sched: Get rid of (pre)select shortcuts.
authorAndre Noll <maan@systemlinux.org>
Sat, 30 Mar 2013 22:43:38 +0000 (22:43 +0000)
committerAndre Noll <maan@systemlinux.org>
Wed, 17 Apr 2013 20:15:46 +0000 (22:15 +0200)
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.

sched.c

diff --git a/sched.c b/sched.c
index 7ee7702..0585162 100644 (file)
--- 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;
 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) {
        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);
                        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);
        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);
        }
                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;
        sched_post_select(s);
        if (list_empty(&s->pre_select_list) && list_empty(&s->post_select_list))
                return 0;