Scheduling server (by Bjorn Swift)

In this second phase, scheduling is moved from PM to its own
scheduler (see r6557 for phase one). In the next phase we hope to a)
include useful information in the "out of quantum" message and b)
create some simple scheduling policy that makes use of that
information.

When the system starts up, PM will iterate over its process table and
ask SCHED to take over scheduling unprivileged processes. This is
done by sending a SCHEDULING_START message to SCHED. This message
includes the processes endpoint, the parent's endpoint and its nice
level. The scheduler adds this process to its schedproc table, issues
a schedctl, and returns its own endpoint to PM - as the endpoint of
the effective scheduler. When a process terminates, a SCHEDULING_STOP
message is sent to the scheduler.

The reason for this effective endpoint is for future compatibility.
Some day, we may have a scheduler that, instead of scheduling the
process itself, forwards the SCHEDULING_START message on to another
scheduler.

PM has information on who schedules whom. As such, scheduling
messages from user-land are sent through PM. An example is when
processes change their priority, using nice(). In that case, a
getsetpriority message is sent to PM, which then sends a
SCHEDULING_SET_NICE to the process's effective scheduler.

When a process is forked through PM, it inherits its parent's
scheduler, but is spawned with an empty quantum. As before, a request
to fork a process flows through VM before returning to PM, which then
wakes up the child process. This flow has been modified slightly so
that PM notifies the scheduler of the new process, before waking up
the child process. If the scheduler fails to take over scheduling,
the child process is torn down and the fork fails with an erroneous
value.

Process priority is entirely decided upon using nice levels. PM
stores a copy of each process's nice level and when a child is
forked, its parent's nice level is sent in the SCHEDULING_START
message. How this level is mapped to a priority queue is up to the
scheduler. It should be noted that the nice level is used to
determine the max_priority and the parent could have been in a lower
priority when it was spawned. To prevent a CPU intensive process from
hawking the CPU by continuously forking children that get scheduled
in the max_priority, the scheduler should determine in which queue
the parent is currently scheduled, and schedule the child in that
same queue.

Other fixes: The USER_Q in kernel/proc.h was incorrectly defined as
NR_SCHED_QUEUES/2. That results in a "off by one" error when
converting priority->nice->priority for nice=0. This also had the
side effect that if someone were to set the MAX_USER_Q to something
else than 0, then USER_Q would be off.

include/minix/com.h

@@ -73,8 +73,9 @@
 #define DS_PROC_NR   ((endpoint_t) 6)   /* data store server */
 #define MFS_PROC_NR  ((endpoint_t) 7)   /* minix root filesystem */
 #define VM_PROC_NR   ((endpoint_t) 8)   /* memory server */
-#define PFS_PROC_NR  ((endpoint_t) 9)   /* pipe filesystem */
-#define LAST_SPECIAL_PROC_NR	10	/* An untyped version for
+#define PFS_PROC_NR  ((endpoint_t) 9)  /* pipe filesystem */
+#define SCHED_PROC_NR ((endpoint_t) 10)	/* scheduler */
+#define LAST_SPECIAL_PROC_NR	11	/* An untyped version for
                                            computation in macros.*/
 #define INIT_PROC_NR ((endpoint_t) LAST_SPECIAL_PROC_NR)  /* init
                                                         -- goes multiuser */
@@ -1079,9 +1080,21 @@
 #define SCHEDULING_BASE	0xF00
 
 #define SCHEDULING_NO_QUANTUM	(SCHEDULING_BASE+1)
-#	define SCHEDULING_ENDPOINT	m1_i1
-#	define SCHEDULING_PRIORITY	m1_i2
-#	define SCHEDULING_QUANTUM	m1_i3
+#	define SCHEDULING_ENDPOINT	m9_l1
+#	define SCHEDULING_QUANTUM	m9_l2
+#	define SCHEDULING_PRIORITY	m9_s1
+
+/* SCHEDULING_START uses _ENDPOINT, _PRIORITY and _QUANTUM from
+ * SCHEDULING_NO_QUANTUM */
+#define SCHEDULING_START	(SCHEDULING_BASE+2)
+#	define SCHEDULING_SCHEDULER	m9_l1 /* Overrides _ENDPOINT on return*/
+#	define SCHEDULING_PARENT	m9_l3
+#	define SCHEDULING_NICE		m9_l4
+
+#define SCHEDULING_STOP		(SCHEDULING_BASE+3)
+
+#define SCHEDULING_SET_NICE	(SCHEDULING_BASE+4)
+
 #endif
 
 /* _MINIX_COM_H */