sched: Only setting CPUSchedulingPriority=rr doesn't work

A service that only sets the scheduling policy to round-robin
fails to be started. This is because the cpu_sched_priority is
initialized to 0 and is not adjusted when the policy is changed.

Clamp the cpu_sched_priority when the scheduler policy is set. Use
the current policy to validate the new priority.

Change the manual page to state that the given range only applies
to the real-time scheduling policies.

Add a testcase that verifies this change:

$ make test-sched-prio; ./test-sched-prio
[test/sched_idle_bad.service:6] CPU scheduling priority is out of range, ignoring: 1
[test/sched_rr_bad.service:7] CPU scheduling priority is out of range, ignoring: 0
[test/sched_rr_bad.service:8] CPU scheduling priority is out of range, ignoring: 100

1 files changed, 15 insertions, 3 deletions

diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index b99e70e05a..6759255984 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -4,6 +4,7 @@
   This file is part of systemd.
 
   Copyright 2010 Lennart Poettering
+  Copyright 2012 Holger Hans Peter Freyther
 
   systemd is free software; you can redistribute it and/or modify it
   under the terms of the GNU Lesser General Public License as published by
@@ -689,6 +690,8 @@ int config_parse_exec_cpu_sched_policy(
         }
 
         c->cpu_sched_policy = x;
+        /* Moving to or from real-time policy? We need to adjust the priority */
+        c->cpu_sched_priority = CLAMP(c->cpu_sched_priority, sched_get_priority_min(x), sched_get_priority_max(x));
         c->cpu_sched_set = true;
 
         return 0;
@@ -705,19 +708,28 @@ int config_parse_exec_cpu_sched_prio(
                 void *userdata) {
 
         ExecContext *c = data;
-        int i;
+        int i, min, max;
 
         assert(filename);
         assert(lvalue);
         assert(rvalue);
         assert(data);
 
-        /* On Linux RR/FIFO have the same range */
-        if (safe_atoi(rvalue, &i) < 0 || i < sched_get_priority_min(SCHED_RR) || i > sched_get_priority_max(SCHED_RR)) {
+        if (safe_atoi(rvalue, &i) < 0) {
                 log_error("[%s:%u] Failed to parse CPU scheduling priority, ignoring: %s", filename, line, rvalue);
                 return 0;
         }
 
+
+        /* On Linux RR/FIFO range from 1 to 99 and OTHER/BATCH may only be 0 */
+        min = sched_get_priority_min(c->cpu_sched_policy);
+        max = sched_get_priority_max(c->cpu_sched_policy);
+
+        if (i < min || i > max) {
+                log_error("[%s:%u] CPU scheduling priority is out of range, ignoring: %s", filename, line, rvalue);
+                return 0;
+        }
+
         c->cpu_sched_priority = i;
         c->cpu_sched_set = true;