Improve JobSystem, especially under contention

- only signal waitAndRelease() when the corresponding job finishes and
only if there is waitAndRelease() active -- instead of signaling 
every time a job ends.

- don't surrender time slice when attempting to steal a job and it fails
as long as some queue has jobs.

- check that we have to wait, because taking the lock

- add a benchmark

This change more than doubles the amount of jobs we can handle per
second (~965,000 jobs/s on Pixel3)

libs/utils/benchmark/benchmark_JobSystem.cpp

@@ -0,0 +1,78 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "PerformanceCounters.h"
+
+#include <utils/JobSystem.h>
+#include <utils/compiler.h>
+
+#include <benchmark/benchmark.h>
+
+using namespace utils;
+
+
+static void emptyJob(void*, JobSystem&, JobSystem::Job*) {
+}
+
+static void BM_JobSystem(benchmark::State& state) {
+    JobSystem js;
+    js.adopt();
+
+    {
+        PerformanceCounters pc(state);
+        for (auto _ : state) {
+            js.runAndWait(js.create(nullptr, &emptyJob));
+        }
+    }
+    state.SetItemsProcessed((int64_t)state.iterations());
+}
+
+static void BM_JobSystemAsChildren4k(benchmark::State& state) {
+    JobSystem js;
+    js.adopt();
+
+    {
+        PerformanceCounters pc(state);
+        for (auto _ : state) {
+            auto root = js.create(nullptr, &emptyJob);
+            for (size_t i = 0; i < 4095; i++) {
+                js.run(js.create(root, &emptyJob));
+            }
+            js.runAndWait(root);
+        }
+    }
+    state.SetItemsProcessed((int64_t)state.iterations() * 4096);
+}
+
+static void BM_JobSystemParallelFor(benchmark::State& state) {
+    JobSystem js;
+    js.adopt();
+
+    {
+        PerformanceCounters pc(state);
+        for (auto _ : state) {
+            auto job = jobs::parallel_for(js, nullptr, 0, 4096, [](uint32_t start, uint32_t count) {
+            }, jobs::CountSplitter<1>());
+            js.runAndWait(job);
+        }
+    }
+    state.SetItemsProcessed((int64_t)state.iterations() * 4096);
+}
+
+
+BENCHMARK(BM_JobSystem);
+BENCHMARK(BM_JobSystemAsChildren4k);
+BENCHMARK(BM_JobSystemParallelFor);