[wasm] Delay taking PKU sample until first module creation

We currently take the sample at the moment the isolate is created. At
that point, the embedder callback for taking samples is not installed
yet. Hence delay taking the sample until the first module is created.
This will only take samples for isolates that actually use wasm, which
will reduce the overall number of samples, but will give a better
picture of PKU support for Wasm.

R=jkummerow@chromium.org
CC=dlehmann@google.com

Bug: v8:11714
Change-Id: I8a4163961c06076efd6c5dde5751682b53863c2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2944429Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74975}

src/wasm/wasm-engine.cc

@@ -391,6 +391,10 @@ struct WasmEngine::IsolateInfo {
   // Keep new modules in tiered down state.
   bool keep_tiered_down = false;
 
+  // Keep track whether we already added a sample for PKU support (we only want
+  // one sample per Isolate).
+  bool pku_support_sampled = false;
+
   // Elapsed time since last throw/rethrow/catch event.
   base::ElapsedTimer throw_timer;
   base::ElapsedTimer rethrow_timer;
@@ -990,15 +994,6 @@ void WasmEngine::AddIsolate(Isolate* isolate) {
   DCHECK_EQ(0, isolates_.count(isolate));
   isolates_.emplace(isolate, std::make_unique<IsolateInfo>(isolate));
 
-  // Record memory protection key support.
-  if (FLAG_wasm_memory_protection_keys) {
-    auto* histogram =
-        isolate->counters()->wasm_memory_protection_keys_support();
-    bool has_mpk =
-        code_manager()->memory_protection_key_ != kNoMemoryProtectionKey;
-    histogram->AddSample(has_mpk ? 1 : 0);
-  }
-
   // Install sampling GC callback.
   // TODO(v8:7424): For now we sample module sizes in a GC callback. This will
   // bias samples towards apps with high memory pressure. We should switch to
@@ -1146,13 +1141,24 @@ std::shared_ptr<NativeModule> WasmEngine::NewNativeModule(
       native_module.get(), std::make_unique<NativeModuleInfo>(native_module)));
   DCHECK(pair.second);  // inserted new entry.
   pair.first->second.get()->isolates.insert(isolate);
-  auto& modules_per_isolate = isolates_[isolate]->native_modules;
-  modules_per_isolate.insert(native_module.get());
-  if (isolates_[isolate]->keep_tiered_down) {
+  auto* isolate_info = isolates_[isolate].get();
+  isolate_info->native_modules.insert(native_module.get());
+  if (isolate_info->keep_tiered_down) {
     native_module->SetTieringState(kTieredDown);
   }
+
+  // Record memory protection key support.
+  if (FLAG_wasm_memory_protection_keys && !isolate_info->pku_support_sampled) {
+    isolate_info->pku_support_sampled = true;
+    auto* histogram =
+        isolate->counters()->wasm_memory_protection_keys_support();
+    bool has_mpk =
+        code_manager_.memory_protection_key_ != kNoMemoryProtectionKey;
+    histogram->AddSample(has_mpk ? 1 : 0);
+  }
+
   isolate->counters()->wasm_modules_per_isolate()->AddSample(
-      static_cast<int>(modules_per_isolate.size()));
+      static_cast<int>(isolate_info->native_modules.size()));
   isolate->counters()->wasm_modules_per_engine()->AddSample(
       static_cast<int>(native_modules_.size()));
   return native_module;