Add more comment for core affinity in prvSelectHighestPriorityTask (#801)
* Add more comment for core affinity in prvSelectHighestPriorityTask
Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
---------
Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>
Co-authored-by: kar-rahul-aws <118818625+kar-rahul-aws@users.noreply.github.com>
Co-authored-by: Gaurav Aggarwal <aggarg@amazon.com>
diff --git a/tasks.c b/tasks.c
index bea306e..c8a8cde 100644
--- a/tasks.c
+++ b/tasks.c
@@ -1134,16 +1134,31 @@
if( ( uxCoreMap & ( ( UBaseType_t ) 1U << ( UBaseType_t ) xCoreID ) ) != 0U )
{
- /* The ready task that was removed from this core is not excluded from it.
- * Only look at the intersection of the cores the removed task is allowed to run
- * on with the cores that the new task is excluded from. It is possible that the
- * new task was only placed onto this core because it is excluded from another.
- * Check to see if the previous task could run on one of those cores. */
+ /* pxPreviousTCB was removed from this core and this core is not excluded
+ * from it's core affinity mask.
+ *
+ * pxPreviousTCB is preempted by the new higher priority task
+ * pxCurrentTCBs[ xCoreID ]. When searching a new core for pxPreviousTCB,
+ * we do not need to look at the cores on which pxCurrentTCBs[ xCoreID ]
+ * is allowed to run. The reason is - when more than one cores are
+ * eligible for an incoming task, we preempt the core with the minimum
+ * priority task. Because this core (i.e. xCoreID) was preempted for
+ * pxCurrentTCBs[ xCoreID ], this means that all the others cores
+ * where pxCurrentTCBs[ xCoreID ] can run, are running tasks with priority
+ * no lower than pxPreviousTCB's priority. Therefore, the only cores where
+ * which can be preempted for pxPreviousTCB are the ones where
+ * pxCurrentTCBs[ xCoreID ] is not allowed to run (and obviously,
+ * pxPreviousTCB is allowed to run).
+ *
+ * This is an optimization which reduces the number of cores needed to be
+ * searched for pxPreviousTCB to run. */
uxCoreMap &= ~( pxCurrentTCBs[ xCoreID ]->uxCoreAffinityMask );
}
else
{
- /* The ready task that was removed from this core is excluded from it. */
+ /* pxPreviousTCB's core affinity mask is changed and it is no longer
+ * allowed to run on this core. Searching all the cores in pxPreviousTCB's
+ * new core affinity mask to find a core on which it can run. */
}
uxCoreMap &= ( ( 1U << configNUMBER_OF_CORES ) - 1U );
