Date:   Tue, 9 Feb 2021 11:27:05 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Wei Wang <weiwan@...gle.com>, David Miller <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi


On 2021/2/9 上午3:08, Willem de Bruijn wrote:
> On Sun, Feb 7, 2021 at 10:29 PM Jason Wang <jasowang@...hat.com> wrote:
>>
>> On 2021/2/5 上午4:50, Willem de Bruijn wrote:
>>> On Wed, Feb 3, 2021 at 10:06 PM Jason Wang <jasowang@...hat.com> wrote:
>>>> On 2021/2/4 上午2:28, Willem de Bruijn wrote:
>>>>> On Wed, Feb 3, 2021 at 12:33 AM Jason Wang <jasowang@...hat.com> wrote:
>>>>>> On 2021/2/2 下午10:37, Willem de Bruijn wrote:
>>>>>>> On Mon, Feb 1, 2021 at 10:09 PM Jason Wang <jasowang@...hat.com> wrote:
>>>>>>>> On 2021/1/29 上午8:21, Wei Wang 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 could introduce a race where tx complete
>>>>>>>>> interrupt has been raised, but the handler found there is no work to do
>>>>>>>>> because we have done the work in the previous rx interrupt handler.
>>>>>>>>> This could 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 patch adds a new param to struct vring_virtqueue, and we set it for
>>>>>>>>> tx virtqueues if napi-tx is enabled, to suppress the warning in such
>>>>>>>>> case.
>>>>>>>>>
>>>>>>>>> Fixes: 7b0411ef4aa6 ("virtio-net: clean tx descriptors from rx napi")
>>>>>>>>> Reported-by: Rick Jones <jonesrick@...gle.com>
>>>>>>>>> Signed-off-by: Wei Wang <weiwan@...gle.com>
>>>>>>>>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>>>>>>>> Please use get_maintainer.pl to make sure Michael and me were cced.
>>>>>>> Will do. Sorry about that. I suggested just the virtualization list, my bad.
>>>>>>>
>>>>>>>>> ---
>>>>>>>>>       drivers/net/virtio_net.c     | 19 ++++++++++++++-----
>>>>>>>>>       drivers/virtio/virtio_ring.c | 16 ++++++++++++++++
>>>>>>>>>       include/linux/virtio.h       |  2 ++
>>>>>>>>>       3 files changed, 32 insertions(+), 5 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>>>>>>>> index 508408fbe78f..e9a3f30864e8 100644
>>>>>>>>> --- a/drivers/net/virtio_net.c
>>>>>>>>> +++ b/drivers/net/virtio_net.c
>>>>>>>>> @@ -1303,13 +1303,22 @@ static void virtnet_napi_tx_enable(struct virtnet_info *vi,
>>>>>>>>>                   return;
>>>>>>>>>           }
>>>>>>>>>
>>>>>>>>> +     /* With napi_tx enabled, free_old_xmit_skbs() could be called from
>>>>>>>>> +      * rx napi handler. Set work_steal to suppress bad irq warning for
>>>>>>>>> +      * IRQ_NONE case from tx complete interrupt handler.
>>>>>>>>> +      */
>>>>>>>>> +     virtqueue_set_work_steal(vq, true);
>>>>>>>>> +
>>>>>>>>>           return virtnet_napi_enable(vq, napi);
>>>>>>>> Do we need to force the ordering between steal set and napi enable?
>>>>>>> The warning only occurs after one hundred spurious interrupts, so not
>>>>>>> really.
>>>>>> Ok, so it looks like a hint. Then I wonder how much value do we need to
>>>>>> introduce helper like virtqueue_set_work_steal() that allows the caller
>>>>>> to toggle. How about disable the check forever during virtqueue
>>>>>> initialization?
>>>>> Yes, that is even simpler.
>>>>>
>>>>> We still need the helper, as the internal variables of vring_virtqueue
>>>>> are not accessible from virtio-net. An earlier patch added the
>>>>> variable to virtqueue itself, but I think it belongs in
>>>>> vring_virtqueue. And the helper is not a lot of code.
>>>> It's better to do this before the allocating the irq. But it looks not
>>>> easy unless we extend find_vqs().
>>> Can you elaborate why that is better? At virtnet_open the interrupts
>>> are not firing either.
>>
>> I think you meant NAPI actually?
> I meant interrupt: we don't have to worry about the spurious interrupt
> warning when no interrupts will be firing. Until virtnet_open
> completes, the device is down.


Ok.


>
>
>>> I have no preference. Just curious, especially if it complicates the patch.
>>>
>> My understanding is that. It's probably ok for net. But we probably need
>> to document the assumptions to make sure it was not abused in other drivers.
>>
>> Introduce new parameters for find_vqs() can help to eliminate the subtle
>> stuffs but I agree it looks like a overkill.
>>
>> (Btw, I forget the numbers but wonder how much difference if we simple
>> remove the free_old_xmits() from the rx NAPI path?)
> The committed patchset did not record those numbers, but I found them
> in an earlier iteration:
>
>    [PATCH net-next 0/3] virtio-net tx napi
>    https://lists.openwall.net/netdev/2017/04/02/55
>
> It did seem to significantly reduce compute cycles ("Gcyc") at the
> time. For instance:
>
>      TCP_RR Latency (us):
>      1x:
>        p50              24       24       21
>        p99              27       27       27
>        Gcycles         299      432      308
>
> I'm concerned that removing it now may cause a regression report in a
> few months. That is higher risk than the spurious interrupt warning
> that was only reported after years of use.


Right.

So if Michael is fine with this approach, I'm ok with it. But we 
probably need to a TODO to invent the interrupt handlers that can be 
used for more than one virtqueues. When MSI-X is enabled, the interrupt 
handler (vring_interrup()) assumes the interrupt is used by a single 
virtqueue.

Thanks


>

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