
To be sure not to break smp_nice, we have defined over-utilization
conservatively as when any cpu in the system is fully utilized at its
highest frequency instead:
cpu_rq(any).cfs.avg.util_avg + margin > cpu_rq(any).capacity
IOW, as soon as one cpu is (nearly) 100% utilized, we switch to load_avg
to factor in priority to preserve smp_nice.
With this definition, we can skip periodic load-balance as no cpu has an
always-running task when the system is not over-utilized. All tasks will
be periodic and we can balance them at wake-up. This conservative
condition does however mean that some scenarios that could benefit from
energy-aware decisions even if one cpu is fully utilized would not get
those benefits.
For systems where some cpus might have reduced capacity on some cpus
(RT-pressure and/or big.LITTLE), we want periodic load-balance checks as
soon a just a single cpu is fully utilized as it might one of those with
reduced capacity and in that case we want to migrate it.
cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
[ Added a comment explaining why new tasks are not accounted during
overutilization detection ]
Signed-off-by: Quentin Perret <quentin.perret@arm.com>
---
kernel/sched/fair.c | 59 ++++++++++++++++++++++++++++++++++++++++++--
kernel/sched/sched.h | 4 +++
2 files changed, 61 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e21f37129395..c3b2dad72c9c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5082,6 +5082,24 @@ static inline void hrtick_update(struct rq *rq)
}
#endif
+#ifdef CONFIG_SMP
+static inline unsigned long cpu_util(int cpu);
+static unsigned long capacity_of(int cpu);
+
+static inline bool cpu_overutilized(int cpu)
+{
+ return (capacity_of(cpu) * 1024) < (cpu_util(cpu) * capacity_margin);
+}
+
+static inline void update_overutilized_status(struct rq *rq)
+{
+ if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu))
+ WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED);
+}
+#else
+static inline void update_overutilized_status(struct rq *rq) { }
+#endif
+
/*
* The enqueue_task method is called before nr_running is
* increased. Here we update the fair scheduling stats and
@@ -5139,8 +5157,26 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
update_cfs_group(se);
}
- if (!se)
+ if (!se) {
add_nr_running(rq, 1);
+ /*
+ * Since new tasks are assigned an initial util_avg equal to
+ * half of the spare capacity of their CPU, tiny tasks have the
+ * ability to cross the overutilized threshold, which will
+ * result in the load balancer ruining all the task placement
+ * done by EAS. As a way to mitigate that effect, do not account
+ * for the first enqueue operation of new tasks during the
+ * overutilized flag detection.
+ *
+ * A better way of solving this problem would be to wait for
+ * the PELT signals of tasks to converge before taking them
+ * into account, but that is not straightforward to implement,
+ * and the following generally works well enough in practice.
+ */
+ if (flags & ENQUEUE_WAKEUP)
+ update_overutilized_status(rq);
+
+ }
hrtick_update(rq);
}
@@ -7940,6 +7976,9 @@ static inline void update_sg_lb_stats(struct lb_env *env,
if (nr_running > 1)
*sg_status |= SG_OVERLOAD;
+ if (cpu_overutilized(i))
+ *sg_status |= SG_OVERUTILIZED;
+
#ifdef CONFIG_NUMA_BALANCING
sgs->nr_numa_running += rq->nr_numa_running;
sgs->nr_preferred_running += rq->nr_preferred_running;
@@ -8170,8 +8209,15 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
env->fbq_type = fbq_classify_group(&sds->busiest_stat);
if (!env->sd->parent) {
+ struct root_domain *rd = env->dst_rq->rd;
+
/* update overload indicator if we are at root domain */
- WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);
+ WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);
+
+ /* Update over-utilization (tipping point, U >= 0) indicator */
+ WRITE_ONCE(rd->overutilized, sg_status & SG_OVERUTILIZED);
+ } else if (sg_status & SG_OVERUTILIZED) {
+ WRITE_ONCE(env->dst_rq->rd->overutilized, SG_OVERUTILIZED);
}
}
@@ -8398,6 +8444,14 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
* this level.
*/
update_sd_lb_stats(env, &sds);
+
+ if (static_branch_unlikely(&sched_energy_present)) {
+ struct root_domain *rd = env->dst_rq->rd;
+
+ if (rcu_dereference(rd->pd) && !READ_ONCE(rd->overutilized))
+ goto out_balanced;
+ }
+
local = &sds.local_stat;
busiest = &sds.busiest_stat;
@@ -9798,6 +9852,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
task_tick_numa(rq, curr);
update_misfit_status(curr, rq);
+ update_overutilized_status(task_rq(curr));
}
/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4c1e4b73f40d..28d9209554dc 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -717,6 +717,7 @@ struct perf_domain {
/* Scheduling group status flags */
#define SG_OVERLOAD 0x1 /* More than one runnable task on a CPU. */
+#define SG_OVERUTILIZED 0x2 /* One or more CPUs are over-utilized. */
/*
* We add the notion of a root-domain which will be used to define per-domain
@@ -740,6 +741,9 @@ struct root_domain {
*/
int overload;
+ /* Indicate one or more cpus over-utilized (tipping point) */
+ int overutilized;
+
/*
* The bit corresponding to a CPU gets set here if such CPU has more
* than one runnable -deadline task (as it is below for RT tasks).
--
2.19.2
next prev parent reply other threads:[~2018-12-03 9:57 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-03 9:56 [PATCH v10 00/15] Energy Aware Scheduling Quentin Perret
2018-12-03 9:56 ` [PATCH v10 01/15] sched: Relocate arch_scale_cpu_capacity Quentin Perret
2018-12-11 15:32 ` [tip:sched/core] sched/topology: Relocate arch_scale_cpu_capacity() to the internal header tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 02/15] sched/cpufreq: Prepare schedutil for Energy Aware Scheduling Quentin Perret
2018-12-11 12:01 ` Rafael J. Wysocki
2018-12-11 12:17 ` Quentin Perret
2018-12-11 12:22 ` Rafael J. Wysocki
2018-12-11 12:24 ` Quentin Perret
2018-12-11 15:33 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 03/15] PM: Introduce an Energy Model management framework Quentin Perret
2018-12-11 15:33 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 04/15] PM / EM: Expose the Energy Model in sysfs Quentin Perret
2018-12-11 14:18 ` Ingo Molnar
2018-12-11 15:04 ` Quentin Perret
2018-12-03 9:56 ` [PATCH v10 05/15] sched/topology: Reference the Energy Model of CPUs when available Quentin Perret
2018-12-11 15:34 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 06/15] sched/topology: Lowest CPU asymmetry sched_domain level pointer Quentin Perret
2018-12-11 15:34 ` [tip:sched/core] sched/topology: Add lowest " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 07/15] sched/topology: Disable EAS on inappropriate platforms Quentin Perret
2018-12-11 15:35 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 08/15] sched/topology: Make Energy Aware Scheduling depend on schedutil Quentin Perret
2018-12-11 15:36 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 09/15] sched: Introduce sched_energy_present static key Quentin Perret
2018-12-11 15:36 ` [tip:sched/core] sched/toplogy: Introduce the 'sched_energy_present' " tip-bot for Quentin Perret
2018-12-13 13:56 ` Quentin Perret
2018-12-03 9:56 ` [PATCH v10 10/15] sched: Introduce a sysctl for Energy Aware Scheduling Quentin Perret
2018-12-11 14:15 ` Ingo Molnar
2018-12-11 14:49 ` Quentin Perret
2018-12-13 14:03 ` Peter Zijlstra
2019-01-21 11:35 ` [tip:sched/core] sched/topology: " tip-bot for Quentin Perret
2019-01-21 13:51 ` tip-bot for Quentin Perret
2019-01-27 11:34 ` tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 11/15] sched/fair: Clean-up update_sg_lb_stats parameters Quentin Perret
2018-12-11 15:37 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` Quentin Perret [this message]
2018-12-11 15:37 ` [tip:sched/core] sched/fair: Add over-utilization/tipping point indicator tip-bot for Morten Rasmussen
2018-12-03 9:56 ` [PATCH v10 13/15] sched/fair: Introduce an energy estimation helper function Quentin Perret
2018-12-11 15:38 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 14/15] sched/fair: Select an energy-efficient CPU on task wake-up Quentin Perret
2018-12-11 15:39 ` [tip:sched/core] " tip-bot for Quentin Perret
2018-12-03 9:56 ` [PATCH v10 15/15] OPTIONAL: cpufreq: dt: Register an Energy Model Quentin Perret
2019-01-08 20:38 ` Matthias Kaehlcke
2019-01-09 10:57 ` Quentin Perret
2019-01-09 18:14 ` Matthias Kaehlcke
2019-01-10 9:08 ` Quentin Perret
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