netdev - [PATCH net-next] net: sched: shrink struct Qdisc

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Message-Id: <607936fe39bf1e78ca8b520e2ef25b7b326a767f.1527258390.git.pabeni@redhat.com>
Date:   Fri, 25 May 2018 16:28:44 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: [PATCH net-next] net: sched: shrink struct Qdisc

The struct Qdisc has a lot of holes, especially after commit
a53851e2c321 ("net: sched: explicit locking in gso_cpu fallback"),
which as a side effect, moved the fields just after 'busylock'
on a new cacheline.

Since both 'padded' and 'refcnt' are not updated frequently, and
there is a hole before 'gso_skb', we can move such fields there,
saving a cacheline without any performance side effect.

Before this commit:

pahole -C Qdisc net/sche/sch_generic.o
	# ...
        /* size: 384, cachelines: 6, members: 25 */
        /* sum members: 236, holes: 3, sum holes: 92 */
        /* padding: 56 */

After this commit:
pahole -C Qdisc net/sche/sch_generic.o
	# ...
	/* size: 320, cachelines: 5, members: 25 */
	/* sum members: 236, holes: 2, sum holes: 28 */
	/* padding: 56 */

Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 include/net/sch_generic.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index 98c10a28cd01..827a3711dc68 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -85,6 +85,8 @@ struct Qdisc {
 	struct net_rate_estimator __rcu *rate_est;
 	struct gnet_stats_basic_cpu __percpu *cpu_bstats;
 	struct gnet_stats_queue	__percpu *cpu_qstats;
+	int			padded;
+	refcount_t		refcnt;
 
 	/*
 	 * For performance sake on SMP, we put highly modified fields at the end
@@ -97,8 +99,6 @@ struct Qdisc {
 	unsigned long		state;
 	struct Qdisc            *next_sched;
 	struct sk_buff_head	skb_bad_txq;
-	int			padded;
-	refcount_t		refcnt;
 
 	spinlock_t		busylock ____cacheline_aligned_in_smp;
 	spinlock_t		seqlock;
-- 
2.17.0