| Message ID | d7fbf3d3a2490d0a9e99945593ada243da58e0f8.1619000255.git.cdleonard@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | [RFC] tcp: Delay sending non-probes for RFC4821 mtu probing | expand |
On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote: > > According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes > in order to accumulate enough data" but linux almost never does that. > > Linux waits for probe_size + (1 + retries) * mss_cache to be available > in the send buffer and if that condition is not met it will send anyway > using the current MSS. The feature can be made to work by sending very > large chunks of data from userspace (for example 128k) but for small writes > on fast links probes almost never happen. > > This patch tries to implement the "MAY" by adding an extra flag > "wait_data" to icsk_mtup which is set to 1 if a probe is possible but > insufficient data is available. Then data is held back in > tcp_write_xmit until a probe is sent, probing conditions are no longer > met, or 500ms pass. > > Signed-off-by: Leonard Crestez <cdleonard@gmail.com> > > --- > Documentation/networking/ip-sysctl.rst | 4 ++ > include/net/inet_connection_sock.h | 7 +++- > include/net/netns/ipv4.h | 1 + > include/net/tcp.h | 2 + > net/ipv4/sysctl_net_ipv4.c | 7 ++++ > net/ipv4/tcp_ipv4.c | 1 + > net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++-- > 7 files changed, 71 insertions(+), 5 deletions(-) > > My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing > > This patch makes the test pass quite reliably with > ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while > before it only worked with much higher IPERF_LEN=256k > > In my loopback tests I also observed another issue when tcp_retries > increases because of SACKReorder. This makes the original problem worse > (since the retries amount factors in buffer requirement) and seems to be > unrelated issue. Maybe when loss happens due to MTU shrinkage the sender > sack logic is confused somehow? > > I know it's towards the end of the cycle but this is mostly just intended for > discussion. Thanks for raising the question of how to trigger PMTU probes more often! AFAICT this approach would cause unacceptable performance impacts by often injecting unnecessary 500ms delays when there is no need to do so. If the goal is to increase the frequency of PMTU probes, which seems like a valid goal, I would suggest that we rethink the Linux heuristic for triggering PMTU probes in the light of the fact that the loss detection mechanism is now RACK-TLP, which provides quick recovery in a much wider variety of scenarios. After all, https://tools.ietf.org/html/rfc4821#section-7.4 says: In addition, the timely loss detection algorithms in most protocols have pre-conditions that SHOULD be satisfied before sending a probe. And we know that the "timely loss detection algorithms" have advanced since this RFC was written in 2007. You mention: > Linux waits for probe_size + (1 + retries) * mss_cache to be available The code in question seems to be: size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache; How about just changing this to: size_needed = probe_size + tp->mss_cache; The rationale would be that if that amount of data is available, then the sender can send one probe and one following current-mss-size packet. If the path MTU has not increased to allow the probe of size probe_size to pass through the network, then the following current-mss-size packet will likely pass through the network, generate a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the RACK reorder timer fires. A secondary rationale for this heuristic would be: if the flow never accumulates roughly two packets worth of data, then does the flow really need a bigger packet size? IMHO, just reducing the size_needed seems far preferable to needlessly injecting 500ms delays. best, neal
(Resending in plain text mode)
Surely there is a way to adapt tcp_tso_should_defer(), it is trying to
solve a similar problem.
If I were to implement PLPMTUD today, I would more deeply entwine it
into TCP's support for TSO. e.g. successful deferring segments
sometimes enables TSO and sometimes enables PLPMTUD.
But there is a deeper question: John Heffner and I invested a huge
amount of energy in trying to make PLPMTUD work for opportunistic
Jumbo discovery, only to discover that we had moved the problem down
to the device driver/nic, were it isn't so readily solvable.
The driver needs to carve nic buffer memory before it can communicate
with a switch (to either ask or measure the MTU), and once it has done
that it needs to either re-carve the memory or run with suboptimal
carving. Both of these are problematic.
There is also a problem that many link technologies will
non-deterministically deliver jumbo frames at greatly increased error
rates. This issue requires a long conversation on it's own.
Thanks,
--MM--
The best way to predict the future is to create it. - Alan Kay
We must not tolerate intolerance;
however our response must be carefully measured:
too strong would be hypocritical and risks spiraling out of control;
too weak risks being mistaken for tacit approval.
On Wed, Apr 21, 2021 at 5:48 AM Neal Cardwell <ncardwell@google.com> wrote:
>
> On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote:
> >
> > According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes
> > in order to accumulate enough data" but linux almost never does that.
> >
> > Linux waits for probe_size + (1 + retries) * mss_cache to be available
> > in the send buffer and if that condition is not met it will send anyway
> > using the current MSS. The feature can be made to work by sending very
> > large chunks of data from userspace (for example 128k) but for small writes
> > on fast links probes almost never happen.
> >
> > This patch tries to implement the "MAY" by adding an extra flag
> > "wait_data" to icsk_mtup which is set to 1 if a probe is possible but
> > insufficient data is available. Then data is held back in
> > tcp_write_xmit until a probe is sent, probing conditions are no longer
> > met, or 500ms pass.
> >
> > Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
> >
> > ---
> > Documentation/networking/ip-sysctl.rst | 4 ++
> > include/net/inet_connection_sock.h | 7 +++-
> > include/net/netns/ipv4.h | 1 +
> > include/net/tcp.h | 2 +
> > net/ipv4/sysctl_net_ipv4.c | 7 ++++
> > net/ipv4/tcp_ipv4.c | 1 +
> > net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++--
> > 7 files changed, 71 insertions(+), 5 deletions(-)
> >
> > My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing
> >
> > This patch makes the test pass quite reliably with
> > ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while
> > before it only worked with much higher IPERF_LEN=256k
> >
> > In my loopback tests I also observed another issue when tcp_retries
> > increases because of SACKReorder. This makes the original problem worse
> > (since the retries amount factors in buffer requirement) and seems to be
> > unrelated issue. Maybe when loss happens due to MTU shrinkage the sender
> > sack logic is confused somehow?
> >
> > I know it's towards the end of the cycle but this is mostly just intended for
> > discussion.
>
> Thanks for raising the question of how to trigger PMTU probes more often!
>
> AFAICT this approach would cause unacceptable performance impacts by
> often injecting unnecessary 500ms delays when there is no need to do
> so.
>
> If the goal is to increase the frequency of PMTU probes, which seems
> like a valid goal, I would suggest that we rethink the Linux heuristic
> for triggering PMTU probes in the light of the fact that the loss
> detection mechanism is now RACK-TLP, which provides quick recovery in
> a much wider variety of scenarios.
>
> After all, https://tools.ietf.org/html/rfc4821#section-7.4 says:
>
> In addition, the timely loss detection algorithms in most protocols
> have pre-conditions that SHOULD be satisfied before sending a probe.
>
> And we know that the "timely loss detection algorithms" have advanced
> since this RFC was written in 2007.
>
> You mention:
> > Linux waits for probe_size + (1 + retries) * mss_cache to be available
>
> The code in question seems to be:
>
> size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
>
> How about just changing this to:
>
> size_needed = probe_size + tp->mss_cache;
>
> The rationale would be that if that amount of data is available, then
> the sender can send one probe and one following current-mss-size
> packet. If the path MTU has not increased to allow the probe of size
> probe_size to pass through the network, then the following
> current-mss-size packet will likely pass through the network, generate
> a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the
> RACK reorder timer fires.
>
> A secondary rationale for this heuristic would be: if the flow never
> accumulates roughly two packets worth of data, then does the flow
> really need a bigger packet size?
>
> IMHO, just reducing the size_needed seems far preferable to needlessly
> injecting 500ms delays.
>
> best,
> neal
On 4/21/21 7:45 PM, Matt Mathis wrote: > (Resending in plain text mode) > > Surely there is a way to adapt tcp_tso_should_defer(), it is trying to > solve a similar problem. > > If I were to implement PLPMTUD today, I would more deeply entwine it > into TCP's support for TSO. e.g. successful deferring segments > sometimes enables TSO and sometimes enables PLPMTUD. The mechanisms for delaying sending are difficult to understand, this RFC just added a brand-new unrelated timer. Intertwining it with existing mechanisms would indeed be better. On a closer look it seems that they're not actually based on a timer but other heuristics. It seems that tcp_sendmsg will "tcp_push_one" once the skb at the head of the queue reaches tcp_xmit_size_goal and tcp_xmit_size_goal does not take mtu probing into account. In practice this would mean that application-limited streams won't perform mtu probing unless a single write is 5*mss + probe_size (1*mss over size_needed) I sent a different RFC which tries to modify tcp_xmit_size_goal. > But there is a deeper question: John Heffner and I invested a huge > amount of energy in trying to make PLPMTUD work for opportunistic > Jumbo discovery, only to discover that we had moved the problem down > to the device driver/nic, were it isn't so readily solvable. > > The driver needs to carve nic buffer memory before it can communicate > with a switch (to either ask or measure the MTU), and once it has done > that it needs to either re-carve the memory or run with suboptimal > carving. Both of these are problematic. > > There is also a problem that many link technologies will > non-deterministically deliver jumbo frames at greatly increased error > rates. This issue requires a long conversation on it's own. I'm looking to improve this for tunnels that don't correctly send ICMP packet-too-big messages, the hardware is assumed to be fine. -- Regards, Leonard
On 4/21/21 3:47 PM, Neal Cardwell wrote: > On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote: >> >> According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes >> in order to accumulate enough data" but linux almost never does that. >> >> Linux waits for probe_size + (1 + retries) * mss_cache to be available >> in the send buffer and if that condition is not met it will send anyway >> using the current MSS. The feature can be made to work by sending very >> large chunks of data from userspace (for example 128k) but for small writes >> on fast links probes almost never happen. >> >> This patch tries to implement the "MAY" by adding an extra flag >> "wait_data" to icsk_mtup which is set to 1 if a probe is possible but >> insufficient data is available. Then data is held back in >> tcp_write_xmit until a probe is sent, probing conditions are no longer >> met, or 500ms pass. >> >> Signed-off-by: Leonard Crestez <cdleonard@gmail.com> >> >> --- >> Documentation/networking/ip-sysctl.rst | 4 ++ >> include/net/inet_connection_sock.h | 7 +++- >> include/net/netns/ipv4.h | 1 + >> include/net/tcp.h | 2 + >> net/ipv4/sysctl_net_ipv4.c | 7 ++++ >> net/ipv4/tcp_ipv4.c | 1 + >> net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++-- >> 7 files changed, 71 insertions(+), 5 deletions(-) >> >> My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing >> >> This patch makes the test pass quite reliably with >> ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while >> before it only worked with much higher IPERF_LEN=256k >> >> In my loopback tests I also observed another issue when tcp_retries >> increases because of SACKReorder. This makes the original problem worse >> (since the retries amount factors in buffer requirement) and seems to be >> unrelated issue. Maybe when loss happens due to MTU shrinkage the sender >> sack logic is confused somehow? >> >> I know it's towards the end of the cycle but this is mostly just intended for >> discussion. > > Thanks for raising the question of how to trigger PMTU probes more often! > > AFAICT this approach would cause unacceptable performance impacts by > often injecting unnecessary 500ms delays when there is no need to do > so. > > If the goal is to increase the frequency of PMTU probes, which seems > like a valid goal, I would suggest that we rethink the Linux heuristic > for triggering PMTU probes in the light of the fact that the loss > detection mechanism is now RACK-TLP, which provides quick recovery in > a much wider variety of scenarios. > After all, https://tools.ietf.org/html/rfc4821#section-7.4 says: > > In addition, the timely loss detection algorithms in most protocols > have pre-conditions that SHOULD be satisfied before sending a probe. > > And we know that the "timely loss detection algorithms" have advanced > since this RFC was written in 2007. > > You mention: >> Linux waits for probe_size + (1 + retries) * mss_cache to be available > > The code in question seems to be: > > size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache; As far as I understand this is meant to work with classical retransmit: if 3 dupacks are received then the first segment is considered lost and probe success or failure is can determine within roughly 1*rtt. RACK marks segments as lost based on echoed timestamps so it doesn't need multiple segments. The minimum time interval is only a little higher (5/4 rtt). Is this correct? > How about just changing this to: > > size_needed = probe_size + tp->mss_cache; > > The rationale would be that if that amount of data is available, then > the sender can send one probe and one following current-mss-size > packet. If the path MTU has not increased to allow the probe of size > probe_size to pass through the network, then the following > current-mss-size packet will likely pass through the network, generate > a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the > RACK reorder timer fires. This appears to almost work except it stalls after a while. I spend some time investigating it and it seems that cwnd is shrunk on mss increases and does not go back up. This causes probes to be skipped because of a "snd_cwnd < 11" condition. I don't undestand where that magical "11" comes from, could that be shrunk. Maybe it's meant to only send probes when the cwnd is above the default of 10? Then maybe mtu_probe_success shouldn't shrink mss below what is required for an additional probe, or at least round-up. The shrinkage of cwnd is a problem with this "short probes" approach because tcp_is_cwnd_limited returns false because tp->max_packets_out is smaller (4). With longer probes tp->max_packets_out is larger (6) so tcp_is_cwnd_limited returns true even for a cwnd of 10. I'm testing using namespace-to-namespace loopback so my delays are close to zero. I tried to introduce an artificial delay of 30ms (using tc netem) and it works but 20ms does not. > A secondary rationale for this heuristic would be: if the flow never > accumulates roughly two packets worth of data, then does the flow > really need a bigger packet size? The problem is that "accumulating sufficient data" is an extremely fuzzy concept. In particular it seems that at the same traffic level performing shorter writes from userspace (2kb instead of 64k) can prevent mtu probing entirely and this is unreasonable. -- Regards, Leonard
First RFC 4821 was finished in 2007, but most of the work was done
several years earlier. In the intervening years just about
everything else in TCP has been overhauled. The 11 packets was
probably reasonable for Reno with early retransmit and non adaptive
reordering. These are archaic algorithms by today's standards.
You have two problems that you need to address:
1) like TSO you need to hold back transmits that are otherwise
eligible to be sent, such that you have enough data in the backlog to
send a probe or TSO.
2) you need to make sure that TCP recovers promptly when a probe is
lost (which is the most common case). This is not so hard on a well
behaved network, but is likely to make your head hurt in the presence
of reordering (e,g, ECMP w/o flow pinning). RACK helps, I'm sure.
Thanks,
--MM--
The best way to predict the future is to create it. - Alan Kay
We must not tolerate intolerance;
however our response must be carefully measured:
too strong would be hypocritical and risks spiraling out of control;
too weak risks being mistaken for tacit approval.
On Sun, Apr 25, 2021 at 7:34 PM Leonard Crestez <lcrestez@drivenets.com> wrote:
>
> On 4/21/21 3:47 PM, Neal Cardwell wrote:
> > On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote:
> >>
> >> According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes
> >> in order to accumulate enough data" but linux almost never does that.
> >>
> >> Linux waits for probe_size + (1 + retries) * mss_cache to be available
> >> in the send buffer and if that condition is not met it will send anyway
> >> using the current MSS. The feature can be made to work by sending very
> >> large chunks of data from userspace (for example 128k) but for small writes
> >> on fast links probes almost never happen.
> >>
> >> This patch tries to implement the "MAY" by adding an extra flag
> >> "wait_data" to icsk_mtup which is set to 1 if a probe is possible but
> >> insufficient data is available. Then data is held back in
> >> tcp_write_xmit until a probe is sent, probing conditions are no longer
> >> met, or 500ms pass.
> >>
> >> Signed-off-by: Leonard Crestez <cdleonard@gmail.com>
> >>
> >> ---
> >> Documentation/networking/ip-sysctl.rst | 4 ++
> >> include/net/inet_connection_sock.h | 7 +++-
> >> include/net/netns/ipv4.h | 1 +
> >> include/net/tcp.h | 2 +
> >> net/ipv4/sysctl_net_ipv4.c | 7 ++++
> >> net/ipv4/tcp_ipv4.c | 1 +
> >> net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++--
> >> 7 files changed, 71 insertions(+), 5 deletions(-)
> >>
> >> My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing
> >>
> >> This patch makes the test pass quite reliably with
> >> ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while
> >> before it only worked with much higher IPERF_LEN=256k
> >>
> >> In my loopback tests I also observed another issue when tcp_retries
> >> increases because of SACKReorder. This makes the original problem worse
> >> (since the retries amount factors in buffer requirement) and seems to be
> >> unrelated issue. Maybe when loss happens due to MTU shrinkage the sender
> >> sack logic is confused somehow?
> >>
> >> I know it's towards the end of the cycle but this is mostly just intended for
> >> discussion.
> >
> > Thanks for raising the question of how to trigger PMTU probes more often!
> >
> > AFAICT this approach would cause unacceptable performance impacts by
> > often injecting unnecessary 500ms delays when there is no need to do
> > so.
> >
> > If the goal is to increase the frequency of PMTU probes, which seems
> > like a valid goal, I would suggest that we rethink the Linux heuristic
> > for triggering PMTU probes in the light of the fact that the loss
> > detection mechanism is now RACK-TLP, which provides quick recovery in
> > a much wider variety of scenarios.
>
> > After all, https://tools.ietf.org/html/rfc4821#section-7.4 says:
> >
> > In addition, the timely loss detection algorithms in most protocols
> > have pre-conditions that SHOULD be satisfied before sending a probe.
> >
> > And we know that the "timely loss detection algorithms" have advanced
> > since this RFC was written in 2007. >
> > You mention:
> >> Linux waits for probe_size + (1 + retries) * mss_cache to be available
> >
> > The code in question seems to be:
> >
> > size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
>
> As far as I understand this is meant to work with classical retransmit:
> if 3 dupacks are received then the first segment is considered lost and
> probe success or failure is can determine within roughly 1*rtt. RACK
> marks segments as lost based on echoed timestamps so it doesn't need
> multiple segments. The minimum time interval is only a little higher
> (5/4 rtt). Is this correct?
>
> > How about just changing this to:
> >
> > size_needed = probe_size + tp->mss_cache;
> >
> > The rationale would be that if that amount of data is available, then
> > the sender can send one probe and one following current-mss-size
> > packet. If the path MTU has not increased to allow the probe of size
> > probe_size to pass through the network, then the following
> > current-mss-size packet will likely pass through the network, generate
> > a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the
> > RACK reorder timer fires.
>
> This appears to almost work except it stalls after a while. I spend some
> time investigating it and it seems that cwnd is shrunk on mss increases
> and does not go back up. This causes probes to be skipped because of a
> "snd_cwnd < 11" condition.
>
> I don't undestand where that magical "11" comes from, could that be
> shrunk. Maybe it's meant to only send probes when the cwnd is above the
> default of 10? Then maybe mtu_probe_success shouldn't shrink mss below
> what is required for an additional probe, or at least round-up.
>
> The shrinkage of cwnd is a problem with this "short probes" approach
> because tcp_is_cwnd_limited returns false because tp->max_packets_out is
> smaller (4). With longer probes tp->max_packets_out is larger (6) so
> tcp_is_cwnd_limited returns true even for a cwnd of 10.
>
> I'm testing using namespace-to-namespace loopback so my delays are close
> to zero. I tried to introduce an artificial delay of 30ms (using tc
> netem) and it works but 20ms does not.
>
> > A secondary rationale for this heuristic would be: if the flow never
> > accumulates roughly two packets worth of data, then does the flow
> > really need a bigger packet size?
>
> The problem is that "accumulating sufficient data" is an extremely fuzzy
> concept. In particular it seems that at the same traffic level
> performing shorter writes from userspace (2kb instead of 64k) can
> prevent mtu probing entirely and this is unreasonable.
>
> --
> Regards,
> Leonard
On Sun, Apr 25, 2021 at 10:34 PM Leonard Crestez <lcrestez@drivenets.com> wrote: > > On 4/21/21 3:47 PM, Neal Cardwell wrote: > > On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote: > >> > >> According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes > >> in order to accumulate enough data" but linux almost never does that. > >> > >> Linux waits for probe_size + (1 + retries) * mss_cache to be available > >> in the send buffer and if that condition is not met it will send anyway > >> using the current MSS. The feature can be made to work by sending very > >> large chunks of data from userspace (for example 128k) but for small writes > >> on fast links probes almost never happen. > >> > >> This patch tries to implement the "MAY" by adding an extra flag > >> "wait_data" to icsk_mtup which is set to 1 if a probe is possible but > >> insufficient data is available. Then data is held back in > >> tcp_write_xmit until a probe is sent, probing conditions are no longer > >> met, or 500ms pass. > >> > >> Signed-off-by: Leonard Crestez <cdleonard@gmail.com> > >> > >> --- > >> Documentation/networking/ip-sysctl.rst | 4 ++ > >> include/net/inet_connection_sock.h | 7 +++- > >> include/net/netns/ipv4.h | 1 + > >> include/net/tcp.h | 2 + > >> net/ipv4/sysctl_net_ipv4.c | 7 ++++ > >> net/ipv4/tcp_ipv4.c | 1 + > >> net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++-- > >> 7 files changed, 71 insertions(+), 5 deletions(-) > >> > >> My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing > >> > >> This patch makes the test pass quite reliably with > >> ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while > >> before it only worked with much higher IPERF_LEN=256k > >> > >> In my loopback tests I also observed another issue when tcp_retries > >> increases because of SACKReorder. This makes the original problem worse > >> (since the retries amount factors in buffer requirement) and seems to be > >> unrelated issue. Maybe when loss happens due to MTU shrinkage the sender > >> sack logic is confused somehow? > >> > >> I know it's towards the end of the cycle but this is mostly just intended for > >> discussion. > > > > Thanks for raising the question of how to trigger PMTU probes more often! > > > > AFAICT this approach would cause unacceptable performance impacts by > > often injecting unnecessary 500ms delays when there is no need to do > > so. > > > > If the goal is to increase the frequency of PMTU probes, which seems > > like a valid goal, I would suggest that we rethink the Linux heuristic > > for triggering PMTU probes in the light of the fact that the loss > > detection mechanism is now RACK-TLP, which provides quick recovery in > > a much wider variety of scenarios. > > > After all, https://tools.ietf.org/html/rfc4821#section-7.4 says: > > > > In addition, the timely loss detection algorithms in most protocols > > have pre-conditions that SHOULD be satisfied before sending a probe. > > > > And we know that the "timely loss detection algorithms" have advanced > > since this RFC was written in 2007. > > > You mention: > >> Linux waits for probe_size + (1 + retries) * mss_cache to be available > > > > The code in question seems to be: > > > > size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache; > > As far as I understand this is meant to work with classical retransmit: > if 3 dupacks are received then the first segment is considered lost and > probe success or failure is can determine within roughly 1*rtt. Yes, that is my sense as well. > RACK > marks segments as lost based on echoed timestamps so it doesn't need > multiple segments. The minimum time interval is only a little higher > (5/4 rtt). Is this correct? That's basically the case, though RACK doesn't even require echoed timestamps. > > How about just changing this to: > > > > size_needed = probe_size + tp->mss_cache; > > > > The rationale would be that if that amount of data is available, then > > the sender can send one probe and one following current-mss-size > > packet. If the path MTU has not increased to allow the probe of size > > probe_size to pass through the network, then the following > > current-mss-size packet will likely pass through the network, generate > > a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the > > RACK reorder timer fires. > > This appears to almost work except it stalls after a while. I spend some > time investigating it and it seems that cwnd is shrunk on mss increases > and does not go back up. This causes probes to be skipped because of a > "snd_cwnd < 11" condition. > > I don't undestand where that magical "11" comes from, could that be > shrunk. Maybe it's meant to only send probes when the cwnd is above the > default of 10? Then maybe mtu_probe_success shouldn't shrink mss below > what is required for an additional probe, or at least round-up. > > The shrinkage of cwnd is a problem with this "short probes" approach > because tcp_is_cwnd_limited returns false because tp->max_packets_out is > smaller (4). With longer probes tp->max_packets_out is larger (6) so > tcp_is_cwnd_limited returns true even for a cwnd of 10. > > I'm testing using namespace-to-namespace loopback so my delays are close > to zero. I tried to introduce an artificial delay of 30ms (using tc > netem) and it works but 20ms does not. I agree the magic 11 seems outdated and unnecessarily high, given RACK-TLP. I think it would be fine to change the magic 11 to a magic (TCP_FASTRETRANS_THRESH+1), aka 3+1=4: - tp->snd_cwnd < 11 || + p->snd_cwnd < (TCP_FASTRETRANS_THRESH + 1) || As long as the cwnd is >= TCP_FASTRETRANS_THRESH+1 then the sender should usually be able to send the 1 probe packet and then 3 additional packets beyond the probe, and in the common case (with no reordering) then with failed probes this should allow the sender to quickly receive 3 SACKed segments and enter fast recovery quickly. Even if the sender doesn't have 3 additional packets, or if reordering has been detected, then RACK-TLP should be able to start recovery quickly (5/4*RTT if there is at least one SACK, or 2*RTT for a TLP if there is no SACK). > > A secondary rationale for this heuristic would be: if the flow never > > accumulates roughly two packets worth of data, then does the flow > > really need a bigger packet size? > > The problem is that "accumulating sufficient data" is an extremely fuzzy > concept. In particular it seems that at the same traffic level > performing shorter writes from userspace (2kb instead of 64k) can > prevent mtu probing entirely and this is unreasonable. Something like your autocorking-enhanced-PMTU patch sounds like a reasonable way to deal with this. thanks, neal
On 26.04.2021 18:59, Neal Cardwell wrote: > On Sun, Apr 25, 2021 at 10:34 PM Leonard Crestez <lcrestez@drivenets.com> wrote: >> On 4/21/21 3:47 PM, Neal Cardwell wrote: >>> On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote: >>> If the goal is to increase the frequency of PMTU probes, which seems >>> like a valid goal, I would suggest that we rethink the Linux heuristic >>> for triggering PMTU probes in the light of the fact that the loss >>> detection mechanism is now RACK-TLP, which provides quick recovery in >>> a much wider variety of scenarios. >> >>> You mention: >>>> Linux waits for probe_size + (1 + retries) * mss_cache to be available >>> >>> The code in question seems to be: >>> >>> size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache; >>> How about just changing this to: >>> >>> size_needed = probe_size + tp->mss_cache; >>> >>> The rationale would be that if that amount of data is available, then >>> the sender can send one probe and one following current-mss-size >>> packet. If the path MTU has not increased to allow the probe of size >>> probe_size to pass through the network, then the following >>> current-mss-size packet will likely pass through the network, generate >>> a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the >>> RACK reorder timer fires. >> >> This appears to almost work except it stalls after a while. I spend some >> time investigating it and it seems that cwnd is shrunk on mss increases >> and does not go back up. This causes probes to be skipped because of a >> "snd_cwnd < 11" condition. >> >> I don't undestand where that magical "11" comes from, could that be >> shrunk. Maybe it's meant to only send probes when the cwnd is above the >> default of 10? Then maybe mtu_probe_success shouldn't shrink mss below >> what is required for an additional probe, or at least round-up. >> >> The shrinkage of cwnd is a problem with this "short probes" approach >> because tcp_is_cwnd_limited returns false because tp->max_packets_out is >> smaller (4). With longer probes tp->max_packets_out is larger (6) so >> tcp_is_cwnd_limited returns true even for a cwnd of 10. >> >> I'm testing using namespace-to-namespace loopback so my delays are close >> to zero. I tried to introduce an artificial delay of 30ms (using tc >> netem) and it works but 20ms does not. > > I agree the magic 11 seems outdated and unnecessarily high, given RACK-TLP. > > I think it would be fine to change the magic 11 to a magic > (TCP_FASTRETRANS_THRESH+1), aka 3+1=4: > > - tp->snd_cwnd < 11 || > + p->snd_cwnd < (TCP_FASTRETRANS_THRESH + 1) || > > As long as the cwnd is >= TCP_FASTRETRANS_THRESH+1 then the sender > should usually be able to send the 1 probe packet and then 3 > additional packets beyond the probe, and in the common case (with no > reordering) then with failed probes this should allow the sender to > quickly receive 3 SACKed segments and enter fast recovery quickly. > Even if the sender doesn't have 3 additional packets, or if reordering > has been detected, then RACK-TLP should be able to start recovery > quickly (5/4*RTT if there is at least one SACK, or 2*RTT for a TLP if > there is no SACK). As far as I understand tp->reordering is a dynamic evaluation of the fastretrans threshold to deal with environments with lots of reordering. Your suggestion seems equivalent to the current size_needed calculation except using packets instead of bytes. Wouldn't it be easier to drop the "11" check and just verify that size_needed fits into cwnd as bytes? -- Regards, Leonard
On Mon, Apr 26, 2021 at 1:09 PM Leonard Crestez <lcrestez@drivenets.com> wrote: > > On 26.04.2021 18:59, Neal Cardwell wrote: > > On Sun, Apr 25, 2021 at 10:34 PM Leonard Crestez <lcrestez@drivenets.com> wrote: > >> On 4/21/21 3:47 PM, Neal Cardwell wrote: > >>> On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote: > > >>> If the goal is to increase the frequency of PMTU probes, which seems > >>> like a valid goal, I would suggest that we rethink the Linux heuristic > >>> for triggering PMTU probes in the light of the fact that the loss > >>> detection mechanism is now RACK-TLP, which provides quick recovery in > >>> a much wider variety of scenarios. > >> > >>> You mention: > >>>> Linux waits for probe_size + (1 + retries) * mss_cache to be available > >>> > >>> The code in question seems to be: > >>> > >>> size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache; > >>> How about just changing this to: > >>> > >>> size_needed = probe_size + tp->mss_cache; > >>> > >>> The rationale would be that if that amount of data is available, then > >>> the sender can send one probe and one following current-mss-size > >>> packet. If the path MTU has not increased to allow the probe of size > >>> probe_size to pass through the network, then the following > >>> current-mss-size packet will likely pass through the network, generate > >>> a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the > >>> RACK reorder timer fires. > >> > >> This appears to almost work except it stalls after a while. I spend some > >> time investigating it and it seems that cwnd is shrunk on mss increases > >> and does not go back up. This causes probes to be skipped because of a > >> "snd_cwnd < 11" condition. > >> > >> I don't undestand where that magical "11" comes from, could that be > >> shrunk. Maybe it's meant to only send probes when the cwnd is above the > >> default of 10? Then maybe mtu_probe_success shouldn't shrink mss below > >> what is required for an additional probe, or at least round-up. > >> > >> The shrinkage of cwnd is a problem with this "short probes" approach > >> because tcp_is_cwnd_limited returns false because tp->max_packets_out is > >> smaller (4). With longer probes tp->max_packets_out is larger (6) so > >> tcp_is_cwnd_limited returns true even for a cwnd of 10. > >> > >> I'm testing using namespace-to-namespace loopback so my delays are close > >> to zero. I tried to introduce an artificial delay of 30ms (using tc > >> netem) and it works but 20ms does not. > > > > I agree the magic 11 seems outdated and unnecessarily high, given RACK-TLP. > > > > I think it would be fine to change the magic 11 to a magic > > (TCP_FASTRETRANS_THRESH+1), aka 3+1=4: > > > > - tp->snd_cwnd < 11 || > > + p->snd_cwnd < (TCP_FASTRETRANS_THRESH + 1) || > > > > As long as the cwnd is >= TCP_FASTRETRANS_THRESH+1 then the sender > > should usually be able to send the 1 probe packet and then 3 > > additional packets beyond the probe, and in the common case (with no > > reordering) then with failed probes this should allow the sender to > > quickly receive 3 SACKed segments and enter fast recovery quickly. > > Even if the sender doesn't have 3 additional packets, or if reordering > > has been detected, then RACK-TLP should be able to start recovery > > quickly (5/4*RTT if there is at least one SACK, or 2*RTT for a TLP if > > there is no SACK). > > As far as I understand tp->reordering is a dynamic evaluation of the > fastretrans threshold to deal with environments with lots of reordering. > Your suggestion seems equivalent to the current size_needed calculation > except using packets instead of bytes. > > Wouldn't it be easier to drop the "11" check and just verify that > size_needed fits into cwnd as bytes? Yes, that sounds good to me (dropping the "11" check in favor of verifying that size_needed fits into the cwnd). neal
diff --git a/Documentation/networking/ip-sysctl.rst b/Documentation/networking/ip-sysctl.rst index c2ecc9894fd0..47945a6b435d 100644 --- a/Documentation/networking/ip-sysctl.rst +++ b/Documentation/networking/ip-sysctl.rst @@ -550,10 +550,14 @@ tcp_probe_interval - UNSIGNED INTEGER tcp_probe_threshold - INTEGER Controls when TCP Packetization-Layer Path MTU Discovery probing will stop in respect to the width of search range in bytes. Default is 8 bytes. +tcp_probe_wait - INTEGER + How long to wait for data for a tcp mtu probe. The default is 500 + milliseconds, zero can be used to disable this feature. + tcp_no_metrics_save - BOOLEAN By default, TCP saves various connection metrics in the route cache when the connection closes, so that connections established in the near future can use these to set initial conditions. Usually, this increases overall performance, but may sometimes cause performance diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h index 3c8c59471bc1..19afcc7a4f4a 100644 --- a/include/net/inet_connection_sock.h +++ b/include/net/inet_connection_sock.h @@ -123,15 +123,20 @@ struct inet_connection_sock { /* Range of MTUs to search */ int search_high; int search_low; /* Information on the current probe. */ - u32 probe_size:31, + u32 probe_size:30, + /* Are we actively accumulating data for an mtu probe? */ + wait_data:1, /* Is the MTUP feature enabled for this connection? */ enabled:1; u32 probe_timestamp; + + /* Timer for wait_data */ + struct timer_list wait_data_timer; } icsk_mtup; u32 icsk_probes_tstamp; u32 icsk_user_timeout; u64 icsk_ca_priv[104 / sizeof(u64)]; diff --git a/include/net/netns/ipv4.h b/include/net/netns/ipv4.h index 87e1612497ea..5af1e8a31a02 100644 --- a/include/net/netns/ipv4.h +++ b/include/net/netns/ipv4.h @@ -126,10 +126,11 @@ struct netns_ipv4 { int sysctl_tcp_mtu_probe_floor; int sysctl_tcp_base_mss; int sysctl_tcp_min_snd_mss; int sysctl_tcp_probe_threshold; u32 sysctl_tcp_probe_interval; + int sysctl_tcp_probe_wait; int sysctl_tcp_keepalive_time; int sysctl_tcp_keepalive_intvl; u8 sysctl_tcp_keepalive_probes; diff --git a/include/net/tcp.h b/include/net/tcp.h index eaea43afcc97..9060cc855363 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -1375,10 +1375,12 @@ static inline void tcp_slow_start_after_idle_check(struct sock *sk) s32 delta; if (!sock_net(sk)->ipv4.sysctl_tcp_slow_start_after_idle || tp->packets_out || ca_ops->cong_control) return; + if (inet_csk(sk)->icsk_mtup.wait_data) + return; delta = tcp_jiffies32 - tp->lsndtime; if (delta > inet_csk(sk)->icsk_rto) tcp_cwnd_restart(sk, delta); } diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c index a62934b9f15a..1b6bbb24138a 100644 --- a/net/ipv4/sysctl_net_ipv4.c +++ b/net/ipv4/sysctl_net_ipv4.c @@ -842,10 +842,17 @@ static struct ctl_table ipv4_net_table[] = { .maxlen = sizeof(u32), .mode = 0644, .proc_handler = proc_douintvec_minmax, .extra2 = &u32_max_div_HZ, }, + { + .procname = "tcp_probe_wait", + .data = &init_net.ipv4.sysctl_tcp_probe_wait, + .maxlen = sizeof(int), + .mode = 0644, + .proc_handler = proc_dointvec_ms_jiffies, + }, { .procname = "igmp_link_local_mcast_reports", .data = &init_net.ipv4.sysctl_igmp_llm_reports, .maxlen = sizeof(u8), .mode = 0644, diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 312184cead57..39f74f04e8b5 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -2889,10 +2889,11 @@ static int __net_init tcp_sk_init(struct net *net) net->ipv4.sysctl_tcp_base_mss = TCP_BASE_MSS; net->ipv4.sysctl_tcp_min_snd_mss = TCP_MIN_SND_MSS; net->ipv4.sysctl_tcp_probe_threshold = TCP_PROBE_THRESHOLD; net->ipv4.sysctl_tcp_probe_interval = TCP_PROBE_INTERVAL; net->ipv4.sysctl_tcp_mtu_probe_floor = TCP_MIN_SND_MSS; + net->ipv4.sysctl_tcp_probe_wait = HZ / 2; net->ipv4.sysctl_tcp_keepalive_time = TCP_KEEPALIVE_TIME; net->ipv4.sysctl_tcp_keepalive_probes = TCP_KEEPALIVE_PROBES; net->ipv4.sysctl_tcp_keepalive_intvl = TCP_KEEPALIVE_INTVL; diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index bde781f46b41..15bbf0c97959 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -1754,10 +1754,39 @@ int tcp_mss_to_mtu(struct sock *sk, int mss) } return mtu; } EXPORT_SYMBOL(tcp_mss_to_mtu); +void tcp_mtu_probe_wait_stop(struct sock *sk) +{ + struct inet_connection_sock *icsk = inet_csk(sk); + + if (icsk->icsk_mtup.wait_data) { + icsk->icsk_mtup.wait_data = false; + sk_stop_timer(sk, &icsk->icsk_mtup.wait_data_timer); + } +} + +static void tcp_mtu_probe_wait_timer(struct timer_list *t) +{ + struct inet_connection_sock *icsk = from_timer(icsk, t, icsk_mtup.wait_data_timer); + struct sock *sk = &icsk->icsk_inet.sk; + + bh_lock_sock(sk); + if (!sock_owned_by_user(sk)) { + /* push pending frames now */ + icsk->icsk_mtup.wait_data = false; + tcp_push_pending_frames(sk); + } else { + /* flush later if sock locked by user */ + sk_reset_timer(sk, &icsk->icsk_mtup.wait_data_timer, jiffies + HZ / 10); + } + bh_unlock_sock(sk); + sock_put(sk); +} + + /* MTU probing init per socket */ void tcp_mtup_init(struct sock *sk) { struct tcp_sock *tp = tcp_sk(sk); struct inet_connection_sock *icsk = inet_csk(sk); @@ -1768,10 +1797,11 @@ void tcp_mtup_init(struct sock *sk) icsk->icsk_af_ops->net_header_len; icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, net->ipv4.sysctl_tcp_base_mss); icsk->icsk_mtup.probe_size = 0; if (icsk->icsk_mtup.enabled) icsk->icsk_mtup.probe_timestamp = tcp_jiffies32; + timer_setup(&icsk->icsk_mtup.wait_data_timer, tcp_mtu_probe_wait_timer, 0); } EXPORT_SYMBOL(tcp_mtup_init); /* This function synchronize snd mss to current pmtu/exthdr set. @@ -2366,16 +2396,18 @@ static int tcp_mtu_probe(struct sock *sk) */ tcp_mtu_check_reprobe(sk); return -1; } + /* Can probe ever fit inside window? */ + if (tp->snd_wnd < size_needed) + return -1; + /* Have enough data in the send queue to probe? */ if (tp->write_seq - tp->snd_nxt < size_needed) - return -1; + return net->ipv4.sysctl_tcp_probe_wait ? 0 : -1; - if (tp->snd_wnd < size_needed) - return -1; if (after(tp->snd_nxt + size_needed, tcp_wnd_end(tp))) return 0; /* Do we need to wait to drain cwnd? With none in flight, don't stall */ if (tcp_packets_in_flight(tp) + 2 > tp->snd_cwnd) { @@ -2596,28 +2628,42 @@ void tcp_chrono_stop(struct sock *sk, const enum tcp_chrono type) * but cannot send anything now because of SWS or another problem. */ static bool tcp_write_xmit(struct sock *sk, unsigned int mss_now, int nonagle, int push_one, gfp_t gfp) { + struct inet_connection_sock *icsk = inet_csk(sk); struct tcp_sock *tp = tcp_sk(sk); + struct net *net = sock_net(sk); struct sk_buff *skb; unsigned int tso_segs, sent_pkts; int cwnd_quota; int result; bool is_cwnd_limited = false, is_rwnd_limited = false; u32 max_segs; sent_pkts = 0; tcp_mstamp_refresh(tp); - if (!push_one) { + /* + * Waiting for tcp probe data also applies when push_one=1 + * If user does many small writes we hold them until we have have enough + * for a probe. + */ + if (!push_one || (push_one < 2 && net->ipv4.sysctl_tcp_probe_wait)) { /* Do MTU probing. */ result = tcp_mtu_probe(sk); if (!result) { + if (net->ipv4.sysctl_tcp_probe_wait && !icsk->icsk_mtup.wait_data) { + icsk->icsk_mtup.wait_data = true; + sk_reset_timer(sk, &icsk->icsk_mtup.wait_data_timer, jiffies + net->ipv4.sysctl_tcp_probe_wait); + } return false; } else if (result > 0) { + tcp_mtu_probe_wait_stop(sk); sent_pkts = 1; + } else { + tcp_mtu_probe_wait_stop(sk); } } max_segs = tcp_tso_segs(sk, mss_now); while ((skb = tcp_send_head(sk))) {
According to RFC4821 Section 7.4 "Protocols MAY delay sending non-probes in order to accumulate enough data" but linux almost never does that. Linux waits for probe_size + (1 + retries) * mss_cache to be available in the send buffer and if that condition is not met it will send anyway using the current MSS. The feature can be made to work by sending very large chunks of data from userspace (for example 128k) but for small writes on fast links probes almost never happen. This patch tries to implement the "MAY" by adding an extra flag "wait_data" to icsk_mtup which is set to 1 if a probe is possible but insufficient data is available. Then data is held back in tcp_write_xmit until a probe is sent, probing conditions are no longer met, or 500ms pass. Signed-off-by: Leonard Crestez <cdleonard@gmail.com> --- Documentation/networking/ip-sysctl.rst | 4 ++ include/net/inet_connection_sock.h | 7 +++- include/net/netns/ipv4.h | 1 + include/net/tcp.h | 2 + net/ipv4/sysctl_net_ipv4.c | 7 ++++ net/ipv4/tcp_ipv4.c | 1 + net/ipv4/tcp_output.c | 54 ++++++++++++++++++++++++-- 7 files changed, 71 insertions(+), 5 deletions(-) My tests are here: https://github.com/cdleonard/test-tcp-mtu-probing This patch makes the test pass quite reliably with ICMP_BLACKHOLE=1 TCP_MTU_PROBING=1 IPERF_WINDOW=256k IPERF_LEN=8k while before it only worked with much higher IPERF_LEN=256k In my loopback tests I also observed another issue when tcp_retries increases because of SACKReorder. This makes the original problem worse (since the retries amount factors in buffer requirement) and seems to be unrelated issue. Maybe when loss happens due to MTU shrinkage the sender sack logic is confused somehow? I know it's towards the end of the cycle but this is mostly just intended for discussion.