| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <D42AE34E.1FF9D%brakmo@fb.com>
Date: Tue, 18 Oct 2016 03:35:44 +0000
From: Lawrence Brakmo <brakmo@...com>
To: Yuchung Cheng <ycheng@...gle.com>
CC: netdev <netdev@...r.kernel.org>, Kernel Team <Kernel-team@...com>,
"Eric Dumazet" <eric.dumazet@...il.com>,
Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: Change txhash on some non-RTO retransmits
Yuchung and Eric, thank you for your comments.
It looks like I need to think more about this patch. I was trying
to reduce the likelihood of reordering (which seems even more
important based on Eric¹s comment on pacing), but it seems like
the only way to prevent reordering is to only re-hash after an RTO
or when there are no packets in flight (which may not occur).
On 10/11/16, 8:56 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>On Tue, Oct 11, 2016 at 6:01 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
>> On Tue, Oct 11, 2016 at 2:08 PM, Lawrence Brakmo <brakmo@...com> wrote:
>>> Yuchung, thank you for your comments. Responses inline.
>>>
>>> On 10/11/16, 12:49 PM, "Yuchung Cheng" <ycheng@...gle.com> wrote:
>>>
>>>>On Mon, Oct 10, 2016 at 5:18 PM, Lawrence Brakmo <brakmo@...com> wrote:
>>>>>
>>>>> The purpose of this patch is to help balance flows across paths. A
>>>>>new
>>>>> sysctl "tcp_retrans_txhash_prob" specifies the probability (0-100)
>>>>>that
>>>>> the txhash (IPv6 flowlabel) will be changed after a non-RTO
>>>>>retransmit.
>>>>> A probability is used in order to control how many flows are moved
>>>>> during a congestion event and prevent the congested path from
>>>>>becoming
>>>>> under utilized (which could occur if too many flows leave the current
>>>>> path). Txhash changes may be delayed in order to decrease the
>>>>>likelihood
>>>>> that it will trigger retransmists due to too much reordering.
>>>>>
>>>>> Another sysctl "tcp_retrans_txhash_mode" determines the behavior
>>>>>after
>>>>> RTOs. If the sysctl is 0, then after an RTO, only RTOs can trigger
>>>>> txhash changes. The idea is to decrease the likelihood of going back
>>>>> to a broken path. That is, we don't want flow balancing to trigger
>>>>> changes to broken paths. The drawback is that flow balancing does
>>>>> not work as well. If the sysctl is greater than 1, then we always
>>>>> do flow balancing, even after RTOs.
>>>>>
>>>>> Tested with packedrill tests (for correctness) and performance
>>>>> experiments with 2 and 3 paths. Performance experiments looked at
>>>>> aggregate goodput and fairness. For each run, we looked at the ratio
>>>>>of
>>>>> the goodputs for the fastest and slowest flows. These were averaged
>>>>>for
>>>>> all the runs. A fairness of 1 means all flows had the same goodput, a
>>>>> fairness of 2 means the fastest flow was twice as fast as the slowest
>>>>> flow.
>>>>>
>>>>> The setup for the performance experiments was 4 or 5 serves in a
>>>>>rack,
>>>>> 10G links. I tested various probabilities, but 20 seemed to have the
>>>>> best tradeoff for my setup (small RTTs).
>>>>>
>>>>> --- node1 -----
>>>>> sender --- switch --- node2 ----- switch ---- receiver
>>>>> --- node3 -----
>>>>>
>>>>> Scenario 1: One sender sends to one receiver through 2 routes (node1
>>>>>or
>>>>> node 2). The output from node1 and node2 is 1G (1gbit/sec). With
>>>>>only 2
>>>>> flows, without flow balancing (prob=0) the average goodput is 1.6G
>>>>>vs.
>>>>> 1.9G with flow balancing due to 2 flows ending up in one link and
>>>>>either
>>>>> not moving and taking some time to move. Fairness was 1 in all cases.
>>>>> For 7 flows, goodput was 1.9G for all, but fairness was 1.5, 1.4 or
>>>>>1.2
>>>>> for prob=0, prob=20,mode=0 and prob=20,mode=1 respectively. That is,
>>>>> flow balancing increased fairness.
>>>>>
>>>>> Scenario 2: One sender to one receiver, through 3 routes (node1,...
>>>>> node2). With 6 or 16 flows the goodput was the same for all, but
>>>>> fairness was 1.8, 1.5 and 1.2 respectively. Interestingly, the worst
>>>>> case fairness out of 10 runs were 2.2, 1.8 and 1.4 repectively. That
>>>>>is,
>>>>> prob=20,mode=1 improved average and worst case fairness.
>>>>I am wondering if we can build better API with routing layer to
>>>>implement this type of feature, instead of creeping the tx_rehashing
>>>>logic scatter in TCP. For example, we call dst_negative_advice on TCP
>>>>write timeouts.
>>>
>>> Not sure. The route is not necessarily bad, may be temporarily
>>>congested
>>> or they may all be congested. If all we want to do is change the txhash
>>> (unlike dst_negative_advice), then calling a tx_rehashing function may
>>> be the appropriate call.
>>>
>>>>
>>>>On the patch itself, it seems aggressive to (attempt to) rehash every
>>>>post-RTO retranmission. Also you can just use ca_state (==CA_Loss) to
>>>>identify post-RTO retransmission directly.
>>>
>>> Thanks, I will add the test.
>>>
>>>>
>>>>is this an implementation of the Flow Bender ?
>>>>https://urldefense.proofpoint.com/v2/url?u=http-3A__dl.acm.org_citation
>>>>.cf
>>>>m-3Fid-3D2674985&d=DQIBaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=pq_Mqvzfy-C8ltkgyx
>>>>1u_
>>>>g&m=Q4nONH7kQ5AvQguw9UxpcHd79jfdDdrXj1YSJs7Ezhk&s=MA4fWBLMTGgRS0eGvBjxf
>>>>7BJ
>>>>Ol3-oxAzZDEYUG4cE-s&e=
>>>
>>> Part of flow bender, although there are also some similarities to
>>>flowlet
>>> switching.
>>>
>>>>
>>>>>
>>>>> Scenario 3: One sender to one receiver, 2 routes, one route drops
>>>>>50% of
>>>>> the packets. With 7 flows, goodput was the same 1.1G, but fairness
>>>>>was
>>>>> 1.8, 2.0 and 2.1 respectively. That is, if there is a bad route, then
>>>>> balancing, which does more re-routes, is less fair.
>>>>>
>>>>> Signed-off-by: Lawrence Brakmo <brakmo@...com>
>>>>> ---
>>>>> Documentation/networking/ip-sysctl.txt | 15 +++++++++++++++
>>>>> include/linux/tcp.h | 4 +++-
>>>>> include/net/tcp.h | 2 ++
>>>>> net/ipv4/sysctl_net_ipv4.c | 18 ++++++++++++++++++
>>>>> net/ipv4/tcp_input.c | 10 ++++++++++
>>>>> net/ipv4/tcp_output.c | 23 ++++++++++++++++++++++-
>>>>> net/ipv4/tcp_timer.c | 4 ++++
>>>>> 7 files changed, 74 insertions(+), 2 deletions(-)
>>>>>
>>>>> diff --git a/Documentation/networking/ip-sysctl.txt
>>>>>b/Documentation/networking/ip-sysctl.txt
>>>>> index 3db8c67..87a984c 100644
>>>>> --- a/Documentation/networking/ip-sysctl.txt
>>>>> +++ b/Documentation/networking/ip-sysctl.txt
>>>>> @@ -472,6 +472,21 @@ tcp_max_reordering - INTEGER
>>>>> if paths are using per packet load balancing (like bonding rr
>>>>>mode)
>>>>> Default: 300
>>>>>
>>>>> +tcp_retrans_txhash_mode - INTEGER
>>>>> + If zero, disable txhash recalculation due to non-RTO
>>>>>retransmissions
>>>>> + after an RTO. The idea is that broken paths will trigger an
>>>>>RTO
>>>>>and
>>>>> + we don't want going back to that path due to standard
>>>>>retransmissons
>>>>> + (flow balancing). The drawback is that balancing is less
>>>>>robust.
>>>>> + If greater than zero, can always (probabilistically)
>>>>>recalculate
>>>>> + txhash after non-RTO retransmissions.
>>>>> +
>>>>> +tcp_retrans_txhash_prob - INTEGER
>>>>> + Probability [0 to 100] that we will recalculate txhash when a
>>>>> + packet is resent not due to RTO (for RTO txhash is always
>>>>>recalculated).
>>>>> + The recalculation of the txhash may be delayed to decrease
>>>>>the
>>>>> + likelihood that reordering will trigger retransmissons.
>>>>> + The purpose is to help balance the flows among the possible
>>>>>paths.
>>>>> +
>>>>> tcp_retrans_collapse - BOOLEAN
>>>>> Bug-to-bug compatibility with some broken printers.
>>>>> On retransmit try to send bigger packets to work around bugs
>>>>>in
>>>>> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
>>>>> index a17ae7b..e0e3b7d 100644
>>>>> --- a/include/linux/tcp.h
>>>>> +++ b/include/linux/tcp.h
>>>>> @@ -214,7 +214,9 @@ struct tcp_sock {
>>>>> } rack;
>>>>> u16 advmss; /* Advertised MSS
>>>>>*/
>>>>> u8 rate_app_limited:1, /* rate_{delivered,interval_us}
>>>>>limited? */
>>>>> - unused:7;
>>>>> + txhash_rto:1, /* If set, don't do flow balancing
>>>>>*/
>>>>> + txhash_want:1, /* We want to change txhash when safe
>>>>>*/
>>>>> + unused:5;
>>>>> u8 nonagle : 4,/* Disable Nagle algorithm?
>>>>>*/
>>>>> thin_lto : 1,/* Use linear timeouts for thin
>>>>>streams
>>>>>*/
>>>>> thin_dupack : 1,/* Fast retransmit on first dupack
>>>>>*/
>>>>> diff --git a/include/net/tcp.h b/include/net/tcp.h
>>>>> index f83b7f2..3abd304 100644
>>>>> --- a/include/net/tcp.h
>>>>> +++ b/include/net/tcp.h
>>>>> @@ -271,6 +271,8 @@ extern int sysctl_tcp_autocorking;
>>>>> extern int sysctl_tcp_invalid_ratelimit;
>>>>> extern int sysctl_tcp_pacing_ss_ratio;
>>>>> extern int sysctl_tcp_pacing_ca_ratio;
>>>>> +extern int sysctl_tcp_retrans_txhash_prob;
>>>>> +extern int sysctl_tcp_retrans_txhash_mode;
>>>>>
>>>>> extern atomic_long_t tcp_memory_allocated;
>>>>> extern struct percpu_counter tcp_sockets_allocated;
>>>>> diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
>>>>> index 1cb67de..00d6f26 100644
>>>>> --- a/net/ipv4/sysctl_net_ipv4.c
>>>>> +++ b/net/ipv4/sysctl_net_ipv4.c
>>>>> @@ -28,6 +28,7 @@
>>>>> static int zero;
>>>>> static int one = 1;
>>>>> static int four = 4;
>>>>> +static int hundred = 100;
>>>>> static int thousand = 1000;
>>>>> static int gso_max_segs = GSO_MAX_SEGS;
>>>>> static int tcp_retr1_max = 255;
>>>>> @@ -624,6 +625,23 @@ static struct ctl_table ipv4_table[] = {
>>>>> .proc_handler = proc_dointvec_ms_jiffies,
>>>>> },
>>>>> {
>>>>> + .procname = "tcp_retrans_txhash_prob",
>>>>> + .data = &sysctl_tcp_retrans_txhash_prob,
>>>>> + .maxlen = sizeof(int),
>>>>> + .mode = 0644,
>>>>> + .proc_handler = proc_dointvec_minmax,
>>>>> + .extra1 = &zero,
>>>>> + .extra2 = &hundred,
>>>>> + },
>>>>> + {
>>>>> + .procname = "tcp_retrans_txhash_mode",
>>>>> + .data = &sysctl_tcp_retrans_txhash_mode,
>>>>> + .maxlen = sizeof(int),
>>>>> + .mode = 0644,
>>>>> + .proc_handler = proc_dointvec_minmax,
>>>>> + .extra1 = &zero,
>>>>> + },
>>>>> + {
>>>>> .procname = "icmp_msgs_per_sec",
>>>>> .data = &sysctl_icmp_msgs_per_sec,
>>>>> .maxlen = sizeof(int),
>>>>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>>>>> index a27b9c0..fed5366 100644
>>>>> --- a/net/ipv4/tcp_input.c
>>>>> +++ b/net/ipv4/tcp_input.c
>>>>> @@ -101,6 +101,9 @@ int sysctl_tcp_moderate_rcvbuf __read_mostly = 1;
>>>>> int sysctl_tcp_early_retrans __read_mostly = 3;
>>>>> int sysctl_tcp_invalid_ratelimit __read_mostly = HZ/2;
>>>>>
>>>>> +int sysctl_tcp_retrans_txhash_prob __read_mostly;
>>>>> +int sysctl_tcp_retrans_txhash_mode __read_mostly;
>>>>> +
>>>>> #define FLAG_DATA 0x01 /* Incoming frame contained
>>>>>data.
>>>>> */
>>>>> #define FLAG_WIN_UPDATE 0x02 /* Incoming ACK was a
>>>>>window update. */
>>>>> #define FLAG_DATA_ACKED 0x04 /* This ACK acknowledged
>>>>>new data. */
>>>>> @@ -3674,6 +3677,13 @@ static int tcp_ack(struct sock *sk, const
>>>>>struct
>>>>>sk_buff *skb, int flag)
>>>>> flag |= tcp_clean_rtx_queue(sk, prior_fackets, prior_snd_una,
>>>>>&acked,
>>>>> &sack_state, &now);
>>>>>
>>>>> + /* Check if we should set txhash (would not cause
>>>>>reordering) */
>>>>> + if (tp->txhash_want &&
>>>>> + (tp->packets_out - tp->sacked_out) < tp->reordering) {
>>>>> + sk_set_txhash(sk);
>>>>> + tp->txhash_want = 0;
>>>>> + }
>>>>> +
>>>>> if (tcp_ack_is_dubious(sk, flag)) {
>>>>> is_dupack = !(flag & (FLAG_SND_UNA_ADVANCED |
>>>>>FLAG_NOT_DUP));
>>>>> tcp_fastretrans_alert(sk, acked, is_dupack, &flag,
>>>>>&rexmit);
>>>>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>>>>> index 896e9df..58490ac 100644
>>>>> --- a/net/ipv4/tcp_output.c
>>>>> +++ b/net/ipv4/tcp_output.c
>>>>> @@ -2738,9 +2738,30 @@ int tcp_retransmit_skb(struct sock *sk, struct
>>>>>sk_buff *skb, int segs)
>>>>> tp->retrans_out += tcp_skb_pcount(skb);
>>>>>
>>>>> /* Save stamp of the first retransmit. */
>>>>> - if (!tp->retrans_stamp)
>>>>> + if (!tp->retrans_stamp) {
>>>>> tp->retrans_stamp = tcp_skb_timestamp(skb);
>>>>>
>>>>> + /* Determine if we should reset hash, only
>>>>>done
>>>>>once
>>>>> + * per recovery
>>>>> + */
>>>>> + if ((!tp->txhash_rto ||
>>>>> + sysctl_tcp_retrans_txhash_mode > 0) &&
>>>>> + sk->sk_txhash &&
>>>>> + (prandom_u32_max(100) <
>>>>> + sysctl_tcp_retrans_txhash_prob)) {
>>>>> + /* If not too much reordering, or
>>>>>RTT is
>>>>> + * small enough that we don't care
>>>>>about
>>>>> + * reordering, then change it now.
>>>>> + * Else, wait until it is safe.
>>>>> + */
>>>>> + if ((tp->packets_out -
>>>>>tp->sacked_out) <
>>>>> + tp->reordering)
>>>>I don't parse this logic ... suppose reordering is 100 (not uncommon
>>>>today due to the last packet being delivered slightly earlier than the
>>>>rest), and cwnd==packets_out =~200,we only want to rehash until half
>>>>of the packets are sacked, so we are still rehashing even when
>>>>reordering is heavy?
>>>
>>> In your scenario, there would be no re-hashing until sacked_out is 101
>>> ((packets_out - sacked_out) < 100). This code would mark txhash_want.
>>> Then when an ACK is received and the conditional is true, txhash
>>> would be changed.
>>>
>>> Now, the test does not prevent retransmissions due to reordering in all
>>> cases, but hopefully in most. I will also add the test you recommended,
>>> checking for CA_Loss, to prevent too much re-hashing.
>> If the whole point is to rehash at most once between recovery events,
>> why do we need this complicated change at per packet level in
>> tcp_retransmit_skb()? there are functions that start and end recovery
>> (tcp_enter_recovery, tcp_end_cwnd_reduction). our retransmission logic
>> is already very complicated.
>>
>> I still don't understand the incentive of starting the rehashing
>> half-way retransmitting depending on the sacking and reordering
>> status, for temporarily congestion as you've mentioned.
>>
>> is this feature unique for intra-DC connections?
>I thought more about this patch on my way home and have more
>questions: why do we exclude RTO retransmission specifically? also
>when we rehash, we'll introduce reordering either in recovery or after
>recovery, as some TCP CC like bbr would continue sending regardlessly,
>so starting in tcp_ack() with tp->txhash_want does not really prevent
>causing more reordering.
>
>so if we want to rehash upon a recovery event (fast or timeout), then
>we can make a helper function ie tcp_retry_alt_path, and just call it
>in tcp_enter_loss and tcp_recovery (which are called once per
>recovery). Since it'll cause reordering anyways, I doubt we need to
>check tp->reordering at all. If packets already traverse different
>paths, rehashing can make it better or worse. if packets weren't
>traversing different paths, the rehashing itself can introduce
>reordering. so in the end there is much benefit to make decision based
>on current inflight and reordering IMO...
>
>
>>
>>>
>>>>
>>>>also where do we check RTT is small?
>>>
>>> The RTT comment is left over from a previous version, I will remove it.
>>>
>>>>
>>>>> + sk_set_txhash(sk);
>>>>> + else
>>>>> + tp->txhash_want = 1;
>>>>> + }
>>>>> + }
>>>>> +
>>>>> } else if (err != -EBUSY) {
>>>>> NET_INC_STATS(sock_net(sk),
>>>>>LINUX_MIB_TCPRETRANSFAIL);
>>>>> }
>>>>> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
>>>>> index 3ea1cf8..e66baad 100644
>>>>> --- a/net/ipv4/tcp_timer.c
>>>>> +++ b/net/ipv4/tcp_timer.c
>>>>> @@ -186,6 +186,8 @@ static int tcp_write_timeout(struct sock *sk)
>>>>>
>>>>> if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
>>>>> if (icsk->icsk_retransmits) {
>>>>> + tp->txhash_rto = 1;
>>>>> + tp->txhash_want = 0;
>>>>> dst_negative_advice(sk);
>>>>> if (tp->syn_fastopen || tp->syn_data)
>>>>> tcp_fastopen_cache_set(sk, 0, NULL,
>>>>>true, 0);
>>>>> @@ -218,6 +220,8 @@ static int tcp_write_timeout(struct sock *sk)
>>>>> } else {
>>>>> sk_rethink_txhash(sk);
>>>>> }
>>>>> + tp->txhash_rto = 1;
>>>>> + tp->txhash_want = 0;
>>>>>
>>>>> retry_until = net->ipv4.sysctl_tcp_retries2;
>>>>> if (sock_flag(sk, SOCK_DEAD)) {
>>>>> --
>>>>> 2.9.3
>>>>>
>>>
Powered by blists - more mailing lists