netdev - [PATCH RFC net-next 4/5] net: sched: claim one cache line in Qdisc

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Message-ID: <20251006193103.2684156-5-edumazet@google.com>
Date: Mon,  6 Oct 2025 19:31:02 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com, Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH RFC net-next 4/5] net: sched: claim one cache line in Qdisc

Replace state2 field with a boolean.

Move it to a hole between qstats and state so that
we shrink Qdisc by a full cache line.

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/sch_generic.h | 18 +++++++-----------
 1 file changed, 7 insertions(+), 11 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 32e9961570b467b6066f1bb2c00ff1a270e342bc..31561291bc92fd70d4d3ca8f5f7dbc4c94c895a0 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -41,13 +41,6 @@ enum qdisc_state_t {
 	__QDISC_STATE_DRAINING,
 };
 
-enum qdisc_state2_t {
-	/* Only for !TCQ_F_NOLOCK qdisc. Never access it directly.
-	 * Use qdisc_run_begin/end() or qdisc_is_running() instead.
-	 */
-	__QDISC_STATE2_RUNNING,
-};
-
 #define QDISC_STATE_MISSED	BIT(__QDISC_STATE_MISSED)
 #define QDISC_STATE_DRAINING	BIT(__QDISC_STATE_DRAINING)
 
@@ -117,8 +110,8 @@ struct Qdisc {
 	struct qdisc_skb_head	q;
 	struct gnet_stats_basic_sync bstats;
 	struct gnet_stats_queue	qstats;
+	bool			running; /* must be written under qdisc spinlock */
 	unsigned long		state;
-	unsigned long		state2; /* must be written under qdisc spinlock */
 	struct Qdisc            *next_sched;
 	struct sk_buff_head	skb_bad_txq;
 
@@ -167,7 +160,7 @@ static inline bool qdisc_is_running(struct Qdisc *qdisc)
 {
 	if (qdisc->flags & TCQ_F_NOLOCK)
 		return spin_is_locked(&qdisc->seqlock);
-	return test_bit(__QDISC_STATE2_RUNNING, &qdisc->state2);
+	return READ_ONCE(qdisc->running);
 }
 
 static inline bool nolock_qdisc_is_empty(const struct Qdisc *qdisc)
@@ -210,7 +203,10 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 		 */
 		return spin_trylock(&qdisc->seqlock);
 	}
-	return !__test_and_set_bit(__QDISC_STATE2_RUNNING, &qdisc->state2);
+	if (READ_ONCE(qdisc->running))
+		return false;
+	WRITE_ONCE(qdisc->running, true);
+	return true;
 }
 
 static inline void qdisc_run_end(struct Qdisc *qdisc)
@@ -228,7 +224,7 @@ static inline void qdisc_run_end(struct Qdisc *qdisc)
 				      &qdisc->state)))
 			__netif_schedule(qdisc);
 	} else {
-		__clear_bit(__QDISC_STATE2_RUNNING, &qdisc->state2);
+		WRITE_ONCE(qdisc->running, false);
 	}
 }
 
-- 
2.51.0.618.g983fd99d29-goog