Messages in this thread | | | From | Valentin Schneider <> | Subject | [PATCH v2 0/3] sched/fair: Capacity aware wakeup rework | Date | Fri, 24 Jan 2020 13:02:10 +0000 |
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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 |
Revisions =========
v1 -> v2: Removed unrelated select_idle_core() change
[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 | 67 +++++++++++++++++++++++------------------ kernel/sched/topology.c | 15 ++------- 2 files changed, 41 insertions(+), 41 deletions(-)
-- 2.24.0
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