On 9/26/22 06:18, Konstantin Ananyev wrote:
Hi everyone,
Sorry for late reply.
The main problem is hard to design a tx_prepare for bonding device:
1. as Chas Williams said, there maybe twice hash calc to get target slave
devices.
2. also more important, if the slave devices have changes(e.g. slave device
link down or remove), and if the changes happens between bond-tx-prepare
and
bond-tx-burst, the output slave will changes, and this may lead to
checksum
failed. (Note: a bond device with slave devices may from different
vendors,
and slave devices may have different requirements, e.g. slave-A support
calc
IPv4 pseudo-head automatic (no need driver pre-calc), but slave-B need
driver
pre-calc).
Current design cover the above two scenarios by using in-place tx-prepare. and
in addition, bond devices are not transparent to applications, I think it's a
practical method to provide tx-prepare support in this way.
I don't think you need to export an enable/disable routine for the use of
rte_eth_tx_prepare. It's safe to just call that routine, even if it isn't
implemented. You are just trading one branch in DPDK librte_eth_dev for a
branch in drivers/net/bonding.
Our first patch was just like yours (just add tx-prepare default), but community
is concerned about impacting performance.
As a trade-off, I think we can add the enable/disable API.
IMHO, that's a bad idea. If the rte_eth_dev_tx_prepare API affects
performance adversly, that is not a bonding problem. All applications
should be calling rte_eth_dev_tx_prepare. There's no defined API
to determine if rte_eth_dev_tx_prepare should be called. Therefore,
applications should always call rte_eth_dev_tx_prepare. Regardless,
as I previously mentioned, you are just trading the location of
the branch, especially in the bonding case.
If rte_eth_dev_tx_prepare is causing a performance drop, then that API
should be improved or rewritten. There are PMD that require you to use
that API. Locally, we had maintained a patch to eliminate the use of
rte_eth_dev_tx_prepare. However, that has been getting harder and harder
to maintain. The performance lost by just calling rte_eth_dev_tx_prepare
was marginal.
I think you missed fixing tx_machine in 802.3ad support. We have been using
the following patch locally which I never got around to submitting.
You are right, I will send V3 fix it.
From a458654d68ff5144266807ef136ac3dd2adfcd98 Mon Sep 17 00:00:00 2001
From: "Charles (Chas) Williams" <chwil...@ciena.com>
Date: Tue, 3 May 2022 16:52:37 -0400
Subject: [PATCH] net/bonding: call rte_eth_tx_prepare before rte_eth_tx_burst
Some PMDs might require a call to rte_eth_tx_prepare before sending the
packets for transmission. Typically, the prepare step handles the VLAN
headers, but it may need to do other things.
Signed-off-by: Chas Williams <chwil...@ciena.com>
...
* ring if transmission fails so the packet isn't lost.
@@ -1322,8 +1350,12 @@ bond_ethdev_tx_burst_broadcast(void *queue, struct
rte_mbuf **bufs,
/* Transmit burst on each active slave */
for (i = 0; i < num_of_slaves; i++) {
- slave_tx_total[i] = rte_eth_tx_burst(slaves[i], bd_tx_q->queue_id,
+ uint16_t nb_prep;
+
+ nb_prep = rte_eth_tx_prepare(slaves[i], bd_tx_q->queue_id,
bufs, nb_pkts);
+ slave_tx_total[i] = rte_eth_tx_burst(slaves[i], bd_tx_q->queue_id,
+ bufs, nb_prep);
The tx-prepare may edit packet data, and the broadcast mode will send a packet
to all slaves,
the packet data is sent and edited at the same time. Is this likely to cause
problems ?
This routine is already broken. You can't just increment the refcount
and send the packet into a PMD's transmit routine. Nothing guarantees
that a transmit routine will not modify the packet. Many PMDs perform an
rte_vlan_insert.
Hmm interesting....
My uderstanding was quite opposite - tx_burst() can't modify packet data and
metadata
(except when refcnt==1 and tx_burst() going to free the mbuf and put it back to
the mempool).
While tx_prepare() can - actually as I remember that was one of the reasons why
a separate routine
was introduced.
Is that documented anywhere?
I looked through, but couldn't find too much except what was already mentioned
by Fengcheng:
rte_eth_tx_prepare() notes:
* Since this function can modify packet data, provided mbufs must be safely
* writable (e.g. modified data cannot be in shared segment).
Probably that's not explicit enough, as it doesn't forbid modifying packets in
tx_burst clearly.
This certainly seems like one of those gray areas in the DPDK APIs. It
should be made clear what is expected as far as behavior.
It's been my experience that the device PMD
can do practically anything and you need to protect yourself. Currently,
the af_packet, dpaa2, and vhost driver call rte_vlan_insert. Before 2019,
the virtio driver also used to call rte_vlan_insert during its transmit
path. Of course, rte_vlan_insert modifies the packet data and the mbuf
header.
Interesting, usually apps that trying to use zero-copy multi-cast TX have
packet-header portion
in a separate segment, so it might even keep working.. But definetly doesn't
look right to me:
if mbuf->refnct > 1, I think it should be treated as read-only.
rte_vlan_insert might be a problem with broadcast mode. If the refcnt is
1, rte_vlan_insert is going to fail. So, the current broadcast mode
implementation probably doesn't work if any PMD uses rte_vlan_insert.
So again, a solution is call tx_pkt_prepare once, then increment the
reference count, and send to the all the members. That works if your
PMD correctly implements tx_pkt_prepare. If it doesn't and call rte_vlan_insert
in the transmit routine, that PMD will need to be fixed to work with
bonding.
Regardless, it looks like rte_eth_dev_tx_prepare should always be
called.
Again, as I remember, initial agreement was: if any TX offload is enabled,
tx_prepare() needs to be called (or user has implement similar stuff on his
own).
If no TX offload flags were specified for the packet, tx_prepare() is not
necessary.
For the bonding driver, we potentially have a mix of PMDs for the members.
It's difficult to know in advance if your packets will have TX offload flags
or not. If you have a tx_pkt_prepare stub, there's a good chance that your
packets will have some TX offload flags. So, calling tx_pkt_prepare is likely
the "best" intermediate solution.
> Handling that correctly in broadcast mode probably means always
make a deep copy of the packet, or check to see if all the members are
the same PMD type. If so, you can just call prepare once. You could track
the mismatched nature during additional/removal of the members. Or just
assume people aren't going to mismatch bonding members.
You should at least perform a clone of the packet so
that the mbuf headers aren't mangled by each PMD.
Usually you don't need to clone the whole packet. In many cases it is enough to
just attach
as first segment l2/l3/l4 header portion of the packet.
At least that's how ip_multicast sample works.
Yes, that's what I meant by deep copy the packet headers. You just copy
enough to modify what you need and keep the bulk of the packet otherwise.
Just to be safe you
should perform a partial deep copy of the packet headers in case some
PMD does an rte_vlan_insert and the other PMDs in the bonding group do
not need an rte_vlan_insert.
So doing a blind rte_eth_dev_tx_preprare isn't making anything much
worse.
if (unlikely(slave_tx_total[i] < nb_pkts))
tx_failed_flag = 1;
