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Message-Id: <20200124124255.1095-1-valentin.schneider@arm.com>
Date: Fri, 24 Jan 2020 12:42:52 +0000
From: Valentin Schneider <valentin.schneider@....com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
dietmar.eggemann@....com, morten.rasmussen@....com,
qperret@...gle.com, adharmap@...eaurora.org
Subject: [PATCH 0/3] sched/fair: Capacity aware wakeup rework
This series is about replacing the current wakeup logic for asymmetric CPU
capacity topologies, i.e. wake_cap().
Details are in patch 1, the TL;DR is that wake_cap() works fine for
"legacy" big.LITTLE systems (e.g. Juno), since the Last Level Cache (LLC)
domain of a CPU only spans CPUs of the same capacity, but somewhat broken
for newer DynamIQ systems (e.g. Dragonboard 845C), since the LLC domain of
a CPU can span all CPUs in the system. Both example boards are supported in
mainline.
A bit of history
================
Due to the old Energy Model (EM) used until Android Common Kernel v4.14
which grafted itself onto the sched domain hierarchy, mobile topologies
have been represented with "phantom domains"; IOW we'd make a DynamIQ
topology look like a big.LITTLE one:
actual hardware:
+-------------------+
| L3 |
+----+----+----+----+
| L2 | L2 | L2 | L2 |
+----+----+----+----+
|CPU0|CPU1|CPU2|CPU3|
+----+----+----+----+
^^^^^ ^^^^^
LITTLEs bigs
vanilla/mainline topology:
MC [ ]
0 1 2 3
phantom domains topology:
DIE [ ]
MC [ ][ ]
0 1 2 3
With the newer, mainline EM this is no longer required, and wake_cap() is
the last sticking point to getting rid of this legacy crud. More details
and examples are in patch 1.
Notes
=====
This removes the use of SD_BALANCE_WAKE for asymmetric CPU capacity
topologies (which are the last mainline users of that flag), as such it
shouldn't be a surprise that this comes with significant improvements to
wake-intensive workloads: wakeups no longer go through the
select_task_rq_fair() slow-path.
Testing
=======
I've picked sysbench --test=threads to mimic Peter's testing mentioned in
commit 182a85f8a119 ("sched: Disable wakeup balancing")
Sysbench results are the number of events handled in a fixed amount of
time, so higher is better. Hackbench results are the usual time taken for
the thing, so lower is better.
Note: the 'X%' stats are the percentiles, so 50% is the 50th percentile.
Juno r0 ("legacy" big.LITTLE)
+++++++++++++++++++++++++++++
This is 2 bigs and 4 LITTLEs:
+---------------+ +-------+
| L2 | | L2 |
+---+---+---+---+ +---+---+
| L | L | L | L | | B | B |
+---+---+---+---+ +---+---+
100 iterations of 'hackbench':
| | -PATCH | +PATCH | DELTA (%) |
|-------+----------+----------+-----------|
| mean | 0.622300 | 0.613000 | -1.494 |
| std | 0.028886 | 0.015178 | -47.456 |
| min | 0.579000 | 0.585000 | +1.036 |
| 50% | 0.619500 | 0.610000 | -1.533 |
| 75% | 0.633250 | 0.621000 | -1.934 |
| 99% | 0.680270 | 0.649110 | -4.581 |
| max | 0.806000 | 0.660000 | -18.114 |
100 iterations of 'sysbench --max-time=5 --max-requests=-1 --test=threads --num-threads=6 run':
| | -PATCH | +PATCH | DELTA(%) |
|-------+--------------+--------------+----------|
| mean | 9695.500000 | 12556.930000 | +29.513 |
| std | 2897.875263 | 2941.268452 | +1.497 |
| min | 7011.000000 | 6800.000000 | -3.010 |
| 50% | 8305.000000 | 13636.500000 | +64.196 |
| 75% | 11924.500000 | 15273.250000 | +28.083 |
| 99% | 15310.140000 | 15558.860000 | +1.625 |
| max | 15522.000000 | 15644.000000 | +0.786 |
Pixel3 (DynamIQ)
++++++++++++++++
Ideally I would have used a DB845C but had a few issues with mine, so I
went with a mainline-ish Pixel3 instead [1]. It's still the same SoC under
the hood (Snapdragon 845), which has 4 bigs and 4 LITTLEs:
+-------------------------------+
| L3 |
+---+---+---+---+---+---+---+---+
| L2| L2| L2| L2| L2| L2| L2| L2|
+---+---+---+---+---+---+---+---+
| L | L | L | L | B | B | B | B |
+---+---+---+---+---+---+---+---+
Default topology (single MC domain)
-----------------------------------
100 iterations of 'hackbench -l 200'
| | -PATCH | +PATCH | DELTA (%) |
|-------+----------+----------+-----------|
| mean | 1.165010 | 1.116370 | -4.175 |
| std | 0.124682 | 0.111952 | -10.210 |
| min | 0.962000 | 0.936000 | -2.703 |
| 50% | 1.133500 | 1.090000 | -3.838 |
| 75% | 1.251500 | 1.186000 | -5.234 |
| 99% | 1.483050 | 1.350040 | -8.969 |
| max | 1.488000 | 1.354000 | -9.005 |
100 iterations of 'sysbench --max-time=5 --max-requests=-1 --test=threads --num-threads=8 run':
| | -PATCH | +PATCH | DELTA (%) |
|-------+-------------+------------+-----------|
| mean | 7108.310000 | 8455.97000 | +18.959 |
| std | 199.431854 | 248.27939 | +24.493 |
| min | 6655.000000 | 7875.00000 | +18.332 |
| 50% | 7107.500000 | 8454.50000 | +18.952 |
| 75% | 7255.500000 | 8622.50000 | +18.841 |
| 99% | 7539.540000 | 8981.21000 | +19.121 |
| max | 7593.000000 | 9101.00000 | +19.860 |
Phantom domains (MC + DIE)
--------------------------
This is mostly included for the sake of completeness.
100 iterations of 'sysbench --max-time=5 --max-requests=-1 --test=threads --num-threads=8 run':
| | -PATCH | +PATCH | DELTA (%) |
|-------+-------------+-------------+-----------|
| mean | 5568.930000 | 7884.180000 | +41.574 |
| std | 238.341587 | 218.808407 | -8.195 |
| min | 4961.000000 | 7222.000000 | +45.575 |
| 50% | 5575.500000 | 7885.500000 | +41.431 |
| 75% | 5711.500000 | 8031.250000 | +40.615 |
| 99% | 6178.000000 | 8395.670000 | +35.896 |
| max | 6178.000000 | 8462.000000 | +36.970 |
[1]: https://git.linaro.org/people/amit.pundir/linux.git/log/?h=blueline-mainline-tracking
Morten Rasmussen (3):
sched/fair: Add asymmetric CPU capacity wakeup scan
sched/topology: Remove SD_BALANCE_WAKE on asymmetric capacity systems
sched/fair: Kill wake_cap()
kernel/sched/fair.c | 69 +++++++++++++++++++++++------------------
kernel/sched/topology.c | 15 ++-------
2 files changed, 42 insertions(+), 42 deletions(-)
--
2.24.0
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