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authorTalal Ahmad <talalahmad@google.com>2021-10-29 22:05:42 -0400
committerJakub Kicinski <kuba@kernel.org>2021-11-01 16:33:27 -0700
commitf1a456f8f3fc5828d8abcad941860380ae147b1d (patch)
tree20453ce2f7741682aabd80e1c21c2db8c8562680 /net/ipv4
parent03271f3a3594c0e88f68d8cfbec0ba250b2c538a (diff)
net: avoid double accounting for pure zerocopy skbs
Track skbs with only zerocopy data and avoid charging them to kernel memory to correctly account the memory utilization for msg_zerocopy. All of the data in such skbs is held in user pages which are already accounted to user. Before this change, they are charged again in kernel in __zerocopy_sg_from_iter. The charging in kernel is excessive because data is not being copied into skb frags. This excessive charging can lead to kernel going into memory pressure state which impacts all sockets in the system adversely. Mark pure zerocopy skbs with a SKBFL_PURE_ZEROCOPY flag and remove charge/uncharge for data in such skbs. Initially, an skb is marked pure zerocopy when it is empty and in zerocopy path. skb can then change from a pure zerocopy skb to mixed data skb (zerocopy and copy data) if it is at tail of write queue and there is room available in it and non-zerocopy data is being sent in the next sendmsg call. At this time sk_mem_charge is done for the pure zerocopied data and the pure zerocopy flag is unmarked. We found that this happens very rarely on workloads that pass MSG_ZEROCOPY. A pure zerocopy skb can later be coalesced into normal skb if they are next to each other in queue but this patch prevents coalescing from happening. This avoids complexity of charging when skb downgrades from pure zerocopy to mixed. This is also rare. In sk_wmem_free_skb, if it is a pure zerocopy skb, an sk_mem_uncharge for SKB_TRUESIZE(MAX_TCP_HEADER) is done for sk_mem_charge in tcp_skb_entail for an skb without data. Testing with the msg_zerocopy.c benchmark between two hosts(100G nics) with zerocopy showed that before this patch the 'sock' variable in memory.stat for cgroup2 that tracks sum of sk_forward_alloc, sk_rmem_alloc and sk_wmem_queued is around 1822720 and with this change it is 0. This is due to no charge to sk_forward_alloc for zerocopy data and shows memory utilization for kernel is lowered. Signed-off-by: Talal Ahmad <talalahmad@google.com> Acked-by: Arjun Roy <arjunroy@google.com> Acked-by: Soheil Hassas Yeganeh <soheil@google.com> Signed-off-by: Willem de Bruijn <willemb@google.com> Signed-off-by: Eric Dumazet <edumazet@google.com> Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Diffstat (limited to 'net/ipv4')
-rw-r--r--net/ipv4/tcp.c22
-rw-r--r--net/ipv4/tcp_output.c7
2 files changed, 25 insertions, 4 deletions
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index bc7f419184aa..2561c14a6e63 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -863,6 +863,7 @@ struct sk_buff *tcp_stream_alloc_skb(struct sock *sk, int size, gfp_t gfp,
if (likely(skb)) {
bool mem_scheduled;
+ skb->truesize = SKB_TRUESIZE(size + MAX_TCP_HEADER);
if (force_schedule) {
mem_scheduled = true;
sk_forced_mem_schedule(sk, skb->truesize);
@@ -1319,6 +1320,15 @@ new_segment:
copy = min_t(int, copy, pfrag->size - pfrag->offset);
+ /* skb changing from pure zc to mixed, must charge zc */
+ if (unlikely(skb_zcopy_pure(skb))) {
+ if (!sk_wmem_schedule(sk, skb->data_len))
+ goto wait_for_space;
+
+ sk_mem_charge(sk, skb->data_len);
+ skb_shinfo(skb)->flags &= ~SKBFL_PURE_ZEROCOPY;
+ }
+
if (!sk_wmem_schedule(sk, copy))
goto wait_for_space;
@@ -1339,8 +1349,16 @@ new_segment:
}
pfrag->offset += copy;
} else {
- if (!sk_wmem_schedule(sk, copy))
- goto wait_for_space;
+ /* First append to a fragless skb builds initial
+ * pure zerocopy skb
+ */
+ if (!skb->len)
+ skb_shinfo(skb)->flags |= SKBFL_PURE_ZEROCOPY;
+
+ if (!skb_zcopy_pure(skb)) {
+ if (!sk_wmem_schedule(sk, copy))
+ goto wait_for_space;
+ }
err = skb_zerocopy_iter_stream(sk, skb, msg, copy, uarg);
if (err == -EMSGSIZE || err == -EEXIST) {
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 6fbbf1558033..287b57aadc37 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -1677,7 +1677,8 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
if (delta_truesize) {
skb->truesize -= delta_truesize;
sk_wmem_queued_add(sk, -delta_truesize);
- sk_mem_uncharge(sk, delta_truesize);
+ if (!skb_zcopy_pure(skb))
+ sk_mem_uncharge(sk, delta_truesize);
}
/* Any change of skb->len requires recalculation of tso factor. */
@@ -2295,7 +2296,9 @@ static bool tcp_can_coalesce_send_queue_head(struct sock *sk, int len)
if (len <= skb->len)
break;
- if (unlikely(TCP_SKB_CB(skb)->eor) || tcp_has_tx_tstamp(skb))
+ if (unlikely(TCP_SKB_CB(skb)->eor) ||
+ tcp_has_tx_tstamp(skb) ||
+ !skb_pure_zcopy_same(skb, next))
return false;
len -= skb->len;