netdev - [RFC PATCH 20/21] core/skbuff: use skb_zdata for testing whether skb is zerocopy

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Date:   Thu, 18 Jun 2020 09:09:40 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     <netdev@...r.kernel.org>
CC:     <kernel-team@...com>, <axboe@...nel.dk>
Subject: [RFC PATCH 20/21] core/skbuff: use skb_zdata for testing whether skb is zerocopy

skb_zcopy() flag indicates whether the skb has a zerocopy ubuf.
netgpu does not use ubufs, so skb_zdata() indicates whether the
skb is carrying zero copy data, and should be left alone, while
skb_zcopy() indicates whhether there is an attached ubuf.

Also, when a write() on a zero-copy socket returns EWOULDBLOCK,
this is not synchronized with select(), which will only look at
the send buffer, and return writability if there is tcp space.

This appears to be caused by some ubuf logic, leading to iperf
spending 70% of its time in select() for ZC transmits.  With this
change, the time spent drops to 20%.

Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
---
 include/linux/skbuff.h | 24 +++++++++++++++++++++++-
 net/core/skbuff.c      | 16 ++++++++++++----
 2 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index ba41d1a383f8..3c2efd45655b 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -443,8 +443,12 @@ enum {
 
 	/* generate software time stamp when entering packet scheduling */
 	SKBTX_SCHED_TSTAMP = 1 << 6,
+
+	/* fragments are accessed only via DMA */
+	SKBTX_DEV_NETDMA = 1 << 7,
 };
 
+#define SKBTX_ZERODATA_FRAG	(SKBTX_DEV_ZEROCOPY | SKBTX_DEV_NETDMA)
 #define SKBTX_ZEROCOPY_FRAG	(SKBTX_DEV_ZEROCOPY | SKBTX_SHARED_FRAG)
 #define SKBTX_ANY_SW_TSTAMP	(SKBTX_SW_TSTAMP    | \
 				 SKBTX_SCHED_TSTAMP)
@@ -1416,6 +1420,24 @@ static inline struct skb_shared_hwtstamps *skb_hwtstamps(struct sk_buff *skb)
 	return &skb_shinfo(skb)->hwtstamps;
 }
 
+static inline bool skb_netdma(struct sk_buff *skb)
+{
+	return skb && skb_shinfo(skb)->tx_flags & SKBTX_DEV_NETDMA;
+}
+
+static inline bool skb_zdata(struct sk_buff *skb)
+{
+	return skb && skb_shinfo(skb)->tx_flags & SKBTX_ZERODATA_FRAG;
+}
+
+static inline void skb_netdma_set(struct sk_buff *skb, bool netdma)
+{
+	if (skb && netdma) {
+		skb_shinfo(skb)->tx_flags |= SKBTX_DEV_NETDMA;
+		skb_shinfo(skb)->destructor_arg = NULL;
+	}
+}
+
 static inline struct ubuf_info *skb_zcopy(struct sk_buff *skb)
 {
 	bool is_zcopy = skb && skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY;
@@ -3260,7 +3282,7 @@ static inline int skb_add_data(struct sk_buff *skb,
 static inline bool skb_can_coalesce(struct sk_buff *skb, int i,
 				    const struct page *page, int off)
 {
-	if (skb_zcopy(skb))
+	if (skb_zdata(skb))
 		return false;
 	if (i) {
 		const skb_frag_t *frag = &skb_shinfo(skb)->frags[i - 1];
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 2b4176cab578..67a421257a27 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1323,6 +1323,8 @@ int skb_zerocopy_iter_stream(struct sock *sk, struct sk_buff *skb,
 	}
 
 	skb_zcopy_set(skb, uarg, NULL);
+	skb_netdma_set(skb, sk->sk_user_data);
+
 	return skb->len - orig_len;
 }
 EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);
@@ -1330,8 +1332,8 @@ EXPORT_SYMBOL_GPL(skb_zerocopy_iter_stream);
 static int skb_zerocopy_clone(struct sk_buff *nskb, struct sk_buff *orig,
 			      gfp_t gfp_mask)
 {
-	if (skb_zcopy(orig)) {
-		if (skb_zcopy(nskb)) {
+	if (skb_zdata(orig)) {
+		if (skb_zdata(nskb)) {
 			/* !gfp_mask callers are verified to !skb_zcopy(nskb) */
 			if (!gfp_mask) {
 				WARN_ON_ONCE(1);
@@ -1343,6 +1345,7 @@ static int skb_zerocopy_clone(struct sk_buff *nskb, struct sk_buff *orig,
 				return -EIO;
 		}
 		skb_zcopy_set(nskb, skb_uarg(orig), NULL);
+		skb_netdma_set(nskb, skb_netdma(orig));
 	}
 	return 0;
 }
@@ -1372,6 +1375,9 @@ int skb_copy_ubufs(struct sk_buff *skb, gfp_t gfp_mask)
 	if (skb_shared(skb) || skb_unclone(skb, gfp_mask))
 		return -EINVAL;
 
+	if (!skb_has_shared_frag(skb))
+		return -EINVAL;
+
 	if (!num_frags)
 		goto release;
 
@@ -2078,6 +2084,8 @@ void *__pskb_pull_tail(struct sk_buff *skb, int delta)
 	 */
 	int i, k, eat = (skb->tail + delta) - skb->end;
 
+	BUG_ON(skb_netdma(skb));
+
 	if (eat > 0 || skb_cloned(skb)) {
 		if (pskb_expand_head(skb, 0, eat > 0 ? eat + 128 : 0,
 				     GFP_ATOMIC))
@@ -3328,7 +3336,7 @@ int skb_shift(struct sk_buff *tgt, struct sk_buff *skb, int shiftlen)
 
 	if (skb_headlen(skb))
 		return 0;
-	if (skb_zcopy(tgt) || skb_zcopy(skb))
+	if (skb_zdata(tgt) || skb_zdata(skb))
 		return 0;
 
 	todo = shiftlen;
@@ -5171,7 +5179,7 @@ bool skb_try_coalesce(struct sk_buff *to, struct sk_buff *from,
 	from_shinfo = skb_shinfo(from);
 	if (to_shinfo->frag_list || from_shinfo->frag_list)
 		return false;
-	if (skb_zcopy(to) || skb_zcopy(from))
+	if (skb_zdata(to) || skb_zdata(from))
 		return false;
 
 	if (skb_headlen(from) != 0) {
-- 
2.24.1