xen/arinc653: fix delay in the start of major frame

ARINC653 specification requires partition scheduling to be deterministic
and periodic over time.

However, the use of current timestamp (now) as the baseline to calculate
next_major_frame and next_switch_time introduces a delay in the start of
major frame at every period, which breaks determinism and periodicity in
partition scheduling.

For example, we observe 3.5 msec of accumulated delay at the 21st major
frame with the following configuration.

Target : qemuarm64
xen version : 4.19 (43aeacff86, x86/IRQ: constrain creator-domain-ID assertion)
dom1 : 10 msec runtime
dom2 : 10 msec runtime

$ a653_sched -p Pool-arinc dom1:10 dom2:10

0.014553536 ---x d?v? runstate_change d1v0 runnable->running //1st major
frame
0.034629712 ---x d?v? runstate_change d1v0 runnable->running //2nd major
frame
<snip>
0.397747008 |||x d?v? runstate_change d1v0 runnable->running //20th
major frame
0.418066096 -||x d?v? runstate_change d1v0 runnable->running //21st
major frame

This is due to an inherent delta between the time value the scheduler timer
is programmed to be fired with and the time value the schedule function
is executed.

Another observation that breaks the deterministic behavior of partition
scheduling is a delayed execution of schedule(); It was called 14 msec
later than programmed.

1.530603952 ---x d?v? runstate_change d1v0 runnable->running
1.564956784 --|x d?v? runstate_change d1v0 runnable->running

Enforce the periodic behavior of partition scheduling by using the value
next_major_frame as the base to calculate the start of major frame and
the next domain switch time.

Per discussion with Nathan Studer, there are odd cases the first minor
frame can be also missed. In that scenario, iterate through the schedule
after resyncing the expected next major frame.

Per suggestion from Stewart Hildebrand, use a modulo operation to
calculate the start of next major frame.

Fixes: 22787f2e10 ("ARINC 653 scheduler")
Suggested-by: Stewart Hildebrand <stewart.hildebrand@amd.com>
Suggested-by: Nathan Studer <nathan.studer@dornerworks.com>
Signed-off-by: Anderson Choi <anderson.choi@boeing.com>
Reviewed-by: Stewart Hildebrand <stewart@stew.dk>

xen/common/sched/arinc653.c

@@ -363,6 +363,8 @@ a653sched_init(struct scheduler *ops)
     ops->sched_data = prv;
 
     prv->next_major_frame = 0;
+    prv->major_frame = DEFAULT_TIMESLICE;
+    prv->schedule[0].runtime = DEFAULT_TIMESLICE;
     spin_lock_init(&prv->lock);
     INIT_LIST_HEAD(&prv->unit_list);
 
@@ -528,27 +530,31 @@ a653sched_do_schedule(
 
     spin_lock_irqsave(&sched_priv->lock, flags);
 
-    if ( sched_priv->num_schedule_entries < 1 )
-        sched_priv->next_major_frame = now + DEFAULT_TIMESLICE;
-    else if ( now >= sched_priv->next_major_frame )
+    ASSERT(sched_priv->major_frame > 0);
+
+    /* Switch to next major frame using a modulo operation */
+    if ( now >= sched_priv->next_major_frame )
     {
-        /* time to enter a new major frame
-         * the first time this function is called, this will be true */
-        /* start with the first domain in the schedule */
+        s_time_t major_frame = sched_priv->major_frame;
+        s_time_t remainder = (now - sched_priv->next_major_frame) % major_frame;
+
+        /*
+         * major_frame and schedule[0].runtime contain DEFAULT_TIMESLICE
+         * if num_schedule_entries is zero.
+         */
         sched_priv->sched_index = 0;
-        sched_priv->next_major_frame = now + sched_priv->major_frame;
-        sched_priv->next_switch_time = now + sched_priv->schedule[0].runtime;
+        sched_priv->next_major_frame = now - remainder + major_frame;
+        sched_priv->next_switch_time = now - remainder +
+            sched_priv->schedule[0].runtime;
     }
-    else
+
+    /* Switch minor frame or find correct minor frame after a miss */
+    while ( (now >= sched_priv->next_switch_time) &&
+        (sched_priv->sched_index < sched_priv->num_schedule_entries) )
     {
-        while ( (now >= sched_priv->next_switch_time) &&
-                (sched_priv->sched_index < sched_priv->num_schedule_entries) )
-        {
-            /* time to switch to the next domain in this major frame */
-            sched_priv->sched_index++;
-            sched_priv->next_switch_time +=
-                sched_priv->schedule[sched_priv->sched_index].runtime;
-        }
+        sched_priv->sched_index++;
+        sched_priv->next_switch_time +=
+            sched_priv->schedule[sched_priv->sched_index].runtime;
     }
 
     /*