log GRPC_POLL_STRATEGY when fail to find polling engine

also log reason to skip a polling engine

src/core/lib/iomgr/ev_epoll1_linux.cc

@@ -1230,6 +1230,7 @@ static const grpc_event_engine_vtable vtable = {
  * support is available */
 const grpc_event_engine_vtable *grpc_init_epoll1_linux(bool explicit_request) {
   if (!grpc_has_wakeup_fd()) {
+    gpr_log(GPR_ERROR, "Skipping epoll1 because of no wakeup fd.");
     return NULL;
   }
 
@@ -1254,6 +1255,8 @@ const grpc_event_engine_vtable *grpc_init_epoll1_linux(bool explicit_request) {
 /* If GRPC_LINUX_EPOLL is not defined, it means epoll is not available. Return
  * NULL */
 const grpc_event_engine_vtable *grpc_init_epoll1_linux(bool explicit_request) {
+  gpr_log(GPR_ERROR,
+          "Skipping epollsig becuase GRPC_LINUX_EPOLL is not defined.");
   return NULL;
 }
 #endif /* defined(GRPC_POSIX_SOCKET) */

src/core/lib/iomgr/ev_epollex_linux.cc

@@ -1447,10 +1447,12 @@ const grpc_event_engine_vtable *grpc_init_epollex_linux(
   }
 
   if (!grpc_has_wakeup_fd()) {
+    gpr_log(GPR_ERROR, "Skipping epollex because of no wakeup fd.");
     return NULL;
   }
 
   if (!grpc_is_epollexclusive_available()) {
+    gpr_log(GPR_INFO, "Skipping epollex because it is not supported.");
     return NULL;
   }
 
@@ -1476,6 +1478,8 @@ const grpc_event_engine_vtable *grpc_init_epollex_linux(
  * NULL */
 const grpc_event_engine_vtable *grpc_init_epollex_linux(
     bool explicitly_requested) {
+  gpr_log(GPR_ERROR,
+          "Skipping epollex becuase GRPC_LINUX_EPOLL is not defined.");
   return NULL;
 }
 #endif /* defined(GRPC_POSIX_SOCKET) */

src/core/lib/iomgr/ev_epollsig_linux.cc

@@ -1708,16 +1708,20 @@ static bool is_epoll_available() {
 
 const grpc_event_engine_vtable *grpc_init_epollsig_linux(
     bool explicit_request) {
+  const char *error_msg = NULL;
   /* If use of signals is disabled, we cannot use epoll engine*/
   if (is_grpc_wakeup_signal_initialized && grpc_wakeup_signal < 0) {
+    gpr_log(GPR_ERROR, "Skipping epollsig because use of signals is disabled.");
     return NULL;
   }
 
   if (!grpc_has_wakeup_fd()) {
+    gpr_log(GPR_ERROR, "Skipping epollsig because of no wakeup fd.");
     return NULL;
   }
 
   if (!is_epoll_available()) {
+    gpr_log(GPR_ERROR, "Skipping epollsig because epoll is unavailable.");
     return NULL;
   }
 
@@ -1725,6 +1729,8 @@ const grpc_event_engine_vtable *grpc_init_epollsig_linux(
     if (explicit_request) {
       grpc_use_signal(SIGRTMIN + 6);
     } else {
+      gpr_log(GPR_ERROR,
+              "Skipping epollsig because uninitialized wakeup signal.");
       return NULL;
     }
   }
@@ -1750,6 +1756,8 @@ const grpc_event_engine_vtable *grpc_init_epollsig_linux(
  * NULL */
 const grpc_event_engine_vtable *grpc_init_epollsig_linux(
     bool explicit_request) {
+  gpr_log(GPR_ERROR,
+          "Skipping epollsig becuase GRPC_LINUX_EPOLL is not defined.");
   return NULL;
 }
 #endif /* defined(GRPC_POSIX_SOCKET) */

src/core/lib/iomgr/ev_poll_posix.cc

@@ -1713,6 +1713,7 @@ static const grpc_event_engine_vtable vtable = {
 
 const grpc_event_engine_vtable *grpc_init_poll_posix(bool explicit_request) {
   if (!grpc_has_wakeup_fd()) {
+    gpr_log(GPR_ERROR, "Skipping poll because of no wakeup fd.");
     return NULL;
   }
   if (!GRPC_LOG_IF_ERROR("pollset_global_init", pollset_global_init())) {

src/core/lib/iomgr/ev_posix.cc

@@ -172,12 +172,12 @@ void grpc_event_engine_init(void) {
     gpr_free(strings[i]);
   }
   gpr_free(strings);
-  gpr_free(s);
 
   if (g_event_engine == NULL) {
-    gpr_log(GPR_ERROR, "No event engine could be initialized");
+    gpr_log(GPR_ERROR, "No event engine could be initialized from %s", s);
     abort();
   }
+  gpr_free(s);
 }
 
 void grpc_event_engine_shutdown(void) {