Hi Marc,

On Tue, Jun 15, 2021 at 4:15 AM Marc Kleine-Budde [off-list ref] wrote:

The imx SPI driver has quite some overhead, when it comes to small SPI
transfers. The mcp251fd driver performs much better with the SPI IP
cores on the raspi, which have quite good optimized drivers.

Hook up a scope to the SPI's clock and chip select lines of the imx,
you'll see the time between end of transfer until the chip select is
inactive is longer than the SPI transfer itself.

I expect most bang for the buck can be archived by adding an IRQ less
busy polling transfer mode, which kicks in below a certain SPI transfer
length.

On the mcp251xfd driver side, there is some room for optimization. The
basic idea is to reduce the number of SPI transfers by combining several
reads into one transfer. This can be done in some places.

For peak loads in CAN-2.0 mode it would be interesting to make use of
the remaining RAM for a 2nd FIFO.

Thanks for your reply.

I do see some RCU related errors every time the application is launched:

# ./cantest.sh start
root@verdin-imx8mm:~# interface = can1, family = 29, ty[   17.484220]
NOHZ tick-stop error: Non-RCU local softirq work is pending, handler
#08!!!
[   17.484240] NOHZ tick-stop error: Non-RCU local softirq work is
pending, handler #08!!!
pe = 3, proto = 1
interface = ca[   17.502870] NOHZ tick-stop error: Non-RCU local
softirq work is pending, handler #08!!!
n0, family = 29, type = 3, proto [   17.502912] NOHZ tick-stop error:
Non-RCU local softirq work is pending, handler #08!!!
= 1
interface = can1, family = 2[   17.524457] NOHZ tick-stop error:
Non-RCU local softirq work is pending, handler #08!!!
9, type = 3, proto = 1
interface[   17.524476] NOHZ tick-stop error: Non-RCU local softirq
work is pending, handler #08!!!
 = can0, family = 29, type = 3, p[   17.535223] NOHZ tick-stop error:
Non-RCU local softirq work is pending, handler #08!!!
roto = 1
[   17.557284] NOHZ tick-stop error: Non-RCU local softirq work is
pending, handler #08!!!
[   17.557284] NOHZ tick-stop error: Non-RCU local softirq work is
pending, handler #08!!!
[   17.574035] NOHZ tick-stop error: Non-RCU local softirq work is
pending, handler #08!!!
[   17.574037] NOHZ tick-stop error: Non-RCU local softirq work is
pending, handler #08!!!
[   18.435652] sched: RT throttling activated

After some time:

[  292.197058] rcu: 0-....: (1 GPs behind)
idle=6db/1/0x4000000000000002 softirq=2974/2975 fqs=7882
[  292.206039] (t=21003 jiffies g=1249 q=1317)
[  292.210316] Task dump for CPU 0:
[  292.213549] task:cansequence     state:R  running task     stack:
 0 pid:  374 ppid:     1 flags:0x00000202
[  292.223485] Call trace:
[  292.225932]  dump_backtrace+0x0/0x1a8
[  292.229613]  show_stack+0x18/0x28
[  292.232936]  sched_show_task+0x150/0x170
[  292.236869]  dump_cpu_task+0x44/0x54
[  292.240453]  rcu_dump_cpu_stacks+0xf4/0x13c
[  292.244648]  rcu_sched_clock_irq+0x844/0xdc0
[  292.248929]  update_process_times+0x98/0xe8
[  292.253125]  tick_nohz_handler+0xac/0x110
[  292.257142]  arch_timer_handler_phys+0x34/0x48
[  292.261598]  handle_percpu_devid_irq+0x84/0x148
[  292.266138]  handle_domain_irq+0x60/0x90
[  292.270071]  gic_handle_irq+0x54/0x120
[  292.273833]  call_on_irq_stack+0x28/0x50
[  292.277767]  do_interrupt_handler+0x54/0x60
[  292.281964]  el1_interrupt+0x30/0x78
[  292.285550]  el1h_64_irq_handler+0x18/0x28
[  292.289653]  el1h_64_irq+0x78/0x7c
[  292.293061]  __audit_syscall_exit+0x8/0x238
[  292.297256]  el0_svc_common+0x60/0xd8
[  292.300927]  do_el0_svc+0x28/0x90
[  292.304249]  el0_svc+0x24/0x38
[  292.307312]  el0t_64_sync_handler+0xb0/0xb8
[  292.311502]  el0t_64_sync+0x198/0x19c
2020-02-12 19:28:40:388 sequence CNT: 1586297, RX:     93    expected:
121    missing:  228    skt overfl d:    0 a:    0    delta: 228
incident: 1    seq_wrap RX: 6196     sequ_wrap_expected: 6196
overall lost: 4294967268
2020-02-12 19:28:40:389 sequence CNT:     95, RX:    121    expected:
95    missing:   26    skt overfl d:    0 a:    0    delta:  26
incident: 2    seq_wrap RX: 6196     sequ_wrap_expected: 6196
overall lost: 4294967294
2020-02-12 19:28:40:389 sequence CNT:    125, RX:    127    expected:
125    missing:    2    skt overfl d:    0 a:    0    delta:   2
incident: 3    seq_wrap RX: 6196     sequ_wrap_expected: 6196
overall lost: 0

Any ideas how these RCU errors could be fixed?

Thanks,

Fabio Estevam