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

[patchbot@xxxxxxx]

commit f2927d8343aef714339fa455074bf56d2b878b3e
Author:     Anderson Choi <anderson.choi@xxxxxxxxxx>
AuthorDate: Mon Jul 28 13:05:56 2025 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Jul 28 13:05:56 2025 +0200

    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: 22787f2e107c ("ARINC 653 scheduler")
    Suggested-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
    Suggested-by: Nathan Studer <nathan.studer@xxxxxxxxxxxxxxx>
    Signed-off-by: Anderson Choi <anderson.choi@xxxxxxxxxx>
    Reviewed-by: Stewart Hildebrand <stewart@xxxxxxx>
---
 xen/common/sched/arinc653.c | 40 +++++++++++++++++++++++-----------------
 1 file changed, 23 insertions(+), 17 deletions(-)

diff --git a/xen/common/sched/arinc653.c b/xen/common/sched/arinc653.c
index 3c014c9934..8a4f4259d8 100644
--- a/xen/common/sched/arinc653.c
+++ b/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;
     }
 
     /*
--
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