Introduce multiple clocks to GPR

27 files changed, 84 insertions, 49 deletions

diff --git a/src/core/channel/census_filter.c b/src/core/channel/census_filter.c
index 7e393a01a6..0aa3cc3710 100644
--- a/src/core/channel/census_filter.c
+++ b/src/core/channel/census_filter.c
@@ -149,7 +149,7 @@ static void client_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) client_mutate_op(elem, initial_op);
 }
@@ -167,7 +167,7 @@ static void server_init_call_elem(grpc_call_element* elem,
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   init_rpc_stats(&d->stats);
-  d->start_ts = gpr_now();
+  d->start_ts = gpr_now(GPR_CLOCK_REALTIME);
   d->op_id = census_tracing_start_op();
   if (initial_op) server_mutate_op(elem, initial_op);
 }
@@ -176,7 +176,7 @@ static void server_destroy_call_elem(grpc_call_element* elem) {
   call_data* d = elem->call_data;
   GPR_ASSERT(d != NULL);
   d->stats.elapsed_time_ms =
-      gpr_timespec_to_micros(gpr_time_sub(gpr_now(), d->start_ts));
+      gpr_timespec_to_micros(gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), d->start_ts));
   census_record_rpc_server_stats(d->op_id, &d->stats);
   census_tracing_end_op(d->op_id);
 }
diff --git a/src/core/channel/client_setup.c b/src/core/channel/client_setup.c
index 5be8fa66e9..9844f4e43e 100644
--- a/src/core/channel/client_setup.c
+++ b/src/core/channel/client_setup.c
@@ -94,7 +94,7 @@ static void setup_initiate(grpc_transport_setup *sp) {
   int in_alarm = 0;
 
   r->setup = s;
-  r->deadline = gpr_time_add(gpr_now(), gpr_time_from_seconds(60));
+  r->deadline = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(60));
 
   gpr_mu_lock(&s->mu);
   GPR_ASSERT(s->refs > 0);
@@ -221,7 +221,7 @@ void grpc_client_setup_create_and_attach(
 int grpc_client_setup_request_should_continue(grpc_client_setup_request *r,
                                               const char *reason) {
   int result;
-  if (gpr_time_cmp(gpr_now(), r->deadline) > 0) {
+  if (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), r->deadline) > 0) {
     result = 0;
   } else {
     gpr_mu_lock(&r->setup->mu);
@@ -275,7 +275,7 @@ void grpc_client_setup_request_finish(grpc_client_setup_request *r,
     /* TODO(klempner): Replace these values with further consideration. 2x is
        probably too aggressive of a backoff. */
     gpr_timespec max_backoff = gpr_time_from_minutes(2);
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     gpr_timespec deadline = gpr_time_add(s->current_backoff_interval, now);
     GPR_ASSERT(!s->in_alarm);
     s->in_alarm = 1;
diff --git a/src/core/iomgr/iomgr.c b/src/core/iomgr/iomgr.c
index c47528aa94..39ba4f5264 100644
--- a/src/core/iomgr/iomgr.c
+++ b/src/core/iomgr/iomgr.c
@@ -59,7 +59,7 @@ static void background_callback_executor(void *ignored) {
   while (!g_shutdown) {
     gpr_timespec deadline = gpr_inf_future;
     gpr_timespec short_deadline =
-        gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
     if (g_cbs_head) {
       grpc_iomgr_closure *closure = g_cbs_head;
       g_cbs_head = closure->next;
@@ -67,7 +67,7 @@ static void background_callback_executor(void *ignored) {
       gpr_mu_unlock(&g_mu);
       closure->cb(closure->cb_arg, closure->success);
       gpr_mu_lock(&g_mu);
-    } else if (grpc_alarm_check(&g_mu, gpr_now(), &deadline)) {
+    } else if (grpc_alarm_check(&g_mu, gpr_now(GPR_CLOCK_REALTIME), &deadline)) {
     } else {
       gpr_mu_unlock(&g_mu);
       gpr_sleep_until(gpr_time_min(short_deadline, deadline));
@@ -89,7 +89,7 @@ void grpc_iomgr_init(void) {
   gpr_thd_id id;
   gpr_mu_init(&g_mu);
   gpr_cv_init(&g_rcv);
-  grpc_alarm_list_init(gpr_now());
+  grpc_alarm_list_init(gpr_now(GPR_CLOCK_REALTIME));
   g_root_object.next = g_root_object.prev = &g_root_object;
   g_root_object.name = "root";
   grpc_iomgr_platform_init();
@@ -110,7 +110,7 @@ void grpc_iomgr_shutdown(void) {
   grpc_iomgr_object *obj;
   grpc_iomgr_closure *closure;
   gpr_timespec shutdown_deadline =
-      gpr_time_add(gpr_now(), gpr_time_from_seconds(10));
+      gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_seconds(10));
 
   gpr_mu_lock(&g_mu);
   g_shutdown = 1;
@@ -145,9 +145,9 @@ void grpc_iomgr_shutdown(void) {
     if (g_root_object.next != &g_root_object) {
       int timeout = 0;
       gpr_timespec short_deadline =
-          gpr_time_add(gpr_now(), gpr_time_from_millis(100));
+          gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100));
       while (gpr_cv_wait(&g_rcv, &g_mu, short_deadline) && g_cbs_head == NULL) {
-        if (gpr_time_cmp(gpr_now(), shutdown_deadline) > 0) {
+        if (gpr_time_cmp(gpr_now(GPR_CLOCK_REALTIME), shutdown_deadline) > 0) {
           timeout = 1;
           break;
         }
diff --git a/src/core/iomgr/pollset_posix.c b/src/core/iomgr/pollset_posix.c
index 15ed8e75e6..852f2f4740 100644
--- a/src/core/iomgr/pollset_posix.c
+++ b/src/core/iomgr/pollset_posix.c
@@ -122,7 +122,7 @@ static void finish_shutdown(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   /* pollset->mu already held */
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0;
   }
diff --git a/src/core/iomgr/pollset_windows.c b/src/core/iomgr/pollset_windows.c
index 8d6bc79c96..c0573030e0 100644
--- a/src/core/iomgr/pollset_windows.c
+++ b/src/core/iomgr/pollset_windows.c
@@ -70,7 +70,7 @@ void grpc_pollset_destroy(grpc_pollset *pollset) {
 
 int grpc_pollset_work(grpc_pollset *pollset, gpr_timespec deadline) {
   gpr_timespec now;
-  now = gpr_now();
+  now = gpr_now(GPR_CLOCK_REALTIME);
   if (gpr_time_cmp(now, deadline) > 0) {
     return 0 /* GPR_FALSE */;
   }
diff --git a/src/core/iomgr/tcp_client_posix.c b/src/core/iomgr/tcp_client_posix.c
index bbf7711588..e0b0fd6e53 100644
--- a/src/core/iomgr/tcp_client_posix.c
+++ b/src/core/iomgr/tcp_client_posix.c
@@ -246,7 +246,7 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *ep),
   ac->write_closure.cb_arg = ac;
 
   gpr_mu_lock(&ac->mu);
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now(GPR_CLOCK_REALTIME));
   grpc_fd_notify_on_write(ac->fd, &ac->write_closure);
   gpr_mu_unlock(&ac->mu);
 
diff --git a/src/core/iomgr/tcp_client_windows.c b/src/core/iomgr/tcp_client_windows.c
index b1a169b519..2ae08f2435 100644
--- a/src/core/iomgr/tcp_client_windows.c
+++ b/src/core/iomgr/tcp_client_windows.c
@@ -215,7 +215,7 @@ void grpc_tcp_client_connect(void (*cb)(void *arg, grpc_endpoint *tcp),
   ac->refs = 2;
   ac->aborted = 0;
 
-  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now());
+  grpc_alarm_init(&ac->alarm, deadline, on_alarm, ac, gpr_now(GPR_CLOCK_REALTIME));
   socket->write_info.outstanding = 1;
   grpc_socket_notify_on_write(socket, on_connect, ac);
   return;
diff --git a/src/core/profiling/timers_preciseclock.h b/src/core/profiling/timers_preciseclock.h
index 163d52b797..5c251b47e6 100644
--- a/src/core/profiling/timers_preciseclock.h
+++ b/src/core/profiling/timers_preciseclock.h
@@ -82,7 +82,7 @@ struct grpc_precise_clock {
   gpr_timespec clock;
 };
 static void grpc_precise_clock_now(grpc_precise_clock* clk) {
-  clk->clock = gpr_now();
+  clk->clock = gpr_now(GPR_CLOCK_REALTIME);
 }
 #define GRPC_PRECISE_CLOCK_FORMAT "%ld.%09d"
 #define GRPC_PRECISE_CLOCK_PRINTF_ARGS(clk) \
diff --git a/src/core/security/credentials.c b/src/core/security/credentials.c
index cf663faf2d..4481554d74 100644
--- a/src/core/security/credentials.c
+++ b/src/core/security/credentials.c
@@ -384,7 +384,7 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
     if (c->cached.service_url != NULL &&
         strcmp(c->cached.service_url, service_url) == 0 &&
         c->cached.jwt_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration, gpr_now()),
+        (gpr_time_cmp(gpr_time_sub(c->cached.jwt_expiration, gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
       jwt_md = grpc_credentials_md_store_ref(c->cached.jwt_md);
     }
@@ -401,7 +401,7 @@ static void jwt_get_request_metadata(grpc_credentials *creds,
       char *md_value;
       gpr_asprintf(&md_value, "Bearer %s", jwt);
       gpr_free(jwt);
-      c->cached.jwt_expiration = gpr_time_add(gpr_now(), c->jwt_lifetime);
+      c->cached.jwt_expiration = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), c->jwt_lifetime);
       c->cached.service_url = gpr_strdup(service_url);
       c->cached.jwt_md = grpc_credentials_md_store_create(1);
       grpc_credentials_md_store_add_cstrings(
@@ -586,7 +586,7 @@ static void on_oauth2_token_fetcher_http_response(
   status = grpc_oauth2_token_fetcher_credentials_parse_server_response(
       response, &c->access_token_md, &token_lifetime);
   if (status == GRPC_CREDENTIALS_OK) {
-    c->token_expiration = gpr_time_add(gpr_now(), token_lifetime);
+    c->token_expiration = gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), token_lifetime);
     r->cb(r->user_data, c->access_token_md->entries,
           c->access_token_md->num_entries, status);
   } else {
@@ -608,7 +608,7 @@ static void oauth2_token_fetcher_get_request_metadata(
   {
     gpr_mu_lock(&c->mu);
     if (c->access_token_md != NULL &&
-        (gpr_time_cmp(gpr_time_sub(c->token_expiration, gpr_now()),
+        (gpr_time_cmp(gpr_time_sub(c->token_expiration, gpr_now(GPR_CLOCK_REALTIME)),
                       refresh_threshold) > 0)) {
       cached_access_token_md =
           grpc_credentials_md_store_ref(c->access_token_md);
@@ -623,7 +623,7 @@ static void oauth2_token_fetcher_get_request_metadata(
     c->fetch_func(
         grpc_credentials_metadata_request_create(creds, cb, user_data),
         &c->httpcli_context, pollset, on_oauth2_token_fetcher_http_response,
-        gpr_time_add(gpr_now(), refresh_threshold));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), refresh_threshold));
   }
 }
 
diff --git a/src/core/security/google_default_credentials.c b/src/core/security/google_default_credentials.c
index 5822ce6337..93e609456e 100644
--- a/src/core/security/google_default_credentials.c
+++ b/src/core/security/google_default_credentials.c
@@ -105,7 +105,7 @@ static int is_stack_running_on_compute_engine(void) {
   grpc_httpcli_context_init(&context);
 
   grpc_httpcli_get(&context, &detector.pollset, &request,
-                   gpr_time_add(gpr_now(), max_detection_delay),
+                   gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), max_detection_delay),
                    on_compute_engine_detection_http_response, &detector);
 
   /* Block until we get the response. This is not ideal but this should only be
diff --git a/src/core/security/json_token.c b/src/core/security/json_token.c
index 6116f1d767..3c411d76cf 100644
--- a/src/core/security/json_token.c
+++ b/src/core/security/json_token.c
@@ -207,7 +207,7 @@ static char *encoded_jwt_claim(const grpc_auth_json_key *json_key,
   grpc_json *child = NULL;
   char *json_str = NULL;
   char *result = NULL;
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   gpr_timespec expiration = gpr_time_add(now, token_lifetime);
   char now_str[GPR_LTOA_MIN_BUFSIZE];
   char expiration_str[GPR_LTOA_MIN_BUFSIZE];
diff --git a/src/core/statistics/census_rpc_stats.c b/src/core/statistics/census_rpc_stats.c
index 0491c91947..3e571b1143 100644
--- a/src/core/statistics/census_rpc_stats.c
+++ b/src/core/statistics/census_rpc_stats.c
@@ -157,7 +157,7 @@ static void record_stats(census_ht* store, census_op_id op_id,
         key.ptr = gpr_strdup(key.ptr);
         census_ht_insert(store, key, (void*)window_stats);
       }
-      census_window_stats_add(window_stats, gpr_now(), stats);
+      census_window_stats_add(window_stats, gpr_now(GPR_CLOCK_REALTIME), stats);
     } else {
       census_internal_unlock_trace_store();
     }
@@ -185,7 +185,7 @@ static void get_stats(census_ht* store, census_aggregated_rpc_stats* data) {
   if (store != NULL) {
     size_t n;
     unsigned i, j;
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_kv* kv = census_ht_get_all_elements(store, &n);
     if (kv != NULL) {
       data->num_entries = n;
diff --git a/src/core/statistics/census_tracing.c b/src/core/statistics/census_tracing.c
index 05e72b99c0..3b31a02cb3 100644
--- a/src/core/statistics/census_tracing.c
+++ b/src/core/statistics/census_tracing.c
@@ -94,7 +94,7 @@ census_op_id census_tracing_start_op(void) {
     g_id++;
     memcpy(&ret->id, &g_id, sizeof(census_op_id));
     ret->rpc_stats.cnt = 1;
-    ret->ts = gpr_now();
+    ret->ts = gpr_now(GPR_CLOCK_REALTIME);
     census_ht_insert(g_trace_store, op_id_as_key(&ret->id), (void*)ret);
     gpr_log(GPR_DEBUG, "Start tracing for id %lu", g_id);
     gpr_mu_unlock(&g_mu);
@@ -122,7 +122,7 @@ void census_tracing_print(census_op_id op_id, const char* anno_txt) {
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
     census_trace_annotation* anno = gpr_malloc(sizeof(census_trace_annotation));
-    anno->ts = gpr_now();
+    anno->ts = gpr_now(GPR_CLOCK_REALTIME);
     {
       char* d = anno->txt;
       const char* s = anno_txt;
@@ -144,7 +144,7 @@ void census_tracing_end_op(census_op_id op_id) {
   trace = census_ht_find(g_trace_store, op_id_as_key(&op_id));
   if (trace != NULL) {
     trace->rpc_stats.elapsed_time_ms =
-        gpr_timespec_to_micros(gpr_time_sub(gpr_now(), trace->ts));
+        gpr_timespec_to_micros(gpr_time_sub(gpr_now(GPR_CLOCK_REALTIME), trace->ts));
     gpr_log(GPR_DEBUG, "End tracing for id %lu, method %s, latency %f us",
             op_id_2_uint64(&op_id), trace->method,
             trace->rpc_stats.elapsed_time_ms);
diff --git a/src/core/statistics/window_stats.h b/src/core/statistics/window_stats.h
index d733d8d247..9651198c71 100644
--- a/src/core/statistics/window_stats.h
+++ b/src/core/statistics/window_stats.h
@@ -90,11 +90,11 @@
     // Record a new event, taking 15.3ms, transferring 1784 bytes.
     stat.latency = 0.153;
     stat.bytes = 1784;
-    census_window_stats_add(stats, gpr_now(), &stat);
+    census_window_stats_add(stats, gpr_now(GPR_CLOCK_REALTIME), &stat);
     // Get sums and print them out
     result[kMinInterval].statistic = &sums[kMinInterval];
     result[kHourInterval].statistic = &sums[kHourInterval];
-    census_window_stats_get_sums(stats, gpr_now(), result);
+    census_window_stats_get_sums(stats, gpr_now(GPR_CLOCK_REALTIME), result);
     printf("%d events/min, average time %gs, average bytes %g\n",
            result[kMinInterval].count,
            (my_stat*)result[kMinInterval].statistic->latency /
diff --git a/src/core/support/cancellable.c b/src/core/support/cancellable.c
index 5a4d488dd3..4f8f237f40 100644
--- a/src/core/support/cancellable.c
+++ b/src/core/support/cancellable.c
@@ -122,7 +122,7 @@ void gpr_cancellable_cancel(gpr_cancellable *c) {
           gpr_event ev;
           gpr_event_init(&ev);
           gpr_event_wait(&ev,
-                         gpr_time_add(gpr_now(), gpr_time_from_micros(1000)));
+                         gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_micros(1000)));
         }
       }
     } while (failures != 0);
diff --git a/src/core/support/log_linux.c b/src/core/support/log_linux.c
index 48349d2c83..26f1feb5b3 100644
--- a/src/core/support/log_linux.c
+++ b/src/core/support/log_linux.c
@@ -73,7 +73,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *final_slash;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_posix.c b/src/core/support/log_posix.c
index afca792c40..940ee20f15 100644
--- a/src/core/support/log_posix.c
+++ b/src/core/support/log_posix.c
@@ -75,7 +75,7 @@ void gpr_default_log(gpr_log_func_args *args) {
   char *final_slash;
   const char *display_file;
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   final_slash = strrchr(args->file, '/');
diff --git a/src/core/support/log_win32.c b/src/core/support/log_win32.c
index d249be7d2e..629781da8a 100644
--- a/src/core/support/log_win32.c
+++ b/src/core/support/log_win32.c
@@ -82,7 +82,7 @@ void gpr_log(const char *file, int line, gpr_log_severity severity,
 /* Simple starter implementation */
 void gpr_default_log(gpr_log_func_args *args) {
   char time_buffer[64];
-  gpr_timespec now = gpr_now();
+  gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
   struct tm tm;
 
   if (localtime_s(&tm, &now.tv_sec)) {
diff --git a/src/core/support/sync_win32.c b/src/core/support/sync_win32.c
index cc31d9b052..22387103fe 100644
--- a/src/core/support/sync_win32.c
+++ b/src/core/support/sync_win32.c
@@ -89,7 +89,7 @@ int gpr_cv_wait(gpr_cv *cv, gpr_mu *mu, gpr_timespec abs_deadline) {
   if (gpr_time_cmp(abs_deadline, gpr_inf_future) == 0) {
     SleepConditionVariableCS(cv, &mu->cs, INFINITE);
   } else {
-    gpr_timespec now = gpr_now();
+    gpr_timespec now = gpr_now(GPR_CLOCK_REALTIME);
     gpr_int64 now_ms = now.tv_sec * 1000 + now.tv_nsec / 1000000;
     gpr_int64 deadline_ms =
         abs_deadline.tv_sec * 1000 + abs_deadline.tv_nsec / 1000000;
diff --git a/src/core/support/time_posix.c b/src/core/support/time_posix.c
index afb58ef231..d5797e0db2 100644
--- a/src/core/support/time_posix.c
+++ b/src/core/support/time_posix.c
@@ -55,22 +55,55 @@ static gpr_timespec gpr_from_timespec(struct timespec ts) {
   return rv;
 }
 
-gpr_timespec gpr_now(void) {
+/** maps gpr_clock_type --> clockid_t for clock_gettime */
+static clockid_t clockid_for_gpr_clock[] = {
+  CLOCK_MONOTONIC,
+  CLOCK_REALTIME
+};
+
+void gpr_time_init(void) {}
+
+gpr_timespec gpr_now(gpr_clock_type clock) {
   struct timespec now;
-  clock_gettime(CLOCK_REALTIME, &now);
+  clock_gettime(clockid_for_gpr_clock[clock], &now);
   return gpr_from_timespec(now);
 }
 #else
 /* For some reason Apple's OSes haven't implemented clock_gettime. */
 
 #include <sys/time.h>
+#include <mach/mach.h>
+#include <mach/mach_time.h>
+
+static double g_time_scale;
+static uint64_t g_time_start;
+
+void gpr_time_init(void) {
+  mach_timebase_info_data_t tb = { 0, 1 };
+  mach_timebase_info(&tb);
+  g_time_scale = tb.numer;
+  g_time_scale /= tb.denom;
+  g_time_start = mach_absolute_time();
+}
 
-gpr_timespec gpr_now(void) {
+gpr_timespec gpr_now(gpr_clock_type clock) {
   gpr_timespec now;
   struct timeval now_tv;
-  gettimeofday(&now_tv, NULL);
-  now.tv_sec = now_tv.tv_sec;
-  now.tv_nsec = now_tv.tv_usec * 1000;
+  double now_dbl;
+
+  switch (clock) {
+    case GPR_CLOCK_REALTIME:
+      gettimeofday(&now_tv, NULL);
+      now.tv_sec = now_tv.tv_sec;
+      now.tv_nsec = now_tv.tv_usec * 1000;
+      break;
+    case GPR_CLOCK_MONOTONIC:
+      now_dbl = (mach_absolute_time() - g_time_start) * g_time_scale;
+      now.tv_sec = now_dbl * 1e-9;
+      now.tv_nsec = now_dbl - now.tv_sec * 1e9;
+      break;
+  }
+
   return now;
 }
 #endif
@@ -83,7 +116,7 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
diff --git a/src/core/support/time_win32.c b/src/core/support/time_win32.c
index f4443b5c2d..827abc741d 100644
--- a/src/core/support/time_win32.c
+++ b/src/core/support/time_win32.c
@@ -58,7 +58,7 @@ void gpr_sleep_until(gpr_timespec until) {
   for (;;) {
     /* We could simplify by using clock_nanosleep instead, but it might be
      * slightly less portable. */
-    now = gpr_now();
+    now = gpr_now(GPR_CLOCK_REALTIME);
     if (gpr_time_cmp(until, now) <= 0) {
       return;
     }
diff --git a/src/core/surface/call.c b/src/core/surface/call.c
index 181617fff8..82bd1b2de2 100644
--- a/src/core/surface/call.c
+++ b/src/core/surface/call.c
@@ -1190,7 +1190,7 @@ static void set_deadline_alarm(grpc_call *call, gpr_timespec deadline) {
   }
   GRPC_CALL_INTERNAL_REF(call, "alarm");
   call->have_alarm = 1;
-  grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now());
+  grpc_alarm_init(&call->alarm, deadline, call_alarm, call, gpr_now(GPR_CLOCK_REALTIME));
 }
 
 /* we offset status by a small amount when storing it into transport metadata
diff --git a/src/core/surface/completion_queue.c b/src/core/surface/completion_queue.c
index 030a8b4e6f..3c074da652 100644
--- a/src/core/surface/completion_queue.c
+++ b/src/core/surface/completion_queue.c
@@ -325,7 +325,7 @@ void grpc_cq_hack_spin_pollset(grpc_completion_queue *cc) {
   gpr_mu_lock(GRPC_POLLSET_MU(&cc->pollset));
   grpc_pollset_kick(&cc->pollset);
   grpc_pollset_work(&cc->pollset,
-                    gpr_time_add(gpr_now(), gpr_time_from_millis(100)));
+                    gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), gpr_time_from_millis(100)));
   gpr_mu_unlock(GRPC_POLLSET_MU(&cc->pollset));
 }
 
diff --git a/src/core/surface/init.c b/src/core/surface/init.c
index ca61a38a35..db896934a3 100644
--- a/src/core/surface/init.c
+++ b/src/core/surface/init.c
@@ -33,6 +33,7 @@
 
 #include <grpc/census.h>
 #include <grpc/grpc.h>
+#include <grpc/support/time.h>
 #include "src/core/channel/channel_stack.h"
 #include "src/core/debug/trace.h"
 #include "src/core/iomgr/iomgr.h"
@@ -56,6 +57,7 @@ void grpc_init(void) {
 
   gpr_mu_lock(&g_init_mu);
   if (++g_initializations == 1) {
+    gpr_time_init();
     grpc_register_tracer("channel", &grpc_trace_channel);
     grpc_register_tracer("surface", &grpc_surface_trace);
     grpc_register_tracer("http", &grpc_http_trace);
diff --git a/src/core/transport/chttp2/parsing.c b/src/core/transport/chttp2/parsing.c
index 4664a0895c..a11b7e6d36 100644
--- a/src/core/transport/chttp2/parsing.c
+++ b/src/core/transport/chttp2/parsing.c
@@ -599,7 +599,7 @@ static void on_header(void *tp, grpc_mdelem *md) {
     }
     grpc_chttp2_incoming_metadata_buffer_set_deadline(
         &stream_parsing->incoming_metadata,
-        gpr_time_add(gpr_now(), *cached_timeout));
+        gpr_time_add(gpr_now(GPR_CLOCK_REALTIME), *cached_timeout));
     grpc_mdelem_unref(md);
   } else {
     grpc_chttp2_incoming_metadata_buffer_add(&stream_parsing->incoming_metadata,
diff --git a/src/core/transport/chttp2/stream_encoder.c b/src/core/transport/chttp2/stream_encoder.c
index cf78ac50cc..db642961ec 100644
--- a/src/core/transport/chttp2/stream_encoder.c
+++ b/src/core/transport/chttp2/stream_encoder.c
@@ -437,7 +437,7 @@ static void deadline_enc(grpc_chttp2_hpack_compressor *c, gpr_timespec deadline,
                          framer_state *st) {
   char timeout_str[GRPC_CHTTP2_TIMEOUT_ENCODE_MIN_BUFSIZE];
   grpc_mdelem *mdelem;
-  grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now()), timeout_str);
+  grpc_chttp2_encode_timeout(gpr_time_sub(deadline, gpr_now(GPR_CLOCK_REALTIME)), timeout_str);
   mdelem = grpc_mdelem_from_metadata_strings(
       c->mdctx, grpc_mdstr_ref(c->timeout_key_str),
       grpc_mdstr_from_string(c->mdctx, timeout_str));
diff --git a/src/core/transport/metadata.c b/src/core/transport/metadata.c
index c80d67823f..4d0d83e7b7 100644
--- a/src/core/transport/metadata.c
+++ b/src/core/transport/metadata.c
@@ -161,7 +161,7 @@ grpc_mdctx *grpc_mdctx_create(void) {
   /* This seed is used to prevent remote connections from controlling hash table
    * collisions. It needs to be somewhat unpredictable to a remote connection.
    */
-  return grpc_mdctx_create_with_seed(gpr_now().tv_nsec);
+  return grpc_mdctx_create_with_seed(gpr_now(GPR_CLOCK_REALTIME).tv_nsec);
 }
 
 static void discard_metadata(grpc_mdctx *ctx) {