Re: [PATCH v3 0/4] virtio net: spurious interrupt related fixes

[Willem de Bruijn]

On Wed, May 26, 2021 at 11:34 AM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> On Wed, May 26, 2021 at 4:24 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> >
> > With the implementation of napi-tx in virtio driver, we clean tx
> > descriptors from rx napi handler, for the purpose of reducing tx
> > complete interrupts. But this introduces a race where tx complete
> > interrupt has been raised, but the handler finds there is no work to do
> > because we have done the work in the previous rx interrupt handler.
> > A similar issue exists with polling from start_xmit, it is however
> > less common because of the delayed cb optimization of the split ring -
> > but will likely affect the packed ring once that is more common.
> >
> > In particular, this was reported to lead to the following warning msg:
> > [ 3588.010778] irq 38: nobody cared (try booting with the
> > "irqpoll" option)
> > [ 3588.017938] CPU: 4 PID: 0 Comm: swapper/4 Not tainted
> > 5.3.0-19-generic #20~18.04.2-Ubuntu
> > [ 3588.017940] Call Trace:
> > [ 3588.017942]  <IRQ>
> > [ 3588.017951]  dump_stack+0x63/0x85
> > [ 3588.017953]  __report_bad_irq+0x35/0xc0
> > [ 3588.017955]  note_interrupt+0x24b/0x2a0
> > [ 3588.017956]  handle_irq_event_percpu+0x54/0x80
> > [ 3588.017957]  handle_irq_event+0x3b/0x60
> > [ 3588.017958]  handle_edge_irq+0x83/0x1a0
> > [ 3588.017961]  handle_irq+0x20/0x30
> > [ 3588.017964]  do_IRQ+0x50/0xe0
> > [ 3588.017966]  common_interrupt+0xf/0xf
> > [ 3588.017966]  </IRQ>
> > [ 3588.017989] handlers:
> > [ 3588.020374] [<000000001b9f1da8>] vring_interrupt
> > [ 3588.025099] Disabling IRQ #38
> >
> > This patchset attempts to fix this by cleaning up a bunch of races
> > related to the handling of sq callbacks (aka tx interrupts).
> > Somewhat tested but I couldn't reproduce the original issues
> > reported, sending out for help with testing.
> >
> > Wei, does this address the spurious interrupt issue you are
> > observing? Could you confirm please?
>
> Thanks for working on this, Michael. Wei is on leave. I'll try to reproduce.

The original report was generated with five GCE virtual machines
sharing a sole-tenant node, together sending up to 160 netperf
tcp_stream connections to 16 other instances. Running Ubuntu 20.04-LTS
with kernel 5.4.0-1034-gcp.

But the issue can also be reproduced with just two n2-standard-16
instances, running neper tcp_stream with high parallelism (-T 16 -F
240).

It's a bit faster to trigger by reducing the interrupt count threshold
from 99.9K/100K to 9.9K/10K. And I added additional logging to report
the unhandled rate even if lower.

Unhandled interrupt rate scales with the number of queue pairs
(`ethtool -L $DEV combined $NUM`). It is essentially absent at 8
queues, at around 90% at 14 queues. By default these GCE instances
have one rx and tx interrupt per core, so 16 each. With the rx and tx
interrupts for a given virtio-queue pinned to the same core.

Unfortunately, commit 3/4 did not have a significant impact on these
numbers. Have to think a bit more about possible mitigations. At least
I'll be able to test the more easily now.