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Message-Id: <20200503025422.219257-2-edumazet@google.com>
Date: Sat, 2 May 2020 19:54:18 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>
Cc: netdev <netdev@...r.kernel.org>,
Eric Dumazet <edumazet@...gle.com>,
Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH net-next 1/5] net_sched: sch_fq: avoid touching f->next from fq_gc()
A significant amount of cpu cycles is spent in fq_gc()
When fq_gc() does its lookup in the rb-tree, it needs the
following fields from struct fq_flow :
f->sk (lookup key in the rb-tree)
f->fq_node (anchor in the rb-tree)
f->next (used to determine if the flow is detached)
f->age (used to determine if the flow is candidate for gc)
This unfortunately spans two cache lines (assuming 64 bytes cache lines)
We can avoid using f->next, if we use the low order bit of f->{age|tail}
This low order bit is 0, if f->tail points to an sk_buff.
We set the low order bit to 1, if the union contains a jiffies value.
Combined with the following patch, this makes sure we only need
to bring into cpu caches one cache line per flow.
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
net/sched/sch_fq.c | 21 +++++++++++++--------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
index fa228df22e5dbf3fb5eb18279b79ab397d36ac58..1649928fe2c1b7476050e5eee3c494c76d114c62 100644
--- a/net/sched/sch_fq.c
+++ b/net/sched/sch_fq.c
@@ -70,14 +70,14 @@ struct fq_flow {
struct sk_buff *head; /* list of skbs for this flow : first skb */
union {
struct sk_buff *tail; /* last skb in the list */
- unsigned long age; /* jiffies when flow was emptied, for gc */
+ unsigned long age; /* (jiffies | 1UL) when flow was emptied, for gc */
};
struct rb_node fq_node; /* anchor in fq_root[] trees */
struct sock *sk;
int qlen; /* number of packets in flow queue */
int credit;
u32 socket_hash; /* sk_hash */
- struct fq_flow *next; /* next pointer in RR lists, or &detached */
+ struct fq_flow *next; /* next pointer in RR lists */
struct rb_node rate_node; /* anchor in q->delayed tree */
u64 time_next_packet;
@@ -130,20 +130,25 @@ struct fq_sched_data {
struct qdisc_watchdog watchdog;
};
-/* special value to mark a detached flow (not on old/new list) */
-static struct fq_flow detached, throttled;
-
+/*
+ * f->tail and f->age share the same location.
+ * We can use the low order bit to differentiate if this location points
+ * to a sk_buff or contains a jiffies value, if we force this value to be odd.
+ * This assumes f->tail low order bit must be 0 since alignof(struct sk_buff) >= 2
+ */
static void fq_flow_set_detached(struct fq_flow *f)
{
- f->next = &detached;
- f->age = jiffies;
+ f->age = jiffies | 1UL;
}
static bool fq_flow_is_detached(const struct fq_flow *f)
{
- return f->next == &detached;
+ return !!(f->age & 1UL);
}
+/* special value to mark a throttled flow (not on old/new list) */
+static struct fq_flow throttled;
+
static bool fq_flow_is_throttled(const struct fq_flow *f)
{
return f->next == &throttled;
--
2.26.2.526.g744177e7f7-goog
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