path: root/net/rxrpc/call_object.c
diff options
authorDavid Howells <dhowells@redhat.com>2020-08-19 23:29:16 +0100
committerDavid Howells <dhowells@redhat.com>2020-08-20 17:59:27 +0100
commit4700c4d80b7bb171f6996016ef121e1508860b42 (patch)
tree56aa716dbd10161efd3e8b4b0ac74f225b7c4f95 /net/rxrpc/call_object.c
parent68528d937dcd675e79973061c1a314db598162d1 (diff)
rxrpc: Fix loss of RTT samples due to interposed ACK
The Rx protocol has a mechanism to help generate RTT samples that works by a client transmitting a REQUESTED-type ACK when it receives a DATA packet that has the REQUEST_ACK flag set. The peer, however, may interpose other ACKs before transmitting the REQUESTED-ACK, as can be seen in the following trace excerpt: rxrpc_tx_data: c=00000044 DATA d0b5ece8:00000001 00000001 q=00000001 fl=07 rxrpc_rx_ack: c=00000044 00000001 PNG r=00000000 f=00000002 p=00000000 n=0 rxrpc_rx_ack: c=00000044 00000002 REQ r=00000001 f=00000002 p=00000001 n=0 ... DATA packet 1 (q=xx) has REQUEST_ACK set (bit 1 of fl=xx). The incoming ping (labelled PNG) hard-acks the request DATA packet (f=xx exceeds the sequence number of the DATA packet), causing it to be discarded from the Tx ring. The ACK that was requested (labelled REQ, r=xx references the serial of the DATA packet) comes after the ping, but the sk_buff holding the timestamp has gone and the RTT sample is lost. This is particularly noticeable on RPC calls used to probe the service offered by the peer. A lot of peers end up with an unknown RTT because we only ever sent a single RPC. This confuses the server rotation algorithm. Fix this by caching the information about the outgoing packet in RTT calculations in the rxrpc_call struct rather than looking in the Tx ring. A four-deep buffer is maintained and both REQUEST_ACK-flagged DATA and PING-ACK transmissions are recorded in there. When the appropriate response ACK is received, the buffer is checked for a match and, if found, an RTT sample is recorded. If a received ACK refers to a packet with a later serial number than an entry in the cache, that entry is presumed lost and the entry is made available to record a new transmission. ACKs types other than REQUESTED-type and PING-type cause any matching sample to be cancelled as they don't necessarily represent a useful measurement. If there's no space in the buffer on ping/data transmission, the sample base is discarded. Fixes: 50235c4b5a2f ("rxrpc: Obtain RTT data by requesting ACKs on DATA packets") Signed-off-by: David Howells <dhowells@redhat.com>
Diffstat (limited to 'net/rxrpc/call_object.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index 38a46167523f..a40fae013942 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -153,6 +153,7 @@ struct rxrpc_call *rxrpc_alloc_call(struct rxrpc_sock *rx, gfp_t gfp,
call->cong_ssthresh = RXRPC_RXTX_BUFF_SIZE - 1;
call->rxnet = rxnet;
+ call->rtt_avail = RXRPC_CALL_RTT_AVAIL_MASK;
return call;