Date:   Wed, 10 Mar 2021 22:56:21 +0100
From:   Daniel Glöckner <dg@...ix.com>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     netdev@...r.kernel.org, linux-can@...r.kernel.org
Subject: Re: Softirq error with mcp251xfd driver

On Wed, Mar 10, 2021 at 10:22:54PM +0100, Daniel Glöckner wrote:
> On Wed, Mar 10, 2021 at 08:13:51AM +0100, Marc Kleine-Budde wrote:
> > On 10.03.2021 07:46:26, Daniel Glöckner wrote:
> > > the mcp251xfd driver uses a threaded irq handler to queue skbs with the
> > > can_rx_offload_* helpers. I get the following error on every packet until
> > > the rate limit kicks in:
> > > 
> > > NOHZ tick-stop error: Non-RCU local softirq work is pending, handler
> > > #08!!!
> > 
> > That's a known problem. But I had no time to investigate it.
> > 
> > > Adding local_bh_disable/local_bh_enable around the can_rx_offload_* calls
> > > gets rid of the error, but is that the correct way to fix this?
> > > Internally the can_rx_offload code uses spin_lock_irqsave to safely
> > > manipulate its queue.
> > 
> > The problem is not the queue handling inside of rx_offload, but the call
> > to napi_schedule(). This boils down to raising a soft IRQ (the NAPI)
> > from the threaded IRQ handler of the mcp251xfd driver.
> > 
> > The local_bh_enable() "fixes" the problem running the softirq if needed.
> > 
> > https://elixir.bootlin.com/linux/v5.11/source/kernel/softirq.c#L1913
> > 
> > I'm not sure how to properly fix the problem, yet.
> 
> If I understand correctly, the point of using can_rx_offload_* in the
> mcp251xfd driver is that it sorts the rx, tx, and error frames according
> to their timestamp. In that case calling local_bh_enable after each packet
> is not correct because there will never be more than one packet in the
> queue. We want to call local_bh_disable + can_rx_offload_schedule +
> local_bh_enable only at the end of mcp251xfd_irq after intf_pending
> indicated that there are no more packets inside the chip. How about adding
> a flag to struct can_rx_offload that suppresses the automatic calls to
> can_rx_offload_schedule?
> 
> If there is the risk that under high load we will never exit the loop in
> mcp251xfd_irq or if can_rx_offload_napi_poll might run again while we add
> more packets to the queue, a more complex scheme is needed. We could
> extend can_rx_offload_napi_poll to process only packets with a timestamp
> below a certain value. That value has to be read from the TBC register
> before we read the INT register. Then the three functions can be run after
> each iteration to empty the queue. We need to update that timestamp limit
> one more time when we finally exit the loop to process those packets that
> have arrived after the reading of the TBC register when the INT register
> still had bits set. Using the timestamp of the tail of the queue is
> probably the easiest way to set the final limit.

Or we leave can_rx_offload unchanged and keep two additional lists of skbs
inside the mcp251xfd driver: One for the packets that arrived before the
timestamp read from TBC and one for the packets that arrived later. At the
end of an iteration we call local_bh_disable, enqueue all packets from the
first list with can_rx_offload_queue_sorted, and the ask the softirq to
process them by calling local_bh_enable. Afterwards we move everything
from the second list to the first list and do the next iteration.

The drawback is that we can't use can_rx_offload_get_echo_skb.

Best regards,

  Daniel

-- 
Dipl.-Math. Daniel Glöckner, emlix GmbH, http://www.emlix.com
Fon +49 551 30664-0, Fax +49 551 30664-11,
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