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Date:   Tue, 21 Jun 2022 10:04:07 +0100
Message-Id: <20220621090414.433602-1-vdonnefort@google.com>
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Subject: [PATCH v11 0/7] feec() energy margin removal
From:   Vincent Donnefort <vdonnefort@google.com>
To:     peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org
Cc:     linux-kernel@vger.kernel.org, dietmar.eggemann@arm.com,
        morten.rasmussen@arm.com, chris.redpath@arm.com,
        qperret@google.com, tao.zhou@linux.dev, kernel-team@android.com,
        vdonnefort@google.com
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Hi,

Here's a new version of the patch-set to get rid of the energy margin in
find_energy_efficient_cpu() (feec()). Many thanks to all for the insightful
comments I got.

feec() will migrate a task to save energy only if it saves at least 6% of
the total energy consumed by the system. This conservative approach is a
problem on a system where a lot of small tasks create a huge load on the
overall: very few of them will be allowed to migrate to a smaller CPU,
wasting a lot of energy. Instead of trying to determine yet another margin,
let's try to remove it.

The first elements of this patch-set are various fixes and improvements
which stabilize task_util and ensures energy comparison fairness across all
CPUs.  Only once those fixed, we can completely remove the margin and let
feec() place tasks more aggressively and save energy.

This has been validated in two different ways:

First using LISA's eas_behaviour test suite. This is composed of a set of
scenarios and verifies if the task placement is optimum. No failure has bee=
n
observed and it also improved some tests such as Ramp-Down (as the
placement is now more energy oriented) and *ThreeSmall (as no bouncing
between clusters happen anymore).

  * Hikey960: 100% PASSED
  * DB-845C:  100% PASSED
  * RB5:      100% PASSED

Second, using an Android benchmark: PCMark2 on a Pixel4, with a lot of
backports to have a scheduler as close as we can from mainline.

  +------------+-----------------+-----------------+
  |    Test    |      Perf       |    Energy [1]   |
  +------------+-----------------+-----------------+
  | Web2       | -0.3% pval 0.03 | -1.8% pval 0.00 |
  | Video2     | -0.3% pval 0.13 | -5.6% pval 0.00 |
  | Photo2 [2] | -3.8% pval 0.00 | -1%   pval 0.00 |
  | Writing2   |  0%   pval 0.13 | -1%   pval 0.00 |
  | Data2      |  0%   pval 0.8  | -0.43 pval 0.00 |
  +------------+-----------------+-----------------+

The margin removal let the kernel make the best use of the Energy Model,
tasks are more likely to be placed where they fit and this saves a
substantial amount of energy, while having a limited impact on
performances.

[1]=C2=A0This is an energy estimation based on the CPU activity and the Ene=
rgy
Model for this device. "All models are wrong but some are useful"; yes,
this is an imperfect estimation that doesn't take into account some idle
states and shared power rails. Nonetheless this is based on the information
the kernel has during runtime and it proves the scheduler can take better
decisions based solely on those data.

[2] This is the only performance impact observed. The debugging of this
test showed no issue with task placement. The better score was solely due
to some critical threads held on better performing CPUs. If a thread needs
a higher capacity CPU, the placement must result from a user input (with
e.g. uclamp min) instead of being artificially held on less efficient CPUs
by feec(). Notice also, the experiment didn't use the Android-only
latency_sensitive feature which would hide this problem on a real-life
device.

v10 -> v11:
  - Cosmetic changes for comments and commit messages.
  - Tested-by tag from Lukasz.

v9 -> v10:
  - Cosmetic changes for comments and commit messages.
  - Renaming timestamp variables =3D> *_idle. (Dietmar)
  - Fix for empty mask in feec() (Dietmar)
  - Collect Reviewed-by tags.

v8 -> v9:
  - PELT migration decay: Fix barriers to prevent overestimation. (Vincent
    G.)
  - PELT migration decay: Fix CONFIG_GROUP_SCHED=3Dn build.
  - Various readbility improvements. (Dietmar)
  - Collect Reviewed-by tags.

v7 -> v8:
  - PELT migration decay: Refine estimation computation. (vincent G.)
  - PELT migration decay: Do not apply estimation if load_avg is decayed
    (Tao)
  - PELT migration decay: throttled_pelt_idle update ordering for the
    update_blocked_load case. (vincent G.)

v6 -> v7:
  - PELT migration decay: Add missing clock_pelt_idle updates.
  - PELT migration decay: Fix PELT scaling delta for CONFIG_CFS_BANDWIDTH.

v4 -> v5:
  - PELT migration decay: timestamp only at idle time (Vincent G.)
  - PELT migration decay: split timestamp values (enter_idle /
    clock_pelt_idle) (Vincent G.)

v3 -> v4:
  - Minor cosmetic changes (Dietmar)

v2 -> v3:
  - feec(): introduce energy_env struct (Dietmar)
  - PELT migration decay: Only apply when src CPU is idle (Vincent G.)
  - PELT migration decay: Do not apply when cfs_rq is throttled
  - PELT migration decay: Snapshot the lag at cfs_rq's level

v1 -> v2:
  - Fix PELT migration last_update_time (previously root cfs_rq's).
  - Add Dietmar's patches to refactor feec()'s CPU loop.
  - feec(): renaming busy time functions get_{pd,tsk}_busy_time()
  - feec(): pd_cap computation in the first for_each_cpu loop.
  - feec(): create get_pd_max_util() function (previously within
    compute_energy())
  - feec(): rename base_energy_pd to base_energy.

Dietmar Eggemann (3):
  sched, drivers: Remove max param from
    effective_cpu_util()/sched_cpu_util()
  sched/fair: Rename select_idle_mask to select_rq_mask
  sched/fair: Use the same cpumask per-PD throughout
    find_energy_efficient_cpu()

Vincent Donnefort (4):
  sched/fair: Provide u64 read for 32-bits arch helper
  sched/fair: Decay task PELT values during wakeup migration
  sched/fair: Remove task_util from effective utilization in feec()
  sched/fair: Remove the energy margin in feec()

 drivers/powercap/dtpm_cpu.c       |  33 +--
 drivers/thermal/cpufreq_cooling.c |   6 +-
 include/linux/sched.h             |   2 +-
 kernel/sched/core.c               |  15 +-
 kernel/sched/cpufreq_schedutil.c  |   5 +-
 kernel/sched/fair.c               | 470 +++++++++++++++++++-----------
 kernel/sched/pelt.h               |  40 ++-
 kernel/sched/sched.h              |  53 +++-
 8 files changed, 400 insertions(+), 224 deletions(-)

--=20
2.37.0.rc0.104.g0611611a94-goog