MDEV-26819 SET GLOBAL innodb_max_dirty_pages_pct=0 occasionally fails to trigger writes

innodb_max_dirty_pages_pct_update(),
innodb_max_dirty_pages_pct_lwm_update():
Invoke buf_pool.page_cleaner_wakeup() in order to wake up
buf_flush_page_cleaner. This allows the test innodb.page_cleaner
to run without any occasional timeouts.

The occasional hangs were introduced by
commit 7b1252c0 (MDEV-24278).

storage/innobase/buf/buf0flu.cc

@@ -123,7 +123,7 @@ static void buf_flush_validate_skip()
 #endif /* UNIV_DEBUG */
 
 /** Wake up the page cleaner if needed */
-inline void buf_pool_t::page_cleaner_wakeup()
+void buf_pool_t::page_cleaner_wakeup()
 {
   if (!page_cleaner_idle())
     return;

storage/innobase/handler/ha_innodb.cc

@@ -17214,7 +17214,12 @@ innodb_max_dirty_pages_pct_update(
 	}
 
 	srv_max_buf_pool_modified_pct = in_val;
-	pthread_cond_signal(&buf_pool.do_flush_list);
+
+	mysql_mutex_unlock(&LOCK_global_system_variables);
+	mysql_mutex_lock(&buf_pool.flush_list_mutex);
+	buf_pool.page_cleaner_wakeup();
+	mysql_mutex_unlock(&buf_pool.flush_list_mutex);
+	mysql_mutex_lock(&LOCK_global_system_variables);
 }
 
 /****************************************************************//**
@@ -17245,7 +17250,12 @@ innodb_max_dirty_pages_pct_lwm_update(
 	}
 
 	srv_max_dirty_pages_pct_lwm = in_val;
-	pthread_cond_signal(&buf_pool.do_flush_list);
+
+	mysql_mutex_unlock(&LOCK_global_system_variables);
+	mysql_mutex_lock(&buf_pool.flush_list_mutex);
+	buf_pool.page_cleaner_wakeup();
+	mysql_mutex_unlock(&buf_pool.flush_list_mutex);
+	mysql_mutex_lock(&LOCK_global_system_variables);
 }
 
 /*************************************************************//**

storage/innobase/include/buf0buf.h

@@ -1935,7 +1935,7 @@ class buf_pool_t
     return page_cleaner_is_idle;
   }
   /** Wake up the page cleaner if needed */
-  inline void page_cleaner_wakeup();
+  void page_cleaner_wakeup();
 
   /** Register whether an explicit wakeup of the page cleaner is needed */
   void page_cleaner_set_idle(bool deep_sleep)