[PATCH 0/2] drivers: android: binder: Introduce Cluster Aware Thread Selection

[trieu2.huynh]

  Motivation
  ----------

On asymmetric multi-cluster ARM64 SoCs, the default binder thread
selection is topology-blind. Cross-cluster thread wakeups incur severe
cache-coherency overhead.

Measured performance test [1], especially with schd-dbg tiered threads [2]
on baseline (10K iter, 16B payload):

  Class       Tier              Avg Latency    Count / % of txns
  -----       ----              -----------    -----------------
  SCHED_FIFO  Same-core         0.089 ms       9,963 / 99.6%
  (max prio)  Same-cluster      0.120 ms          13 /  0.1%
              Cross-cluster     0.140 ms          24 /  0.2%
              -----------------------------------------------------
  Normal      Same-core         0.130 ms       4,623 / 46.2%
  (Other)     Same-cluster      0.140 ms       3,290 / 32.9%
              Cross-cluster     0.190 ms       2,087 / 20.9%

Key Observations:
(*) For SCHED_FIFO tasks, the scheduler aggressively locks the affinity,
    keeping 99.6% of txns on the same core, establishing the hardware's
    ideal latency ceiling (~0.089 ms).
(*) For normal tasks, selection is highly fragmented (only 46.2% on
    same-core). Cross-cluster transactions are ~46% slower (0.19 vs
    0.13 ms) due to interconnect overhead and cache invalidation.
(*) CTABS aims to close this gap for normal threads by enforcing affinity.

  Design
  ------

CTABS optimizes thread dispatching through a two-tiered co-design:

(*) Topology Scoring: Scores waiting threads at selection time:
    - Same CPU: Immediate dispatch (optimal L1/L2 locality via early exit)
    - Score 1 (same cluster): Local cache reuse within the same cluster
    - Score 0 (cross-cluster): Fallback to any available thread

(*) Adaptive Wake-up Nudge: For synchronous transactions yielding a cluster
    match, the caller's CPU is stored as 'preferred_cpu'. Right before
    wakeup, this value is injected into 'task->wake_cpu' to guide the
    scheduler. To prevent runqueue contention under heavy load, this nudge
    is adaptively applied only if 'available_idle_cpu(caller_cpu)' returns
    true.

  Implementation
  --------------

[PATCH 1/2] drivers: android: binder: implement CTABS
  Core scoring in binder_select_thread_topology_ilocked(),
  thread->last_cpu tracking, integration at sync and async paths.

[PATCH 2/2] drivers: android: binder: add CTABS wake-up nudge
  Adaptive scheduler hint via task->wake_cpu on top of scoring,
  safeguarded by available_idle_cpu() to avoid core contention.

  Benchmark Methodology
  ---------------------

The evaluation is conducted on a Qualcomm SA8155P Automotive platform
featuring an 8-core asymmetric Kryo 485 hierarchy organized into 3 distinct
clusters under a 1 + 3 + 4 configuration:
- Cluster 1 (Prime): 1x Kryo Gold Prime core (Cortex-A76 based).
- Cluster 2 (Gold):  3x Kryo Gold cores (Cortex-A76 based).
- Cluster 3 (Silver):4x Kryo Silver cores (Cortex-A55 based).

The test environment runs AAOSP 14 with a Linux Kernel 6.1 GKI under the 
default schedutil governor.

To provide a comprehensive performance profile, two distinct test suites
are leveraged:
(*) schd-dbg [2]: Utilized as a micro-benchmark to isolate and collect
    precise latency granularities across different affinity tiers.
(*) binderThroughputTest [3]: Executed under a heavy stress load (8 workers
    x 100K iterations, 3 runs) paired with simpleperf to capture
    hardware-level metrics (cycles, cache behavior, migrations) that reflect
    the macro-level systemic impact of the optimization.

  Results
  -------

(*) schd-dbg — Tiered Thread Latency (Avg in ms)

  Class      Tier            Baseline   Patch 1/2   Patch 2/2   Δ P1/2   Δ P2/2
  -----      -------------   --------   ---------   ---------   ------   ------
  SCHED_FIFO Same-core       0.089 ms   0.084 ms    0.083 ms    -5.6%    -6.7%
             Same-cluster    0.120 ms   0.110 ms    0.110 ms    -8.3%    -8.3%
             Cross-cluster   0.140 ms   0.130 ms    0.130 ms    -7.1%    -7.1%
             ------------------------------------------------------------------
  Normal     Same-core       0.130 ms   0.140 ms    0.140 ms    +7.7%    +7.7%
  (Other)    Same-cluster    0.140 ms   0.140 ms    0.130 ms     0%      -7.1%
             Cross-cluster   0.190 ms   0.210 ms    0.200 ms    +10.5%   +5.3%

(*) binderThroughputTest — 8 workers x 100K iter

  Metric            Baseline     Patch 1/2    Patch 2/2    Δ P1/2   Δ P2/2
  ------            --------     ---------    ---------    ------   ------
  Throughput        41,045/s     46,252/s     46,179/s     +12.7%   +12.5%
  Avg latency       0.168 ms     0.146 ms     0.145 ms     -13.1%   -13.7%
  p50 latency       0.195 ms     0.195 ms     0.117 ms       0%     -40.0%
  p99 latency       0.586 ms     0.586 ms     0.558 ms       0%      -4.8%
  
  CPU cycles        143.15 B     128.66 B     128.94 B     -10.1%    -9.9%
  Cache refs        21.83 B      21.04 B      21.24 B       -3.6%    -2.7%
  Cache misses      1,078 M      968 M        949 M        -10.2%   -11.9%
  CPU migrations    922,848      730,277      739,576      -20.9%   -19.9%
  Ctx switches      1,697 K      1,691 K      1,692 K       -0.4%    -0.3%

[1] https://source.android.com/docs/core/tests/vts/performance
[2] https://android-review.googlesource.com/c/platform/frameworks/native/+/4164726
[3] https://android.googlesource.com/platform/frameworks/native/+/master/libs/binder/tests/binderThroughputTest.cpp

trieu2.huynh (2):
  drivers: android: binder: implement CTABS heuristic
  drivers: android: binder: add preferred CPU wake-up nudge for waiting
    threads

 drivers/android/binder.c          | 111 ++++++++++++++++++++++++++++--
 drivers/android/binder_internal.h |   2 +
 2 files changed, 109 insertions(+), 4 deletions(-)

-- 
2.43.0