Merge pull request #13344 from muxi/fix-timer-thread-spin

Fix timer loop spinning issue when system clock varies

3 files changed, 70 insertions, 7 deletions

diff --git a/src/core/lib/iomgr/exec_ctx.cc b/src/core/lib/iomgr/exec_ctx.cc
index 0a0ed8a055..423e8d059f 100644
--- a/src/core/lib/iomgr/exec_ctx.cc
+++ b/src/core/lib/iomgr/exec_ctx.cc
@@ -25,6 +25,9 @@
 #include "src/core/lib/iomgr/combiner.h"
 #include "src/core/lib/profiling/timers.h"
 
+#define GRPC_START_TIME_UPDATE_INTERVAL 10000
+extern "C" grpc_tracer_flag grpc_timer_check_trace;
+
 bool grpc_exec_ctx_ready_to_finish(grpc_exec_ctx* exec_ctx) {
   if ((exec_ctx->flags & GRPC_EXEC_CTX_FLAG_IS_FINISHED) == 0) {
     if (exec_ctx->check_ready_to_finish(exec_ctx,
@@ -104,23 +107,49 @@ static void exec_ctx_sched(grpc_exec_ctx* exec_ctx, grpc_closure* closure,
   grpc_closure_list_append(&exec_ctx->closure_list, closure, error);
 }
 
-static gpr_timespec
+/* This time pair is not entirely thread-safe as store/load of tv_sec and
+ * tv_nsec are performed separately. However g_start_time do not need to have
+ * sub-second precision, so it is ok if the value of tv_nsec is off in this
+ * case. */
+typedef struct time_atm_pair {
+  gpr_atm tv_sec;
+  gpr_atm tv_nsec;
+} time_atm_pair;
+
+static time_atm_pair
     g_start_time[GPR_TIMESPAN + 1];  // assumes GPR_TIMESPAN is the
                                      // last enum value in
                                      // gpr_clock_type
+static grpc_millis g_last_start_time_update;
+
+static gpr_timespec timespec_from_time_atm_pair(const time_atm_pair* src,
+                                                gpr_clock_type clock_type) {
+  gpr_timespec time;
+  time.tv_nsec = (int32_t)gpr_atm_no_barrier_load(&src->tv_nsec);
+  time.tv_sec = (int64_t)gpr_atm_no_barrier_load(&src->tv_sec);
+  time.clock_type = clock_type;
+  return time;
+}
+
+static void time_atm_pair_store(time_atm_pair* dst, const gpr_timespec src) {
+  gpr_atm_no_barrier_store(&dst->tv_sec, src.tv_sec);
+  gpr_atm_no_barrier_store(&dst->tv_nsec, src.tv_nsec);
+}
 
 void grpc_exec_ctx_global_init(void) {
   for (int i = 0; i < GPR_TIMESPAN; i++) {
-    g_start_time[i] = gpr_now((gpr_clock_type)i);
+    time_atm_pair_store(&g_start_time[i], gpr_now((gpr_clock_type)i));
   }
   // allows uniform treatment in conversion functions
-  g_start_time[GPR_TIMESPAN] = gpr_time_0(GPR_TIMESPAN);
+  time_atm_pair_store(&g_start_time[GPR_TIMESPAN], gpr_time_0(GPR_TIMESPAN));
 }
 
 void grpc_exec_ctx_global_shutdown(void) {}
 
 static gpr_atm timespec_to_atm_round_down(gpr_timespec ts) {
-  ts = gpr_time_sub(ts, g_start_time[ts.clock_type]);
+  gpr_timespec start_time =
+      timespec_from_time_atm_pair(&g_start_time[ts.clock_type], ts.clock_type);
+  ts = gpr_time_sub(ts, start_time);
   double x =
       GPR_MS_PER_SEC * (double)ts.tv_sec + (double)ts.tv_nsec / GPR_NS_PER_MS;
   if (x < 0) return 0;
@@ -129,7 +158,9 @@ static gpr_atm timespec_to_atm_round_down(gpr_timespec ts) {
 }
 
 static gpr_atm timespec_to_atm_round_up(gpr_timespec ts) {
-  ts = gpr_time_sub(ts, g_start_time[ts.clock_type]);
+  gpr_timespec start_time =
+      timespec_from_time_atm_pair(&g_start_time[ts.clock_type], ts.clock_type);
+  ts = gpr_time_sub(ts, start_time);
   double x = GPR_MS_PER_SEC * (double)ts.tv_sec +
              (double)ts.tv_nsec / GPR_NS_PER_MS +
              (double)(GPR_NS_PER_SEC - 1) / (double)GPR_NS_PER_SEC;
@@ -164,8 +195,9 @@ gpr_timespec grpc_millis_to_timespec(grpc_millis millis,
   if (clock_type == GPR_TIMESPAN) {
     return gpr_time_from_millis(millis, GPR_TIMESPAN);
   }
-  return gpr_time_add(g_start_time[clock_type],
-                      gpr_time_from_millis(millis, GPR_TIMESPAN));
+  gpr_timespec start_time =
+      timespec_from_time_atm_pair(&g_start_time[clock_type], clock_type);
+  return gpr_time_add(start_time, gpr_time_from_millis(millis, GPR_TIMESPAN));
 }
 
 grpc_millis grpc_timespec_to_millis_round_down(gpr_timespec ts) {
@@ -176,6 +208,30 @@ grpc_millis grpc_timespec_to_millis_round_up(gpr_timespec ts) {
   return timespec_to_atm_round_up(ts);
 }
 
+void grpc_exec_ctx_maybe_update_start_time(grpc_exec_ctx* exec_ctx) {
+  grpc_millis now = grpc_exec_ctx_now(exec_ctx);
+  grpc_millis last_start_time_update =
+      gpr_atm_no_barrier_load(&g_last_start_time_update);
+
+  if (now > last_start_time_update &&
+      now - last_start_time_update > GRPC_START_TIME_UPDATE_INTERVAL) {
+    /* Get the current system time and subtract \a now from it, where \a now is
+     * the relative time from grpc_init() from monotonic clock. This calibrates
+     * the time when grpc_exec_ctx_global_init was called based on current
+     * system clock. */
+    gpr_atm_no_barrier_store(&g_last_start_time_update, now);
+    gpr_timespec real_now = gpr_now(GPR_CLOCK_REALTIME);
+    gpr_timespec real_start_time =
+        gpr_time_sub(real_now, gpr_time_from_millis(now, GPR_TIMESPAN));
+    time_atm_pair_store(&g_start_time[GPR_CLOCK_REALTIME], real_start_time);
+
+    if (GRPC_TRACER_ON(grpc_timer_check_trace)) {
+      gpr_log(GPR_DEBUG, "Update realtime clock start time: %" PRId64 "s %dns",
+              real_start_time.tv_sec, real_start_time.tv_nsec);
+    }
+  }
+}
+
 static const grpc_closure_scheduler_vtable exec_ctx_scheduler_vtable = {
     exec_ctx_run, exec_ctx_sched, "exec_ctx"};
 static grpc_closure_scheduler exec_ctx_scheduler = {&exec_ctx_scheduler_vtable};
diff --git a/src/core/lib/iomgr/exec_ctx.h b/src/core/lib/iomgr/exec_ctx.h
index bd27506152..6035e08361 100644
--- a/src/core/lib/iomgr/exec_ctx.h
+++ b/src/core/lib/iomgr/exec_ctx.h
@@ -124,6 +124,8 @@ gpr_timespec grpc_millis_to_timespec(grpc_millis millis, gpr_clock_type clock);
 grpc_millis grpc_timespec_to_millis_round_down(gpr_timespec timespec);
 grpc_millis grpc_timespec_to_millis_round_up(gpr_timespec timespec);
 
+void grpc_exec_ctx_maybe_update_start_time(grpc_exec_ctx* exec_ctx);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/src/core/lib/iomgr/timer_manager.cc b/src/core/lib/iomgr/timer_manager.cc
index 16e9e7e707..bb2a0e0d44 100644
--- a/src/core/lib/iomgr/timer_manager.cc
+++ b/src/core/lib/iomgr/timer_manager.cc
@@ -225,6 +225,11 @@ static void timer_main_loop(grpc_exec_ctx* exec_ctx) {
   for (;;) {
     grpc_millis next = GRPC_MILLIS_INF_FUTURE;
     grpc_exec_ctx_invalidate_now(exec_ctx);
+
+    /* Calibrate g_start_time in exec_ctx.cc with a regular interval in case the
+     * system clock has changed */
+    grpc_exec_ctx_maybe_update_start_time(exec_ctx);
+
     // check timer state, updates next to the next time to run a check
     switch (grpc_timer_check(exec_ctx, &next)) {
       case GRPC_TIMERS_FIRED: