netdev - [PATCH net-next 06/10] net_sched: add Qdisc_read_mostly and Qdisc

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Message-ID: <20251110094505.3335073-7-edumazet@google.com>
Date: Mon, 10 Nov 2025 09:45:01 +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>, 
	"Toke Høiland-Jørgensen" <toke@...hat.com>, 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 net-next 06/10] net_sched: add Qdisc_read_mostly and
 Qdisc_write groups

It is possible to reorg Qdisc to avoid always dirtying 2 cache lines in
fast path by reducing this to a single dirtied cache line.

In current layout, we change only four/six fields in the first cache line:
 - q.spinlock
 - q.qlen
 - bstats.bytes
 - bstats.packets
 - some Qdisc also change q.next/q.prev

In the second cache line we change in the fast path:
 - running
 - state
 - qstats.backlog

        /* --- cacheline 2 boundary (128 bytes) --- */
        struct sk_buff_head        gso_skb __attribute__((__aligned__(64))); /*  0x80  0x18 */
        struct qdisc_skb_head      q;                    /*  0x98  0x18 */
        struct gnet_stats_basic_sync bstats __attribute__((__aligned__(16))); /*  0xb0  0x10 */

        /* --- cacheline 3 boundary (192 bytes) --- */
        struct gnet_stats_queue    qstats;               /*  0xc0  0x14 */
        bool                       running;              /*  0xd4   0x1 */

        /* XXX 3 bytes hole, try to pack */

        unsigned long              state;                /*  0xd8   0x8 */
        struct Qdisc *             next_sched;           /*  0xe0   0x8 */
        struct sk_buff_head        skb_bad_txq;          /*  0xe8  0x18 */
        /* --- cacheline 4 boundary (256 bytes) --- */

Reorganize things to have a first cache line mostly read,
then a mostly written one.

This gives a ~3% increase of performance under tx stress.

Note that there is an additional hols because @qstats now spans over a third cache line.

	/* --- cacheline 2 boundary (128 bytes) --- */
	__u8                       __cacheline_group_begin__Qdisc_read_mostly[0] __attribute__((__aligned__(64))); /*  0x80     0 */
	struct sk_buff_head        gso_skb;              /*  0x80  0x18 */
	struct Qdisc *             next_sched;           /*  0x98   0x8 */
	struct sk_buff_head        skb_bad_txq;          /*  0xa0  0x18 */
	__u8                       __cacheline_group_end__Qdisc_read_mostly[0]; /*  0xb8     0 */

	/* XXX 8 bytes hole, try to pack */

	/* --- cacheline 3 boundary (192 bytes) --- */
	__u8                       __cacheline_group_begin__Qdisc_write[0] __attribute__((__aligned__(64))); /*  0xc0     0 */
	struct qdisc_skb_head      q;                    /*  0xc0  0x18 */
	unsigned long              state;                /*  0xd8   0x8 */
	struct gnet_stats_basic_sync bstats __attribute__((__aligned__(16))); /*  0xe0  0x10 */
	bool                       running;              /*  0xf0   0x1 */

	/* XXX 3 bytes hole, try to pack */

	struct gnet_stats_queue    qstats;               /*  0xf4  0x14 */
	/* --- cacheline 4 boundary (256 bytes) was 8 bytes ago --- */
	__u8                       __cacheline_group_end__Qdisc_write[0]; /* 0x108     0 */

	/* XXX 56 bytes hole, try to pack */

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 include/net/sch_generic.h | 29 ++++++++++++++++++-----------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index ae037e56088208ad17f664c43689aee895569cab..b76436ec3f4aa412bac1be3371f5c7c6245cc362 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -103,17 +103,24 @@ struct Qdisc {
 	int			pad;
 	refcount_t		refcnt;
 
-	/*
-	 * For performance sake on SMP, we put highly modified fields at the end
-	 */
-	struct sk_buff_head	gso_skb ____cacheline_aligned_in_smp;
-	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;
-	struct Qdisc            *next_sched;
-	struct sk_buff_head	skb_bad_txq;
+	/* Cache line potentially dirtied in dequeue() or __netif_reschedule(). */
+	__cacheline_group_begin(Qdisc_read_mostly) ____cacheline_aligned;
+		struct sk_buff_head	gso_skb;
+		struct Qdisc		*next_sched;
+		struct sk_buff_head	skb_bad_txq;
+	__cacheline_group_end(Qdisc_read_mostly);
+
+	/* Fields dirtied in dequeue() fast path. */
+	__cacheline_group_begin(Qdisc_write) ____cacheline_aligned;
+		struct qdisc_skb_head	q;
+		unsigned long		state;
+		struct gnet_stats_basic_sync bstats;
+		bool			running; /* must be written under qdisc spinlock */
+
+		/* Note : we only change qstats.backlog in fast path. */
+		struct gnet_stats_queue	qstats;
+	__cacheline_group_end(Qdisc_write);
+
 
 	atomic_long_t		defer_count ____cacheline_aligned_in_smp;
 	struct llist_head	defer_list;
-- 
2.51.2.1041.gc1ab5b90ca-goog