[SYCL][L0] Add an env var to control buffer migration in context with…

… single root-device (#7178)

Changed the default to perform regular migration, as opposed to have
single root-device allocation to serve all [sub-]devices.
Signed-off-by: Sergey V Maslov <sergey.v.maslov@intel.com>

Signed-off-by: Sergey V Maslov <sergey.v.maslov@intel.com>

sycl/doc/EnvironmentVariables.md

@@ -238,6 +238,7 @@ variables in production code.</span>
 | `SYCL_PI_LEVEL_ZERO_USE_IMMEDIATE_COMMANDLISTS` | Integer | When set to a positive value enables use of Level Zero immediate commandlists, which means there is no batching and all commands are immediately submitted for execution. Default is 0. Note: When immediate commandlist usage is enabled it is necessary to also set SYCL_PI_LEVEL_ZERO_DEVICE_SCOPE_EVENTS to either 0 or 1.  |
 | `SYCL_PI_LEVEL_ZERO_USE_MULTIPLE_COMMANDLIST_BARRIERS` | Integer | When set to a positive value enables use of multiple Level Zero commandlists when submitting barriers. Default is 1. |
 | `SYCL_PI_LEVEL_ZERO_USE_COPY_ENGINE_FOR_FILL` | Integer | When set to a positive value enables use of a copy engine for memory fill operations. Default is 0. |
+| `SYCL_PI_LEVEL_ZERO_SINGLE_ROOT_DEVICE_BUFFER_MIGRATION` | Integer | When set to "0" tells to use single root-device allocation for all devices in a context where all devices have same root. Otherwise performs regular buffer migration. Default is 1. |
 
 ## Debugging variables for CUDA Plugin
 

sycl/plugins/level_zero/pi_level_zero.cpp

@@ -8812,10 +8812,29 @@ pi_result _pi_buffer::getZeHandle(char *&ZeHandle, access_mode_t AccessMode,
     LastDeviceWithValidAllocation = Device;
     return PI_SUCCESS;
   }
+  // Reads user setting on how to deal with buffers in contexts where
+  // all devices have the same root-device. Returns "true" if the
+  // preference is to have allocate on each [sub-]device and migrate
+  // normally (copy) to other sub-devices as needed. Returns "false"
+  // if the preference is to have single root-device allocations
+  // serve the needs of all [sub-]devices, meaning potentially more
+  // cross-tile traffic.
+  //
+  static const bool SingleRootDeviceBufferMigration = [] {
+    const char *EnvStr =
+        std::getenv("SYCL_PI_LEVEL_ZERO_SINGLE_ROOT_DEVICE_BUFFER_MIGRATION");
+    if (EnvStr)
+      return (std::stoi(EnvStr) != 0);
+    // The default is to migrate normally, which may not always be the
+    // best option (depends on buffer access patterns), but is an
+    // overall win on the set of the available benchmarks.
+    return true;
+  }();
 
   // Peform actual device allocation as needed.
   if (!Allocation.ZeHandle) {
-    if (Context->SingleRootDevice && Context->SingleRootDevice != Device) {
+    if (!SingleRootDeviceBufferMigration && Context->SingleRootDevice &&
+        Context->SingleRootDevice != Device) {
       // If all devices in the context are sub-devices of the same device
       // then we reuse root-device allocation by all sub-devices in the
       // context.