[SPARK-27754][K8S] Introduce additional config (spark.kubernetes.driver.request.cores) for driver request cores for spark on k8s

## What changes were proposed in this pull request?

Spark on k8s supports config for specifying the executor cpu requests
(spark.kubernetes.executor.request.cores) but a similar config is missing
for the driver. Instead, currently `spark.driver.cores` value is used for integer value.

Although `pod spec` can have `cpu` for the fine-grained control like the following, this PR proposes additional configuration `spark.kubernetes.driver.request.cores` for driver request cores.
```
resources:
  requests:
    memory: "64Mi"
    cpu: "250m"
```

## How was this patch tested?

Unit tests

Closes #24630 from arunmahadevan/SPARK-27754.

Authored-by: Arun Mahadevan <arunm@apache.org>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

docs/running-on-kubernetes.md

@@ -793,6 +793,15 @@ See the [configuration page](configuration.html) for information on Spark config
     Interval between reports of the current Spark job status in cluster mode.
   </td>
 </tr>
+<tr>
+  <td><code>spark.kubernetes.driver.request.cores</code></td>
+  <td>(none)</td>
+  <td>
+    Specify the cpu request for the driver pod. Values conform to the Kubernetes <a href="https://kubernetes.io/docs/concepts/configuration/manage-compute-resources-container/#meaning-of-cpu">convention</a>.
+    Example values include 0.1, 500m, 1.5, 5, etc., with the definition of cpu units documented in <a href="https://kubernetes.io/docs/tasks/configure-pod-container/assign-cpu-resource/#cpu-units">CPU units</a>.
+    This takes precedence over <code>spark.driver.cores</code> for specifying the driver pod cpu request if set.
+  </td>
+</tr>
 <tr>
   <td><code>spark.kubernetes.driver.limit.cores</code></td>
   <td>(none)</td>

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/Config.scala

@@ -125,6 +125,12 @@ private[spark] object Config extends Logging {
       .stringConf
       .createOptional
 
+  val KUBERNETES_DRIVER_REQUEST_CORES =
+    ConfigBuilder("spark.kubernetes.driver.request.cores")
+      .doc("Specify the cpu request for the driver pod")
+      .stringConf
+      .createOptional
+
   val KUBERNETES_DRIVER_SUBMIT_CHECK =
     ConfigBuilder("spark.kubernetes.submitInDriver")
     .internal()

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/features/BasicDriverFeatureStep.scala

@@ -43,7 +43,10 @@ private[spark] class BasicDriverFeatureStep(conf: KubernetesDriverConf)
     .getOrElse(throw new SparkException("Must specify the driver container image"))
 
   // CPU settings
-  private val driverCpuCores = conf.get(DRIVER_CORES.key, "1")
+  private val driverCpuCores = conf.get(DRIVER_CORES)
+  private val driverCoresRequest = conf
+    .get(KUBERNETES_DRIVER_REQUEST_CORES)
+    .getOrElse(driverCpuCores.toString)
   private val driverLimitCores = conf.get(KUBERNETES_DRIVER_LIMIT_CORES)
 
   // Memory settings
@@ -77,7 +80,7 @@ private[spark] class BasicDriverFeatureStep(conf: KubernetesDriverConf)
       }
 
     val driverCpuQuantity = new QuantityBuilder(false)
-      .withAmount(driverCpuCores)
+      .withAmount(driverCoresRequest)
       .build()
     val driverMemoryQuantity = new QuantityBuilder(false)
       .withAmount(s"${driverMemoryWithOverheadMiB}Mi")

resource-managers/kubernetes/core/src/test/scala/org/apache/spark/deploy/k8s/features/BasicDriverFeatureStepSuite.scala

@@ -117,6 +117,38 @@ class BasicDriverFeatureStepSuite extends SparkFunSuite {
     assert(featureStep.getAdditionalPodSystemProperties() === expectedSparkConf)
   }
 
+  test("Check driver pod respects kubernetes driver request cores") {
+    val sparkConf = new SparkConf()
+      .set(KUBERNETES_DRIVER_POD_NAME, "spark-driver-pod")
+      .set(CONTAINER_IMAGE, "spark-driver:latest")
+
+    val basePod = SparkPod.initialPod()
+    // if spark.driver.cores is not set default is 1
+    val requests1 = new BasicDriverFeatureStep(KubernetesTestConf.createDriverConf(sparkConf))
+      .configurePod(basePod)
+      .container.getResources
+      .getRequests.asScala
+    assert(requests1("cpu").getAmount === "1")
+
+    // if spark.driver.cores is set it should be used
+    sparkConf.set(DRIVER_CORES, 10)
+    val requests2 = new BasicDriverFeatureStep(KubernetesTestConf.createDriverConf(sparkConf))
+      .configurePod(basePod)
+      .container.getResources
+      .getRequests.asScala
+    assert(requests2("cpu").getAmount === "10")
+
+    // spark.kubernetes.driver.request.cores should be preferred over spark.driver.cores
+    Seq("0.1", "100m").foreach { value =>
+      sparkConf.set(KUBERNETES_DRIVER_REQUEST_CORES, value)
+      val requests3 = new BasicDriverFeatureStep(KubernetesTestConf.createDriverConf(sparkConf))
+        .configurePod(basePod)
+        .container.getResources
+        .getRequests.asScala
+      assert(requests3("cpu").getAmount === value)
+    }
+  }
+
   test("Check appropriate entrypoint rerouting for various bindings") {
     val javaSparkConf = new SparkConf()
       .set(DRIVER_MEMORY.key, "4g")