added more comments about cache invalidation logic

Signed-off-by: Byoungro So <[email protected]>

Author: bso-intel

sycl/plugins/level_zero/pi_level_zero.cpp

@@ -632,6 +632,9 @@ pi_result piDevicesGet(pi_platform Platform, pi_device_type DeviceType,
   // Get number of devices supporting Level Zero
   uint32_t ZeDeviceCount = 0;
   std::lock_guard<std::mutex> Lock(Platform->PiDevicesCacheMutex);
+  // As soon as there was a call to piDeviceRelease(), the entire cache is
+  // invalidated by setting the flag CacheInvalidated. We just need to
+  // re-initialize cached pi_devices to reuse them.
   if (Platform->CacheInvalidated) {
     for (const pi_device CachedDevice : Platform->PiDevicesCache) {
       CachedDevice->initialize();
@@ -708,6 +711,14 @@ pi_result piDeviceRelease(pi_device Device) {
   // TODO: OpenCL says root-device ref-count remains unchanged (1),
   // but when would we free the device's data?
   if (--(Device->RefCount) == 0) {
+    // We invalidate the entire cache as soon as any device is released.
+    // The saved pi_devices in cache is still intact but flag CacheInvalided
+    // will not allow the entire cached devices to be reused without
+    // re-initializing them.
+    // TODO: This means the cached pi_device live until the program ends.
+    // If L0 RT does not do its own cleanup for Ze_Device_Handle upon tear-down,
+    // we need to figure out a way to call
+    // ZE_CALL(zeCommandListDestroy(Device->ZeCommandListInit));
     pi_platform Platform = Device->Platform;
     std::lock_guard<std::mutex> Lock(Platform->PiDevicesCacheMutex);
     Platform->CacheInvalidated = true;

sycl/plugins/level_zero/pi_level_zero.hpp

@@ -75,6 +75,10 @@ struct _pi_platform {
   std::vector<pi_device> PiDevicesCache;
   std::mutex PiDevicesCacheMutex;
   // Flag to indicate PiDevicesCache is invalidated
+  // This flag is used in piDeviceRelease to invalidate the entire cache
+  //   whenever there is a call to piDeviceRelease for any cached device.
+  // This flag is used in piDevicesGet to reuse the cache
+  //   without expensive calls to L0 RT.
   bool CacheInvalidated = false;
 };