netdev - Bad performance in RX with sfc 40G

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Date:   Thu, 18 Nov 2021 16:14:35 +0100
From:   Íñigo Huguet <ihuguet@...hat.com>
To:     Edward Cree <ecree.xilinx@...il.com>, habetsm.xilinx@...il.com
Cc:     netdev@...r.kernel.org, Dinan Gunawardena <dinang@...inx.com>,
        Pablo Cascon <pabloc@...inx.com>
Subject: Bad performance in RX with sfc 40G

Hello,

Doing some tests a few weeks ago I noticed a very low performance in
RX using 40G Solarflare NICs. Doing tests with iperf3 I got more than
30Gbps in TX, but just around 15Gbps in RX. Other NICs from other
vendors could send and receive over 30Gbps.

I was doing the tests with multiple threads in iperf3 (-P 8).

The models used are SFC9140 and SFC9220.

Perf showed that most of the time was being expended in
`native_queued_spin_lock_slowpath`. Tracing the calls to it with
bpftrace I got that most of the calls were from __napi_poll > efx_poll
> efx_fast_push_rx_descriptors > __alloc_pages >
get_page_from_freelist > ...

Please can you help me investigate the issue? At first sight, it seems
a not very optimal memory allocation strategy, or maybe a failure in
pages recycling strategy...

This is the output of bpftrace, the 2 call chains that repeat more
times, both from sfc

@[
    native_queued_spin_lock_slowpath+1
    _raw_spin_lock+26
    rmqueue_bulk+76
    get_page_from_freelist+2295
    __alloc_pages+214
    efx_fast_push_rx_descriptors+640
    efx_poll+660
    __napi_poll+42
    net_rx_action+547
    __softirqentry_text_start+208
    __irq_exit_rcu+179
    common_interrupt+131
    asm_common_interrupt+30
    cpuidle_enter_state+199
    cpuidle_enter+41
    do_idle+462
    cpu_startup_entry+25
    start_kernel+2465
    secondary_startup_64_no_verify+194
]: 2650
@[
    native_queued_spin_lock_slowpath+1
    _raw_spin_lock+26
    rmqueue_bulk+76
    get_page_from_freelist+2295
    __alloc_pages+214
    efx_fast_push_rx_descriptors+640
    efx_poll+660
    __napi_poll+42
    net_rx_action+547
    __softirqentry_text_start+208
    __irq_exit_rcu+179
    common_interrupt+131
    asm_common_interrupt+30
    cpuidle_enter_state+199
    cpuidle_enter+41
    do_idle+462
    cpu_startup_entry+25
    secondary_startup_64_no_verify+194
]: 17119

--
Íñigo Huguet