MDEV-24917 Page cleaner wrongly remains idle

commit a9933105 (MDEV-24537)
introduced the regression that the page cleaner will keep sleeping
even if there is work to do.

innodb_max_dirty_pages_pct_update(): Always wake up the page cleaner
on any SET GLOBAL innodb_max_dirty_pages_pct= assignment.

buf_flush_page_cleaner(): If innodb_max_dirty_pages_pct is nonzero,
consult only that parameter when determining whether there is work
to do. Else, consult innodb_max_dirty_pages.

mysql-test/suite/innodb/r/page_cleaner.result

+SET @save_pct= @@GLOBAL.innodb_max_dirty_pages_pct;
+SET @save_pct_lwm= @@GLOBAL.innodb_max_dirty_pages_pct_lwm;
+SET GLOBAL innodb_max_dirty_pages_pct_lwm=0.0;
+SET GLOBAL innodb_max_dirty_pages_pct=0.0;
+SET GLOBAL innodb_max_dirty_pages_pct=90.0;
+CREATE TABLE t ENGINE=InnoDB SELECT * FROM seq_1_to_10000;
+SELECT variable_value>0 FROM information_schema.global_status
+WHERE variable_name = 'INNODB_BUFFER_POOL_PAGES_DIRTY';
+variable_value>0
+1
+SET GLOBAL innodb_max_dirty_pages_pct=0.0;
+DROP TABLE t;
+SET GLOBAL innodb_max_dirty_pages_pct = @save_pct;
+SET GLOBAL innodb_max_dirty_pages_pct_lwm = @save_pct_lwm;

mysql-test/suite/innodb/t/page_cleaner.test

+--source include/have_innodb.inc
+--source include/have_sequence.inc
+
+SET @save_pct= @@GLOBAL.innodb_max_dirty_pages_pct;
+SET @save_pct_lwm= @@GLOBAL.innodb_max_dirty_pages_pct_lwm;
+
+SET GLOBAL innodb_max_dirty_pages_pct_lwm=0.0;
+SET GLOBAL innodb_max_dirty_pages_pct=0.0;
+
+let $wait_condition =
+SELECT variable_value = 0
+FROM information_schema.global_status
+WHERE variable_name = 'INNODB_BUFFER_POOL_PAGES_DIRTY';
+--source include/wait_condition.inc
+
+SET GLOBAL innodb_max_dirty_pages_pct=90.0;
+
+CREATE TABLE t ENGINE=InnoDB SELECT * FROM seq_1_to_10000;
+
+SELECT variable_value>0 FROM information_schema.global_status
+WHERE variable_name = 'INNODB_BUFFER_POOL_PAGES_DIRTY';
+
+SET GLOBAL innodb_max_dirty_pages_pct=0.0;
+
+# Without the MDEV-24917 fix, we would time out here.
+--source include/wait_condition.inc
+DROP TABLE t;
+
+SET GLOBAL innodb_max_dirty_pages_pct = @save_pct;
+SET GLOBAL innodb_max_dirty_pages_pct_lwm = @save_pct_lwm;

storage/innobase/buf/buf0flu.cc

@@ -2136,11 +2136,13 @@ static os_thread_ret_t DECLARE_THREAD(buf_flush_page_cleaner)(void*)
       double(UT_LIST_GET_LEN(buf_pool.LRU) + UT_LIST_GET_LEN(buf_pool.free));
 
     if (lsn_limit);
+    else if (srv_max_dirty_pages_pct_lwm != 0.0)
+    {
+      if (dirty_pct < srv_max_dirty_pages_pct_lwm)
+        goto unemployed;
+    }
     else if (dirty_pct < srv_max_buf_pool_modified_pct)
       goto unemployed;
-    else if (srv_max_dirty_pages_pct_lwm == 0.0 ||
-             dirty_pct < srv_max_dirty_pages_pct_lwm)
-      goto unemployed;
 
     const lsn_t oldest_lsn= buf_pool.get_oldest_modified()
       ->oldest_modification();

storage/innobase/handler/ha_innodb.cc

@@ -17206,10 +17206,10 @@ innodb_max_dirty_pages_pct_update(
 				    in_val);
 
 		srv_max_dirty_pages_pct_lwm = in_val;
-		pthread_cond_signal(&buf_pool.do_flush_list);
 	}
 
 	srv_max_buf_pool_modified_pct = in_val;
+	pthread_cond_signal(&buf_pool.do_flush_list);
 }
 
 /****************************************************************//**