From: Andre Noll <maan@systemlinux.org>
Date: Sat, 30 Mar 2013 22:43:38 +0000 (+0000)
Subject: sched: Get rid of (pre)select shortcuts.
X-Git-Tag: v0.4.13~39^2~43
X-Git-Url: http://git.tuebingen.mpg.de/?p=paraslash.git;a=commitdiff_plain;h=01f8023b7fe59abc29072c7b84caec5961f8f14b;hp=5bb44a414084464f133beb8810027a18b4254d1f

sched: Get rid of (pre)select shortcuts.

These shortcuts let the scheduler skip any subsequent ->pre_select()
calls as well as the actual call to select(2) if the current task's
->pre_select() requested a zero timeout.

This idea turned out to be dubios at best. The main problem is that
->post_select no longer knows whether ->pre_select() and select()
have been called. For example, if ->pre_select() monitors some file
descriptor, ->post_select() might find it not ready for I/O even when
it actually is because ->pre_select() or select() was optimized away.

Another problem are tasks whose ->post_select() method expects that
->pre_select() sets some variable in a shared data structure.

This patch removes the scheduler optimizations so that all
->pre_select() methods of all tasks are called once per scheduler
interval.
---

diff --git a/sched.c b/sched.c
index 7ee77027..05851621 100644
--- a/sched.c
+++ b/sched.c
@@ -50,20 +50,12 @@ static inline bool timeout_is_zero(struct sched *s)
 static void sched_preselect(struct sched *s)
 {
 	struct task *t, *tmp;
+
 	list_for_each_entry_safe(t, tmp, &s->pre_select_list, pre_select_node) {
-		if (t->error < 0) {
-			unregister_task(t);
-			continue;
-		}
-		if (t->notification != 0) {
+		if (t->notification != 0)
 			sched_min_delay(s);
-			break;
-		}
-		if (!t->pre_select)
-			continue;
-		t->pre_select(s, t);
-		if (timeout_is_zero(s))
-			break;
+		if (t->pre_select)
+			t->pre_select(s, t);
 	}
 }
 
@@ -130,28 +122,21 @@ again:
 	s->max_fileno = -1;
 	gettimeofday(now, NULL);
 	sched_preselect(s);
-	if (list_empty(&s->pre_select_list) && list_empty(&s->post_select_list))
-		return 0;
-	if (!timeout_is_zero(s)) {
-		ret = s->select_function(s->max_fileno + 1, &s->rfds, &s->wfds,
-			&s->select_timeout);
-		if (ret < 0)
-			return ret;
-		if (ret == 0) {
-			/*
-			 * APUE: Be careful not to check the descriptor sets on return
-			 * unless the return value is greater than zero. The return
-			 * state of the descriptor sets is implementation dependent if
-			 * either a signal is caught or the timer expires.
-			 */
-			FD_ZERO(&s->rfds);
-			FD_ZERO(&s->wfds);
-		}
-		gettimeofday(now, NULL);
-	} else {
+	ret = s->select_function(s->max_fileno + 1, &s->rfds, &s->wfds,
+		&s->select_timeout);
+	if (ret < 0)
+		return ret;
+	if (ret == 0) {
+		/*
+		 * APUE: Be careful not to check the descriptor sets on return
+		 * unless the return value is greater than zero. The return
+		 * state of the descriptor sets is implementation dependent if
+		 * either a signal is caught or the timer expires.
+		 */
 		FD_ZERO(&s->rfds);
 		FD_ZERO(&s->wfds);
 	}
+	gettimeofday(now, NULL);
 	sched_post_select(s);
 	if (list_empty(&s->pre_select_list) && list_empty(&s->post_select_list))
 		return 0;