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Subject[PATCH v3 0/5] Capacity awareness for SCHED_DEADLINE
Date
The SCHED_DEADLINE (DL) Admission Control (AC) and task placement do
not work correctly on heterogeneous (asymmetric CPU capacity) systems
such as Arm big.LITTLE or DynamIQ.

Let's fix this by explicitly considering CPU capacity in AC and task
placement.

The DL sched class now attempts to avoid missing task deadlines due to
smaller CPU (CPU capacity < 1024) not being capable enough to finish a
task in time. It does so by trying to place a task so that its CPU
capacity scaled deadline is not smaller than its runtime.

This patch-set only supports capacity awareness in the idle scenario
(cpudl::free_cpus not empty). Capacity awareness for the non-idle
case should be added in a later series.

Changes v2 [1] -> v3:

Discussion about that if 'rq->rd == def_root_domain' AC should be
performed against the capacity of the CPU the task is running on rather
the rd CPU capacity sum.
Since this issue already exists w/o capacity awareness a 'XXX Fix:'
comment was added for now.

Per-patch changes:

(1) Patch 'sched/topology: Store root domain CPU capacity sum' removed
since rd->sum_cpu_capacity is not needed anymore [v2 patch 1/6]

(2) Redesign of dl_bw_capacity() and 'XXX Fix:' comment (mentioned
above) added [patch 2/5]

(3) Favor task_cpu(p) if it has max capacity of !fitting CPUs
[patch 5/5]

Changes v1 [2] -> v2:

Discussion about capacity awareness in idle and non-idle scenarios
indicated that the current patch-set only supports the former.

Per-patch changes:

(1) Use rq->cpu_capacity_orig or capacity_orig_of() instead of
arch_scale_cpu_capacity() [patch 1,6/6]

(2) Optimize dl_bw_cpus(), i.e. return weight of rd->span if rd->span
&sube cpu_active_mask [patch 2/6]

(3) Replace rd_capacity() with dl_bw_capacity() [patch 3/6]

Changes RFC [3] -> v1:

Only use static values for CPU bandwidth (sched_dl_entity::dl_runtime,
::dl_deadline) and CPU capacity (arch_scale_cpu_capacity()) to fix AC.

Dynamic values for CPU bandwidth (sched_dl_entity::runtime, ::deadline)
and CPU capacity (capacity_of()) are considered to be more related to
energy trade-off calculations which could be later introduced using the
Energy Model.

Since the design of the DL and RT sched classes are very similar, the
implementation follows the overall design of RT capacity awareness
(commit 804d402fb6f6 ("sched/rt: Make RT capacity-aware")).

Per-patch changes:

(1) Store CPU capacity sum in the root domain during
build_sched_domains() [patch 1/4]

(2) Adjust to RT capacity awareness design [patch 3/4]

(3) Remove CPU capacity aware placement in switched_to_dl()
(dl_migrate callback) [RFC patch 3/6]

Balance callbacks (push, pull) run only in schedule_tail()
__schedule(), rt_mutex_setprio() or __sched_setscheduler().
DL throttling leads to a call to __dequeue_task_dl() which is not a
full task dequeue. The task is still enqueued and only removed from
the rq.
So a queue_balance_callback() call in update_curr_dl()->
__dequeue_task_dl() will not be followed by a balance_callback()
call in one of the 4 functions mentioned above.

(4) Remove 'dynamic CPU bandwidth' consideration and only support
'static CPU bandwidth' (ratio between sched_dl_entity::dl_runtime
and ::dl_deadline) [RFC patch 4/6]

(5) Remove modification to migration logic which tried to schedule
small tasks on LITTLE CPUs [RFC patch 6/6]

[1] https://lore.kernel.org/r/20200427083709.30262-1-dietmar.eggemann@arm.com
[2] https://lore.kernel.org/r/20200408095012.3819-1-dietmar.eggemann@arm.com
[3] https://lore.kernel.org/r/20190506044836.2914-1-luca.abeni@santannapisa.it

The following rt-app testcase tailored to Arm64 Hikey960:

root@h960:~# cat /sys/devices/system/cpu/cpu*/cpu_capacity
462
462
462
462
1024
1024
1024
1024

shows the expected behavior.

According to the following condition in dl_task_fits_capacity()

cap_scale(dl_deadline, arch_scale_cpu_capacity(cpu)) >= dl_runtime

thread0-[0-3] are placed on a big CPUs whereas thread1-[0-3] run on a
LITTLE CPU respectively.

The 'delay' parameter for the little tasks makes sure that they start
later than the big tasks allowing the big tasks to choose big CPUs.

...
"tasks" : {
"thread0" : {
"policy" : "SCHED_DEADLINE",
"instance" : 4,
"timer" : { "ref" : "unique0", "period" : 16000, "mode" : "absolute" },
"run" : 10000,
"dl-runtime" : 11000,
"dl-period" : 16000,
"dl-deadline" : 16000
},
"thread1" : {
"policy" : "SCHED_DEADLINE",
"instance" : 4,
"delay" : 1000,
"timer" : { "ref" : "unique1", "period" : 16000, "mode" : "absolute" },
"run" : 5500,
"dl-runtime" : 6500
"dl-period" : 16000,
"dl-deadline" : 16000
}
...

Tests were run with Performance CPUfreq governor so that the Schedutil
CPUfreq governor DL threads (sugov:[0,4]), necessary on a
slow-switching platform like Hikey960, do not interfere with the
rt-app test tasks. Using Schedutil would require to lower the number of
tasks to 3 instances each.

Dietmar Eggemann (2):
sched/deadline: Optimize dl_bw_cpus()
sched/deadline: Add dl_bw_capacity()

Luca Abeni (3):
sched/deadline: Improve admission control for asymmetric CPU
capacities
sched/deadline: Make DL capacity-aware
sched/deadline: Implement fallback mechanism for !fit case

kernel/sched/cpudeadline.c | 24 ++++++++++
kernel/sched/deadline.c | 89 ++++++++++++++++++++++++++++++--------
kernel/sched/sched.h | 21 +++++++--
3 files changed, 113 insertions(+), 21 deletions(-)

--
2.17.1

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