diff options
| -rw-r--r-- | sys/kern/sched_ule.c | 45 |
1 files changed, 36 insertions, 9 deletions
diff --git a/sys/kern/sched_ule.c b/sys/kern/sched_ule.c index cf0baabfda86..a9fe49b11ff7 100644 --- a/sys/kern/sched_ule.c +++ b/sys/kern/sched_ule.c @@ -484,6 +484,17 @@ sched_shouldpreempt(int pri, int cpri, int remote) 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 @@ tdq_runq_add(struct tdq *tdq, struct thread *td, int flags) /* 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; |