On 10/1/19 12:22 PM, Kurt Van Dijck wrote:
> On di, 01 okt 2019 11:40:09 +0200, Marc Kleine-Budde wrote:
>> On 9/30/19 9:30 PM, Kurt Van Dijck wrote:
>>> The napi-handler defers c_can reception to softirq, but it is hard to
>>> control the RT priority of the CAN recv end inside a softirq.
>>> Using an irqthread allows precise control of it's RT priority.
>>> Having the quota still around in the IRQ thread allows to restrict
>>> the work_done per cycle.
>>>
>>> Signed-off-by: Kurt Van Dijck <dev.kurt@xxxxxxxxxxxxxxxxxxxxxx>
>>
>> NACK, not pushing CAN frames though NAPI results in very strange things,
>> such like package reordering.
>
> This becomes interesting.
> Would you mind elaborating a bit on that.
>
> I'm currently trying to avoid CAN overflows on an RT system, where
> I eleveated the can irq thread above the others.
RT with PREEMPT_RT?
> Then I discovered that the softirqd waits a lot before being scheduled,
> but this one deal with all others too, so I started to question the whole
> softirq thing because its a garbage can for all postponed work.
> Mirgrating to a threaded irq seems wise to me then.
AFAIC you're only allowed to use netif_receive_skb() from softirq, i.e.
the NAPI context, see the documentation:
https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L5265
I'm not sure about threated IRQ handlers....but from hard-IRQ context,
you should use netif_rx():
https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L4544
and netif_rx_ni() from threaded IRQ context:
https://elixir.bootlin.com/linux/latest/source/net/core/dev.c#L4557
Please switch on all lockdep stuff and watch out for "softirq abc
pending" messages.
> If a single thread reads all the incoming messages from the chip,
> the are received in order, I assume. Who would reorder the packets?
> Is synchronizing rx/tx paths handled in napi? they depend on different
> softirqs, if I remember well.
If, for the above reasons you have to use netif_rx(_ni)() and are on a
multicore system, the packets might be delivered on different CPUs and
processed in the wrong order. I have to google for more details...
What about reading the packet from the hardware in IRQ context and
putting them into the networking stack in NAPI. The rx-offload does
basically this.
Marc
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