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rxrpc: Fix loss of RTT samples due to interposed ACK

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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: 50235c4b5a ("rxrpc: Obtain RTT data by requesting ACKs on DATA packets")
Signed-off-by: David Howells <dhowells@redhat.com>
This commit is contained in:
David Howells
2020-08-19 23:29:16 +01:00
parent 68528d937d
commit 4700c4d80b
6 changed files with 154 additions and 76 deletions
Download Patch File Download Diff File Expand all files Collapse all files

104
net/rxrpc/input.c
View File

@@ -608,36 +608,57 @@ unlock:
}
/*
* Process a requested ACK.
* See if there's a cached RTT probe to complete.
*/
static void rxrpc_input_requested_ack(struct rxrpc_call *call,
ktime_t resp_time,
rxrpc_serial_t orig_serial,
rxrpc_serial_t ack_serial)
static void rxrpc_complete_rtt_probe(struct rxrpc_call *call,
ktime_t resp_time,
rxrpc_serial_t acked_serial,
rxrpc_serial_t ack_serial,
enum rxrpc_rtt_rx_trace type)
{
struct rxrpc_skb_priv *sp;
struct sk_buff *skb;
rxrpc_serial_t orig_serial;
unsigned long avail;
ktime_t sent_at;
int ix;
bool matched = false;
int i;
for (ix = 0; ix < RXRPC_RXTX_BUFF_SIZE; ix++) {
skb = call->rxtx_buffer[ix];
if (!skb)
avail = READ_ONCE(call->rtt_avail);
smp_rmb(); /* Read avail bits before accessing data. */
for (i = 0; i < ARRAY_SIZE(call->rtt_serial); i++) {
if (!test_bit(i + RXRPC_CALL_RTT_PEND_SHIFT, &avail))
continue;
sent_at = skb->tstamp;
smp_rmb(); /* Read timestamp before serial. */
sp = rxrpc_skb(skb);
if (sp->hdr.serial != orig_serial)
continue;
goto found;
sent_at = call->rtt_sent_at[i];
orig_serial = call->rtt_serial[i];
if (orig_serial == acked_serial) {
clear_bit(i + RXRPC_CALL_RTT_PEND_SHIFT, &call->rtt_avail);
smp_mb(); /* Read data before setting avail bit */
set_bit(i, &call->rtt_avail);
if (type != rxrpc_rtt_rx_cancel)
rxrpc_peer_add_rtt(call, type, i, acked_serial, ack_serial,
sent_at, resp_time);
else
trace_rxrpc_rtt_rx(call, rxrpc_rtt_rx_cancel, i,
orig_serial, acked_serial, 0, 0);
matched = true;
}
/* If a later serial is being acked, then mark this slot as
* being available.
*/
if (after(acked_serial, orig_serial)) {
trace_rxrpc_rtt_rx(call, rxrpc_rtt_rx_obsolete, i,
orig_serial, acked_serial, 0, 0);
clear_bit(i + RXRPC_CALL_RTT_PEND_SHIFT, &call->rtt_avail);
smp_wmb();
set_bit(i, &call->rtt_avail);
}
}
return;
found:
rxrpc_peer_add_rtt(call, rxrpc_rtt_rx_requested_ack,
orig_serial, ack_serial, sent_at, resp_time);
if (!matched)
trace_rxrpc_rtt_rx(call, rxrpc_rtt_rx_lost, 9, 0, acked_serial, 0, 0);
}
/*
@@ -682,27 +703,11 @@ static void rxrpc_input_check_for_lost_ack(struct rxrpc_call *call)
*/
static void rxrpc_input_ping_response(struct rxrpc_call *call,
ktime_t resp_time,
rxrpc_serial_t orig_serial,
rxrpc_serial_t acked_serial,
rxrpc_serial_t ack_serial)
{
rxrpc_serial_t ping_serial;
ktime_t ping_time;
ping_time = call->ping_time;
smp_rmb();
ping_serial = READ_ONCE(call->ping_serial);
if (orig_serial == call->acks_lost_ping)
if (acked_serial == call->acks_lost_ping)
rxrpc_input_check_for_lost_ack(call);
if (before(orig_serial, ping_serial) ||
!test_and_clear_bit(RXRPC_CALL_PINGING, &call->flags))
return;
if (after(orig_serial, ping_serial))
return;
rxrpc_peer_add_rtt(call, rxrpc_rtt_rx_ping_response,
orig_serial, ack_serial, ping_time, resp_time);
}
/*
@@ -869,12 +874,23 @@ static void rxrpc_input_ack(struct rxrpc_call *call, struct sk_buff *skb)
first_soft_ack, prev_pkt,
summary.ack_reason, nr_acks);
if (buf.ack.reason == RXRPC_ACK_PING_RESPONSE)
switch (buf.ack.reason) {
case RXRPC_ACK_PING_RESPONSE:
rxrpc_input_ping_response(call, skb->tstamp, acked_serial,
ack_serial);
if (buf.ack.reason == RXRPC_ACK_REQUESTED)
rxrpc_input_requested_ack(call, skb->tstamp, acked_serial,
ack_serial);
rxrpc_complete_rtt_probe(call, skb->tstamp, acked_serial, ack_serial,
rxrpc_rtt_rx_ping_response);
break;
case RXRPC_ACK_REQUESTED:
rxrpc_complete_rtt_probe(call, skb->tstamp, acked_serial, ack_serial,
rxrpc_rtt_rx_requested_ack);
break;
default:
if (acked_serial != 0)
rxrpc_complete_rtt_probe(call, skb->tstamp, acked_serial, ack_serial,
rxrpc_rtt_rx_cancel);
break;
}
if (buf.ack.reason == RXRPC_ACK_PING) {
_proto("Rx ACK %%%u PING Request", ack_serial);