MDEV-7952 - clock_gettime() takes 0.24% in OLTP RO

Initialize abs_timeout when it is about to be used. This saves one my_hrtime()
call on hot path (when we acquire MDL lock without waiting).

When filling I_S.PROCESSLIST use THD::start_utime/THD::utime_after_query instead
of THD::start_time. This allows us to save 2 clock_gettime() calls.

Overhead change:
__clock_gettime   0.13% -> 0.11% (122 -> 76 calls per OLTP RO transaction)
my_interval_timer 0.07% -> 0.06%
my_hrtime         0.04% -> 0.01%

sql/mdl.cc

@@ -1972,14 +1972,10 @@ MDL_context::acquire_lock(MDL_request *mdl_request, double lock_wait_timeout)
 {
   MDL_lock *lock;
   MDL_ticket *ticket;
-  struct timespec abs_timeout;
   MDL_wait::enum_wait_status wait_status;
   DBUG_ENTER("MDL_context::acquire_lock");
   DBUG_PRINT("enter", ("lock_type: %d", mdl_request->type));
 
-  /* Do some work outside the critical section. */
-  set_timespec(abs_timeout, lock_wait_timeout);
-
   if (try_acquire_lock_impl(mdl_request, &ticket))
     DBUG_RETURN(TRUE);
 
@@ -2028,7 +2024,8 @@ MDL_context::acquire_lock(MDL_request *mdl_request, double lock_wait_timeout)
 
   find_deadlock();
 
-  struct timespec abs_shortwait;
+  struct timespec abs_timeout, abs_shortwait;
+  set_timespec(abs_timeout, lock_wait_timeout);
   set_timespec(abs_shortwait, 1);
   wait_status= MDL_wait::EMPTY;
 

sql/sql_class.cc

@@ -947,7 +947,7 @@ THD::THD(bool is_wsrep_applier)
   // Must be reset to handle error with THD's created for init of mysqld
   lex->current_select= 0;
   user_time.val= start_time= start_time_sec_part= 0;
-  start_utime= prior_thr_create_utime= 0L;
+  start_utime= utime_after_query= prior_thr_create_utime= 0L;
   utime_after_lock= 0L;
   progress.arena= 0;
   progress.report_to_client= 0;

sql/sql_parse.cc

@@ -1943,7 +1943,6 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
   thd->m_statement_psi= NULL;
   thd->m_digest= NULL;
 
-  thd->set_time();
   dec_thread_running();
   thd->packet.shrink(thd->variables.net_buffer_length);	// Reclaim some memory
   free_root(thd->mem_root,MYF(MY_KEEP_PREALLOC));

sql/sql_show.cc

@@ -2301,7 +2301,7 @@ public:
   { TRASH(ptr, size); }
 
   ulong thread_id;
-  time_t start_time;
+  ulonglong start_time;
   uint   command;
   const char *user,*host,*db,*proc_info,*state_info;
   CSET_STRING query_string;
@@ -2432,7 +2432,10 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
       }
       else
         thd_info->progress= 0.0;
-      thd_info->start_time= tmp->start_time;
+      thd_info->start_time= tmp->start_utime;
+      ulonglong utime_after_query_snapshot= tmp->utime_after_query;
+      if (thd_info->start_time < utime_after_query_snapshot)
+        thd_info->start_time= utime_after_query_snapshot; // COM_SLEEP
       mysql_mutex_unlock(&tmp->LOCK_thd_data);
       thread_infos.append(thd_info);
     }
@@ -2440,7 +2443,7 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
   mysql_mutex_unlock(&LOCK_thread_count);
 
   thread_info *thd_info;
-  time_t now= my_time(0);
+  ulonglong now= microsecond_interval_timer();
   char buff[20];                                // For progress
   String store_buffer(buff, sizeof(buff), system_charset_info);
 
@@ -2455,8 +2458,8 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
       protocol->store(thd_info->proc_info, system_charset_info);
     else
       protocol->store(command_name[thd_info->command].str, system_charset_info);
-    if (thd_info->start_time)
-      protocol->store_long ((longlong) (now - thd_info->start_time));
+    if (thd_info->start_time && now > thd_info->start_time)
+      protocol->store_long(now - thd_info->start_time);
     else
       protocol->store_null();
     protocol->store(thd_info->state_info, system_charset_info);
@@ -2730,7 +2733,7 @@ int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
   TABLE *table= tables->table;
   CHARSET_INFO *cs= system_charset_info;
   char *user;
-  my_hrtime_t unow= my_hrtime();
+  ulonglong unow= microsecond_interval_timer();
   DBUG_ENTER("fill_schema_processlist");
 
   DEBUG_SYNC(thd,"fill_schema_processlist_after_unow");
@@ -2793,9 +2796,12 @@ int fill_schema_processlist(THD* thd, TABLE_LIST* tables, COND* cond)
         table->field[4]->store(command_name[tmp->get_command()].str,
                                command_name[tmp->get_command()].length, cs);
       /* MYSQL_TIME */
-      ulonglong start_utime= tmp->start_time * HRTIME_RESOLUTION + tmp->start_time_sec_part;
-      ulonglong utime= start_utime && start_utime < unow.val
-                       ? unow.val - start_utime : 0;
+      ulonglong utime= tmp->start_utime;
+      ulonglong utime_after_query_snapshot= tmp->utime_after_query;
+      if (utime < utime_after_query_snapshot)
+        utime= utime_after_query_snapshot; // COM_SLEEP
+      utime= utime && utime < unow ? unow - utime : 0;
+
       table->field[5]->store(utime / HRTIME_RESOLUTION, TRUE);
       /* STATE */
       if ((val= thread_state_info(tmp)))