Commit 2802bf3c authored by Morten Rasmussen's avatar Morten Rasmussen Committed by Ingo Molnar

sched/fair: Add over-utilization/tipping point indicator

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.

[ peterz: Added a comment explaining why new tasks are not accounted during
          overutilization detection. ]
Signed-off-by: default avatarMorten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: default avatarQuentin Perret <quentin.perret@arm.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: adharmap@codeaurora.org
Cc: chris.redpath@arm.com
Cc: currojerez@riseup.net
Cc: dietmar.eggemann@arm.com
Cc: edubezval@gmail.com
Cc: gregkh@linuxfoundation.org
Cc: javi.merino@kernel.org
Cc: joel@joelfernandes.org
Cc: juri.lelli@redhat.com
Cc: patrick.bellasi@arm.com
Cc: pkondeti@codeaurora.org
Cc: rjw@rjwysocki.net
Cc: skannan@codeaurora.org
Cc: smuckle@google.com
Cc: srinivas.pandruvada@linux.intel.com
Cc: thara.gopinath@linaro.org
Cc: tkjos@google.com
Cc: valentin.schneider@arm.com
Cc: vincent.guittot@linaro.org
Cc: viresh.kumar@linaro.org
Link: https://lkml.kernel.org/r/20181203095628.11858-13-quentin.perret@arm.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
parent 630246a0
......@@ -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));
}
/*
......
......@@ -718,6 +718,7 @@ 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
......@@ -741,6 +742,9 @@ 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).
......
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