linux-kernel - [PATCH] flow_dissector: introduce skb_get_hash

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Date:   Wed, 31 May 2023 13:21:57 -0400
From:   Jon Kohler <jon@...anix.com>
To:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Pavel Begunkov <asml.silence@...il.com>,
        Richard Gobert <richardbgobert@...il.com>,
        Menglong Dong <imagedong@...cent.com>,
        Wojciech Drewek <wojciech.drewek@...el.com>,
        Guillaume Nault <gnault@...hat.com>,
        John Fastabend <john.fastabend@...il.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Shmulik Ladkani <shmulik.ladkani@...il.com>,
        Qingqing Yang <qingqing.yang@...adcom.com>,
        Daniel Xu <dxu@...uu.xyz>, Felix Fietkau <nbd@....name>,
        Jon Kohler <jon@...anix.com>,
        Ludovic Cintrat <ludovic.cintrat@...ewatcher.com>,
        Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] flow_dissector: introduce skb_get_hash_symmetric()

tun.c changed from skb_get_hash() to __skb_get_hash_symmetric() on
commit feec084a7cf4 ("tun: use symmetric hash"), which exposes an
overhead for OVS datapath, where ovs_dp_process_packet() has to
calculate the hash again because __skb_get_hash_symmetric() does not
retain the hash that it calculates.

Introduce skb_get_hash_symmetric(), which will get and save the hash
in one go, so that calcuation work does not go to waste, and plumb it
into tun.c.

Fixes: feec084a7cf4 ("tun: use symmetric hash")
Signed-off-by: Jon Kohler <jon@...anix.com>
CC: Jason Wang <jasowang@...hat.com>
CC: David S. Miller <davem@...emloft.net>
---
 drivers/net/tun.c         |  8 ++++----
 include/linux/skbuff.h    |  1 +
 net/core/flow_dissector.c | 29 +++++++++++++++++++++++++++++
 3 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index d75456adc62a..27e9be434593 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -528,7 +528,7 @@ static u16 tun_automq_select_queue(struct tun_struct *tun, struct sk_buff *skb)

 	numqueues = READ_ONCE(tun->numqueues);

-	txq = __skb_get_hash_symmetric(skb);
+	txq = skb_get_hash_symmetric(skb);
 	e = tun_flow_find(&tun->flows[tun_hashfn(txq)], txq);
 	if (e) {
 		tun_flow_save_rps_rxhash(e, txq);
@@ -1046,7 +1046,7 @@ static void tun_automq_xmit(struct tun_struct *tun, struct sk_buff *skb)
 		struct tun_flow_entry *e;
 		__u32 rxhash;

-		rxhash = __skb_get_hash_symmetric(skb);
+		rxhash = skb_get_hash_symmetric(skb);
 		e = tun_flow_find(&tun->flows[tun_hashfn(rxhash)], rxhash);
 		if (e)
 			tun_flow_save_rps_rxhash(e, rxhash);
@@ -1933,7 +1933,7 @@ static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
 	 */
 	if (!rcu_access_pointer(tun->steering_prog) && tun->numqueues > 1 &&
 	    !tfile->detached)
-		rxhash = __skb_get_hash_symmetric(skb);
+		rxhash = skb_get_hash_symmetric(skb);

 	rcu_read_lock();
 	if (unlikely(!(tun->dev->flags & IFF_UP))) {
@@ -2515,7 +2515,7 @@ static int tun_xdp_one(struct tun_struct *tun,

 	if (!rcu_dereference(tun->steering_prog) && tun->numqueues > 1 &&
 	    !tfile->detached)
-		rxhash = __skb_get_hash_symmetric(skb);
+		rxhash = skb_get_hash_symmetric(skb);

 	if (tfile->napi_enabled) {
 		queue = &tfile->sk.sk_write_queue;
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0b40417457cd..8112b1ab5735 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1474,6 +1474,7 @@ __skb_set_sw_hash(struct sk_buff *skb, __u32 hash, bool is_l4)

 void __skb_get_hash(struct sk_buff *skb);
 u32 __skb_get_hash_symmetric(const struct sk_buff *skb);
+u32 skb_get_hash_symmetric(struct sk_buff *skb);
 u32 skb_get_poff(const struct sk_buff *skb);
 u32 __skb_get_poff(const struct sk_buff *skb, const void *data,
 		   const struct flow_keys_basic *keys, int hlen);
diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
index 25fb0bbc310f..d8c0e804bbfe 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -1747,6 +1747,35 @@ u32 __skb_get_hash_symmetric(const struct sk_buff *skb)
 }
 EXPORT_SYMBOL_GPL(__skb_get_hash_symmetric);

+/**
+ * skb_get_hash_symmetric: calculate and set a flow hash in @skb, using
+ * flow_keys_dissector_symmetric.
+ * @skb: sk_buff to calculate flow hash from
+ *
+ * This function is similar to __skb_get_hash_symmetric except that it
+ * retains the hash within the skb, such that it can be reused without
+ * being recalculated later.
+ */
+u32 skb_get_hash_symmetric(struct sk_buff *skb)
+{
+	struct flow_keys keys;
+	u32 hash;
+
+	__flow_hash_secret_init();
+
+	memset(&keys, 0, sizeof(keys));
+	__skb_flow_dissect(NULL, skb, &flow_keys_dissector_symmetric,
+			   &keys, NULL, 0, 0, 0,
+			   FLOW_DISSECTOR_F_STOP_AT_FLOW_LABEL);
+
+	hash = __flow_hash_from_keys(&keys, &hashrnd);
+
+	__skb_set_sw_hash(skb, hash, flow_keys_have_l4(&keys));
+
+	return hash;
+}
+EXPORT_SYMBOL_GPL(skb_get_hash_symmetric);
+
 /**
  * __skb_get_hash: calculate a flow hash
  * @skb: sk_buff to calculate flow hash from
--
2.30.1 (Apple Git-130)