tracing: Avoid trace timestamp conversion to CLOCK_BOOTTIME on Posix.

Chrome traces use CLOCK_MONOTONIC as their trace clock. If the trace
only contains Chrome data, it doesn't make sense to convert its
timestamps to CLOCK_BOOTTIME - in fact, this conversion could lead to
cross-thread timestamp skew and misaligned system tracing events on
CrOS/chromecast.

This patch accomplishes this by configuring the correct clock
(CLOCK_MONOTONIC in this case) as trace clock for chrome-only traces,
thereby instructing trace processor not to convert timestamps.

Requires aosp/1315608.

Bug: 1085002,1060400
Change-Id: I5ff8660a6e27f28d27f68d72bb7cf312494431bc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2210427Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771003}

services/tracing/public/cpp/perfetto/perfetto_config.cc

@@ -13,6 +13,7 @@
 #include "base/trace_event/trace_event.h"
 #include "build/build_config.h"
 #include "build/chromecast_buildflags.h"
+#include "services/tracing/public/cpp/perfetto/trace_time.h"
 #include "services/tracing/public/mojom/perfetto_service.mojom.h"
 
 namespace tracing {
@@ -65,9 +66,18 @@ perfetto::TraceConfig GetDefaultPerfettoConfig(
       break;
   }
 
-  // Perfetto uses clock_gettime for its internal snapshotting, which gets
-  // blocked by the sandboxed and isn't needed for Chrome regardless.
   auto* builtin_data_sources = perfetto_config.mutable_builtin_data_sources();
+
+  // Chrome uses CLOCK_MONOTONIC as its trace clock on Posix. To avoid that
+  // trace processor converts Chrome's event timestamps into CLOCK_BOOTTIME
+  // during import, we set the trace clock here (the service will emit it into
+  // the trace's ClockSnapshots). See also crbug.com/1060400, where the
+  // conversion to BOOTTIME caused CrOS and chromecast system data source data
+  // to be misaligned.
+  builtin_data_sources->set_primary_trace_clock(
+      static_cast<perfetto::protos::gen::BuiltinClock>(kTraceClockId));
+
+  // Chrome emits system / trace config metadata itself.
   builtin_data_sources->set_disable_trace_config(privacy_filtering_enabled);
   builtin_data_sources->set_disable_system_info(privacy_filtering_enabled);
   builtin_data_sources->set_disable_service_events(privacy_filtering_enabled);

services/tracing/public/cpp/perfetto/trace_event_data_source_unittest.cc

@@ -305,8 +305,7 @@ class TraceEventDataSourceTest : public testing::Test {
     ASSERT_EQ(packet->clock_snapshot().clocks().size(), 3);
 
     EXPECT_EQ(packet->clock_snapshot().clocks()[0].clock_id(),
-              static_cast<uint32_t>(
-                  perfetto::protos::pbzero::ClockSnapshot::Clock::BOOTTIME));
+              static_cast<uint32_t>(kTraceClockId));
     EXPECT_FALSE(packet->clock_snapshot().clocks()[0].has_unit_multiplier_ns());
     EXPECT_FALSE(packet->clock_snapshot().clocks()[0].has_is_incremental());
 

services/tracing/public/cpp/perfetto/trace_time.h

@@ -6,21 +6,21 @@
 #define SERVICES_TRACING_PUBLIC_CPP_PERFETTO_TRACE_TIME_H_
 
 #include "build/build_config.h"
-#include "third_party/perfetto/protos/perfetto/trace/clock_snapshot.pbzero.h"
+#include "third_party/perfetto/protos/perfetto/common/builtin_clock.pbzero.h"
 
 namespace tracing {
 
 #if defined(OS_LINUX) || defined(OS_ANDROID) || defined(OS_FUCHSIA)
 // Linux, Android, and Fuchsia all use CLOCK_MONOTONIC. See crbug.com/166153
 // about efforts to unify base::TimeTicks across all platforms.
-constexpr perfetto::protos::pbzero::ClockSnapshot::Clock::BuiltinClocks
-    kTraceClockId = perfetto::protos::pbzero::ClockSnapshot::Clock::MONOTONIC;
+constexpr perfetto::protos::pbzero::BuiltinClock kTraceClockId =
+    perfetto::protos::pbzero::BUILTIN_CLOCK_MONOTONIC;
 #else
 // Mac and Windows TimeTicks advance when sleeping, so are closest to BOOTTIME
 // in behavior.
-// TODO(eseckler): Support specifying Mac/Win platform clocks in BuiltinClocks.
-constexpr perfetto::protos::pbzero::ClockSnapshot::Clock::BuiltinClocks
-    kTraceClockId = perfetto::protos::pbzero::ClockSnapshot::Clock::BOOTTIME;
+// TODO(eseckler): Support specifying Mac/Win platform clocks in BuiltinClock.
+constexpr perfetto::protos::pbzero::BuiltinClock kTraceClockId =
+    perfetto::protos::pbzero::BUILTIN_CLOCK_BOOTTIME;
 #endif
 
 // Returns CLOCK_BOOTTIME on systems that support it, otherwise falls back to

services/tracing/public/cpp/perfetto/track_event_thread_local_event_sink.cc

@@ -915,20 +915,10 @@ void TrackEventThreadLocalEventSink::DoResetIncrementalState(
     }
 
     ClockSnapshot* clocks = packet->set_clock_snapshot();
-    // Always reference the boottime timestamps to help trace processor
-    // translate the clocks to boottime more efficiently.
+    // Reference clock is in nanoseconds.
     ClockSnapshot::Clock* clock_reference = clocks->add_clocks();
-    clock_reference->set_clock_id(ClockSnapshot::Clock::BOOTTIME);
-    if (kTraceClockId == ClockSnapshot::Clock::BOOTTIME) {
+    clock_reference->set_clock_id(kTraceClockId);
     clock_reference->set_timestamp(timestamp.since_origin().InNanoseconds());
-    } else {
-      int64_t current_boot_nanos = TraceBootTicksNow();
-      int64_t current_monotonic_nanos =
-          TRACE_TIME_TICKS_NOW().since_origin().InNanoseconds();
-      int64_t diff = current_boot_nanos - current_monotonic_nanos;
-      clock_reference->set_timestamp(timestamp.since_origin().InNanoseconds() +
-                                     diff);
-    }
     // Absolute clock in micros.
     ClockSnapshot::Clock* clock_absolute = clocks->add_clocks();
     clock_absolute->set_clock_id(kClockIdAbsolute);