sched: manipulate __QDISC_STATE_RUNNING in qdisc_run_* helpers

Currently NOLOCK qdiscs pay a measurable overhead to atomically
manipulate the __QDISC_STATE_RUNNING. Such bit is flipped twice per
packet in the uncontended scenario with packet rate below the
line rate: on packed dequeue and on the next, failing dequeue attempt.

This changeset moves the bit manipulation into the qdisc_run_{begin,end}
helpers, so that the bit is now flipped only once per packet, with
measurable performance improvement in the uncontended scenario.

This also allows simplifying the qdisc teardown code path - since
qdisc_is_running() is now effective for each qdisc type - and avoid a
possible race between qdisc_run() and dev_deactivate_many(), as now
the some_qdisc_is_busy() can properly detect NOLOCK qdiscs being busy
dequeuing packets.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 1 deletions

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 5154c8300262..4d2b37226e75 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -113,13 +113,19 @@ static inline void qdisc_refcount_inc(struct Qdisc *qdisc)
 
 static inline bool qdisc_is_running(const struct Qdisc *qdisc)
 {
+	if (qdisc->flags & TCQ_F_NOLOCK)
+		return test_bit(__QDISC_STATE_RUNNING, &qdisc->state);
 	return (raw_read_seqcount(&qdisc->running) & 1) ? true : false;
 }
 
 static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 {
-	if (qdisc_is_running(qdisc))
+	if (qdisc->flags & TCQ_F_NOLOCK) {
+		if (test_and_set_bit(__QDISC_STATE_RUNNING, &qdisc->state))
+			return false;
+	} else if (qdisc_is_running(qdisc)) {
 		return false;
+	}
 	/* Variant of write_seqcount_begin() telling lockdep a trylock
 	 * was attempted.
 	 */
@@ -131,6 +137,8 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 static inline void qdisc_run_end(struct Qdisc *qdisc)
 {
 	write_seqcount_end(&qdisc->running);
+	if (qdisc->flags & TCQ_F_NOLOCK)
+		clear_bit(__QDISC_STATE_RUNNING, &qdisc->state);
 }
 
 static inline bool qdisc_may_bulk(const struct Qdisc *qdisc)