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diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c
--- a/sys/kern/sched_ule.c
+++ b/sys/kern/sched_ule.c
@@ -484,6 +484,17 @@
return (0);
}
+static inline int
+normalize_ts_off(int offset)
+{
+ /*
+ * Adding RQ_TS_POL_MODULO before taking the modulo is to ensure the
+ * dividend is positive (we want a positive result).
+ */
+ MPASS(offset >= -RQ_TS_POL_MODULO);
+ return ((offset + RQ_TS_POL_MODULO) % RQ_TS_POL_MODULO);
+}
+
/*
* Add a thread to the actual run-queue. Keeps transferable counts up to
* date with what is actually on the run-queue. Selects the correct
@@ -517,18 +528,34 @@
/* Current queue from which processes are being run. */
idx = tdq->tdq_ts_deq_off;
else {
- idx = (RQ_PRI_TO_QUEUE_IDX(pri) - RQ_TS_POL_MIN +
- tdq->tdq_ts_off) % RQ_TS_POL_MODULO;
+ idx = normalize_ts_off(
+ /* Offset corresponding to priority. */
+ RQ_PRI_TO_QUEUE_IDX(pri) - RQ_TS_POL_MIN +
+ /* Insertion offset. */
+ tdq->tdq_ts_off);
/*
- * We avoid enqueuing low priority threads in the queue
- * that we are still draining, effectively shortening
- * the runqueue by one queue.
+ * We avoid enqueuing low priority threads in the queues
+ * we still have to drain. This effectively shortens
+ * the runqueue by a few queues (see update of
+ * 'tdq_ts_deq_off' in sched_clock()).
+ *
+ * The expressions that include differences below are
+ * measuring the "distance" from the dequeue offset to
+ * either 'idx' or the insertion offset modulo
+ * RQ_TS_POL_MODULO. Thanks to the arithmetic operators
+ * always performing the usual arithmetic conversions,
+ * all operands are promoted to integers, which is
+ * necessary to accomodate corner cases (else
+ * we would have to be conditional on whether the first
+ * term is greater or lower than the second, in the
+ * second case correcting the result with UCHAR_MAX %
+ * RQ_TS_POL_MODULO).
*/
if (tdq->tdq_ts_deq_off != tdq->tdq_ts_off &&
- idx == tdq->tdq_ts_deq_off)
- /* Ensure the dividend is positive. */
- idx = (idx - 1 + RQ_TS_POL_MODULO) %
- RQ_TS_POL_MODULO;
+ normalize_ts_off(idx - tdq->tdq_ts_deq_off) <
+ normalize_ts_off(tdq->tdq_ts_off -
+ tdq->tdq_ts_deq_off))
+ idx = normalize_ts_off(tdq->tdq_ts_deq_off - 1);
}
/* Absolute queue index. */
idx += RQ_TS_POL_MIN;

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