Add tracing for navigation start, FP, FCP, and DOMContentLoaded (#34373)as-of-2024-08-12

Signed-off-by: Delan Azabani <dazabani@igalia.com>

6 files changed, 48 insertions, 12 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 04f3954b95f..6074fb1e9b0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4009,6 +4009,7 @@ dependencies = [
  "layout_thread_2020",
  "log",
  "media",
+ "metrics",
  "mozangle",
  "net",
  "net_traits",
@@ -4290,6 +4291,7 @@ dependencies = [
  "script_traits",
  "servo_config",
  "servo_url",
+ "tracing",
 ]
 
 [[package]]
diff --git a/components/metrics/Cargo.toml b/components/metrics/Cargo.toml
index f2464ccf24b..7f12c50ccf4 100644
--- a/components/metrics/Cargo.toml
+++ b/components/metrics/Cargo.toml
@@ -10,6 +10,9 @@ publish.workspace = true
 name = "metrics"
 path = "lib.rs"
 
+[features]
+tracing = ["dep:tracing"]
+
 [dependencies]
 base = { workspace = true }
 fonts_traits = { workspace = true }
@@ -21,3 +24,4 @@ profile_traits = { workspace = true }
 script_traits = { workspace = true }
 servo_config = { path = "../config" }
 servo_url = { path = "../url" }
+tracing = { workspace = true, optional = true }
diff --git a/components/metrics/lib.rs b/components/metrics/lib.rs
index 0ac3021c52b..8c19b129209 100644
--- a/components/metrics/lib.rs
+++ b/components/metrics/lib.rs
@@ -322,17 +322,32 @@ impl PaintTimeMetrics {
 
         if let Some(pending_metric) = self.pending_metrics.borrow_mut().remove(&epoch) {
             let profiler_metadata = pending_metric.0;
-            set_metric(
-                self,
-                profiler_metadata.clone(),
-                ProgressiveWebMetricType::FirstPaint,
-                ProfilerCategory::TimeToFirstPaint,
-                &self.first_paint,
-                paint_time,
-                &self.url,
-            );
-
-            if pending_metric.1 {
+
+            if self.first_paint.get().is_none() {
+                // TODO(delan): exclude compositor-to-script IPC time. May require support for
+                // backdating events, unless we make the compositor aware of things like the page
+                // URL for each pipeline and whether the paint was contentful.
+                #[cfg(feature = "tracing")]
+                tracing::info!(name: "First Paint", servo_profiling = true, url = self.url.to_string());
+
+                set_metric(
+                    self,
+                    profiler_metadata.clone(),
+                    ProgressiveWebMetricType::FirstPaint,
+                    ProfilerCategory::TimeToFirstPaint,
+                    &self.first_paint,
+                    paint_time,
+                    &self.url,
+                );
+            }
+
+            if self.first_contentful_paint.get().is_none() && pending_metric.1 {
+                // TODO(delan): exclude compositor-to-script IPC time. May require support for
+                // backdating events, unless we make the compositor aware of things like the page
+                // URL for each pipeline and whether the paint was contentful.
+                #[cfg(feature = "tracing")]
+                tracing::info!(name: "First Contentful Paint", servo_profiling = true, url = self.url.to_string());
+
                 set_metric(
                     self,
                     profiler_metadata,
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index babfffe9046..28b40109778 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -2607,6 +2607,12 @@ impl Document {
             "Complete before DOMContentLoaded?"
         );
 
+        // We only want a tracing event for DOMContentLoaded for actual pages and iframes,
+        // not when a script is just parsing a fragment.
+        if self.has_browsing_context() {
+            #[cfg(feature = "tracing")]
+            tracing::info!(name: "DOMContentLoaded", servo_profiling = true, url = self.url().to_string());
+        }
         update_with_current_instant(&self.dom_content_loaded_event_start);
 
         // Step 4.1.
@@ -2781,6 +2787,10 @@ impl Document {
             return;
         }
         if self.tti_window.borrow().needs_check() {
+            // TODO(delan): include this in Perfetto traces. May require support for backdating
+            // events, unless we include all candidates and filter them in perf-analysis-tools.
+            // Not a high priority, since TTI is now considered to be an outdated metric:
+            // <https://developer.chrome.com/docs/lighthouse/performance/interactive/>
             self.get_interactive_metrics().maybe_set_tti(
                 self,
                 InteractiveFlag::TimeToInteractive(self.tti_window.borrow().get_start()),
diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index 648e3034f36..0f36d85dedc 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -238,6 +238,10 @@ impl InProgressLoad {
         inherited_secure_context: Option<bool>,
     ) -> InProgressLoad {
         let navigation_start = CrossProcessInstant::now();
+
+        #[cfg(feature = "tracing")]
+        tracing::info!(name: "Navigation start", servo_profiling = true, url = url.to_string());
+
         InProgressLoad {
             pipeline_id: id,
             browsing_context_id,
diff --git a/components/servo/Cargo.toml b/components/servo/Cargo.toml
index 7db65516bc4..11d032e2053 100644
--- a/components/servo/Cargo.toml
+++ b/components/servo/Cargo.toml
@@ -24,7 +24,7 @@ no-wgl = ["mozangle/egl", "mozangle/build_dlls", "surfman/sm-angle-default"]
 dynamic_freetype = ["webrender/dynamic_freetype"]
 profilemozjs = ["script/profilemozjs"]
 refcell_backtrace = ["script/refcell_backtrace"]
-tracing = ["dep:tracing", "compositing/tracing", "constellation/tracing", "layout_thread_2013/tracing", "layout_thread_2020/tracing", "profile_traits/tracing", "script/tracing"]
+tracing = ["dep:tracing", "compositing/tracing", "constellation/tracing", "layout_thread_2013/tracing", "layout_thread_2020/tracing", "metrics/tracing", "profile_traits/tracing", "script/tracing"]
 webdriver = ["webdriver_server"]
 webgl_backtrace = [
     "script/webgl_backtrace",
@@ -59,6 +59,7 @@ layout_thread_2013 = { path = "../layout_thread", optional = true }
 layout_thread_2020 = { path = "../layout_thread_2020" }
 log = { workspace = true }
 media = { path = "../media" }
+metrics = { path = "../metrics" }
 mozangle = { workspace = true }
 net = { path = "../net" }
 net_traits = { workspace = true }