RE: [PATCH v1 5/5] examples/l3fwd: enable direct rearm mode

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

(apologies for being late on replying, catching up after vacation)

<snip>
> >>
> >> 25/05/2022 01:24, Honnappa Nagarahalli пишет:
> >>> From: Konstantin Ananyev <konstantin.v.ananyev@yandex.ru>
> >>>
> >>> 20/04/2022 09:16, Feifei Wang пишет:
> >>>>> Enable direct rearm mode. The mapping is decided in the data plane
> >>>>> based on the first packet received.
> >>>>>
> >>>>> Suggested-by: Honnappa Nagarahalli
> <honnappa.nagarahalli@arm.com>
> >>>>> Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
> >>>>> Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
> >>>>> Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
> >>>>> ---
> >>>>>    examples/l3fwd/l3fwd_lpm.c | 16 +++++++++++++++-
> >>>>>    1 file changed, 15 insertions(+), 1 deletion(-)
> >>>>>
> >>>>> diff --git a/examples/l3fwd/l3fwd_lpm.c
> >>>>> b/examples/l3fwd/l3fwd_lpm.c index bec22c44cd..38ffdf4636 100644
> >>>>> --- a/examples/l3fwd/l3fwd_lpm.c
> >>>>> +++ b/examples/l3fwd/l3fwd_lpm.c
> >>>>> @@ -147,7 +147,7 @@ lpm_main_loop(__rte_unused void *dummy)
> >>>>>        unsigned lcore_id;
> >>>>>        uint64_t prev_tsc, diff_tsc, cur_tsc;
> >>>>>        int i, nb_rx;
> >>>>> -    uint16_t portid;
> >>>>> +    uint16_t portid, tx_portid;
> >>>>>        uint8_t queueid;
> >>>>>        struct lcore_conf *qconf;
> >>>>>        const uint64_t drain_tsc = (rte_get_tsc_hz() + US_PER_S -
> >>>>> 1) / @@ -158,6 +158,8 @@ lpm_main_loop(__rte_unused void
> *dummy)
> >>>>>        const uint16_t n_rx_q = qconf->n_rx_queue;
> >>>>>        const uint16_t n_tx_p = qconf->n_tx_port;
> >>>>> +    int direct_rearm_map[n_rx_q];
> >>>>> +
> >>>>>        if (n_rx_q == 0) {
> >>>>>            RTE_LOG(INFO, L3FWD, "lcore %u has nothing to do\n",
> >>>>> lcore_id);
> >>>>>            return 0;
> >>>>> @@ -169,6 +171,7 @@ lpm_main_loop(__rte_unused void *dummy)
> >>>>>            portid = qconf->rx_queue_list[i].port_id;
> >>>>>            queueid = qconf->rx_queue_list[i].queue_id;
> >>>>> +        direct_rearm_map[i] = 0;
> >>>>>            RTE_LOG(INFO, L3FWD,
> >>>>>                " -- lcoreid=%u portid=%u rxqueueid=%hhu\n",
> >>>>>                lcore_id, portid, queueid); @@ -209,6 +212,17 @@
> >>>>> lpm_main_loop(__rte_unused void *dummy)
> >>>>>                if (nb_rx == 0)
> >>>>>                    continue;
> >>>>> +            /* Determine the direct rearm mapping based on the
> >>>>> +first
> >>>>> +             * packet received on the rx queue
> >>>>> +             */
> >>>>> +            if (direct_rearm_map[i] == 0) {
> >>>>> +                tx_portid = lpm_get_dst_port(qconf,
> >>>>> +pkts_burst[0],
> >>>>> +                            portid);
> >>>>> +                rte_eth_direct_rxrearm_map(portid, queueid,
> >>>>> +                                tx_portid, queueid);
> >>>>> +                direct_rearm_map[i] = 1;
> >>>>> +            }
> >>>>> +
> >>>
> >>>> That just doesn't look right to me: why to make decision based on
> >>>> the first packet?
> >>> The TX queue depends on the incoming packet. So, this method covers
> >>> more scenarios than doing it in the control plane where the outgoing
> >>> queue is not known.
> >>>
> >>>
> >>>> What would happen if second and all other packets have to be routed
> >>>> to different ports?
> >>> This is an example application and it should be fine to make this
> >>> assumption.
> >>> More over, it does not cause any problems if packets change in between.
> >>> When
> >>> the packets change back, the feature works again.
> >>>
> >>>> In fact, this direct-rearm mode seems suitable only for hard-coded
> >>>> one to one mapped forwarding (examples/l2fwd, testpmd).
> >>>> For l3fwd it can be used safely only when we have one port in use.
> >>> Can you elaborate more on the safety issue when more than one port
> >>> is
> >> used?
> >>>
> >>>> Also I think it should be selected at init-time and it shouldn't be
> >>>> on by default.
> >>>> To summarize, my opinion:
> >>>> special cmd-line parameter to enable it.
> >>> Can you please elaborate why a command line parameter is required?
> >>> Other similar features like RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE are
> >>> enabled without a command line parameter. IMO, this is how it should
> >>> ber. Essentially we are trying to measure how different PMDs
> >>> perform, the ones that have implemented performance improvement
> >>> features
> >> would
> >>> show better performance (i.e. the PMDs implementing the features
> >>> should not be penalized by asking for additional user input).
> >>
> >>   From my perspective, main purpose of l3fwd application is to
> >> demonstrate DPDK ability to do packet routing based on input packet
> contents.
> >> Making guesses about packet contents is a change in expected behavior.
> >> For some cases it might improve performance, for many others - will
> >> most likely cause performance drop.
> >> I think that performance drop as default behavior (running the same
> >> parameters as before) should not be allowed.
> >> Plus you did not provided ability to switch off that behavior, if undesired.
> > There is no drop in L3fwd performance due to this patch.
> 
> Hmm..
> Are you saying even when your guess is wrong, and you constantly hitting slow-
> path (check tx_queue first - failure, then allocate from mempool) you didn't
> observe any performance drop?
> There is more work to do, and if workload is cpu-bound, my guess - it should be
> noticeable.
Well, you do not have to trust our words. The platform used to test is mentioned in the cover letter. Appreciate if you could test on your platform and provide the results.

> Also, from previous experience, quite often even after tiny changes in rx/tx
> code-path some slowdown was reported.
> Usually that happened on some low-end ARM cpus (Marvell, NXP).
> 
> 
> >>
> >> About comparison with RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE default
> >> enablement - I don't think it is correct.
> >> Within l3fwd app we can safely guarantee that all
> >> RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE pre-requirements are met:
> >> in each TX queue all mbufs will belong to the same mempool and their
> >> refcnt will always equal to one.
> >> Here you are making guesses about contents of input packets, without
> >> any guarantee that you guess will always be valid.
> > This is not a guess. The code understands the incoming traffic and configures
> accordingly. So, it should be correct.
> 
> 
> No, it is not.
> Your code makes guess about all incoming traffic based on just one (first)
> packet.
> 
> Since it is a sample application, we do not expect the traffic to be complex. If it is
> complex, the performance will be the same as before or better.
> 
> I am not talking about anything 'complex' here.
> Doing dynamic routing based on packet contents is a a basic l3fwd functionality.
> 
> >
> >>
> >> BTW, what's wrong with using l2fwd to demonstrate that feature?
> >> Seems like a natural choice to me.
> > The performance of L3fwd application in DPDK has become a industry
> standard, hence we need to showcase the performance in L3fwd application.
> >
> >>
> >>>> allowable only when we run l3fwd over one port.
> >>>
> >>>
> >>>>>    #if defined RTE_ARCH_X86 || defined __ARM_NEON \
> >>>>>                 || defined RTE_ARCH_PPC_64
> >>>>>                l3fwd_lpm_send_packets(nb_rx, pkts_burst,
> >