Date:   Wed, 3 Feb 2021 13:33:13 +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>,
        Willem de Bruijn <willemb@...gle.com>,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi


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?


>
>>>    }
>>>
>>> -static void virtnet_napi_tx_disable(struct napi_struct *napi)
>>> +static void virtnet_napi_tx_disable(struct virtqueue *vq,
>>> +                                 struct napi_struct *napi)
>>>    {
>>> -     if (napi->weight)
>>> +     if (napi->weight) {
>>>                napi_disable(napi);
>>> +             virtqueue_set_work_steal(vq, false);
>>> +     }
>>>    }
>>>
>>>    static void refill_work(struct work_struct *work)
>>> @@ -1835,7 +1844,7 @@ static int virtnet_close(struct net_device *dev)
>>>        for (i = 0; i < vi->max_queue_pairs; i++) {
>>>                xdp_rxq_info_unreg(&vi->rq[i].xdp_rxq);
>>>                napi_disable(&vi->rq[i].napi);
>>> -             virtnet_napi_tx_disable(&vi->sq[i].napi);
>>> +             virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
>>>        }
>>>
>>>        return 0;
>>> @@ -2315,7 +2324,7 @@ static void virtnet_freeze_down(struct virtio_device *vdev)
>>>        if (netif_running(vi->dev)) {
>>>                for (i = 0; i < vi->max_queue_pairs; i++) {
>>>                        napi_disable(&vi->rq[i].napi);
>>> -                     virtnet_napi_tx_disable(&vi->sq[i].napi);
>>> +                     virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
>>>                }
>>>        }
>>>    }
>>> @@ -2440,7 +2449,7 @@ static int virtnet_xdp_set(struct net_device *dev, struct bpf_prog *prog,
>>>        if (netif_running(dev)) {
>>>                for (i = 0; i < vi->max_queue_pairs; i++) {
>>>                        napi_disable(&vi->rq[i].napi);
>>> -                     virtnet_napi_tx_disable(&vi->sq[i].napi);
>>> +                     virtnet_napi_tx_disable(vi->sq[i].vq, &vi->sq[i].napi);
>>>                }
>>>        }
>>>
>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>>> index 71e16b53e9c1..f7c5d697c302 100644
>>> --- a/drivers/virtio/virtio_ring.c
>>> +++ b/drivers/virtio/virtio_ring.c
>>> @@ -105,6 +105,9 @@ struct vring_virtqueue {
>>>        /* Host publishes avail event idx */
>>>        bool event;
>>>
>>> +     /* Tx side napi work could be done from rx side. */
>>> +     bool work_steal;
>>
>> So vring_vritqueue is a general structure, let's avoid mentioning
>> network specific stuffs here. And we need a better name like
>> "no_interrupt_check"?
>>
>> And we need a separate patch for virtio core changes.
> Ack. Will change.
>
>>> +
>>>        /* Head of free buffer list. */
>>>        unsigned int free_head;
>>>        /* Number we've added since last sync. */
>>> @@ -1604,6 +1607,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
>>>        vq->notify = notify;
>>>        vq->weak_barriers = weak_barriers;
>>>        vq->broken = false;
>>> +     vq->work_steal = false;
>>>        vq->last_used_idx = 0;
>>>        vq->num_added = 0;
>>>        vq->packed_ring = true;
>>> @@ -2038,6 +2042,9 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
>>>
>>>        if (!more_used(vq)) {
>>>                pr_debug("virtqueue interrupt with no work for %p\n", vq);
>>
>> Do we still need to keep this warning?
> Come to think of it, I would say no, in this case.
>
>>
>>> +             if (vq->work_steal)
>>> +                     return IRQ_HANDLED;
>>
>> So I wonder instead of doing trick like this, maybe it's time to unify
>> TX/RX NAPI with the help of[1] (virtio-net use queue pairs).
>>
>> Thanks
>>
>> [1] https://lkml.org/lkml/2014/12/25/169
> Interesting idea. It does sound like a good fit for this model. The
> patch in the Fixes line proved effective at suppressing unnecessary TX
> interrupts when processing in RX interrupt handler. So not sure how
> much will help in practice. Might be a nice project to evaluate
> separate for net-next at some point.


Right, basically the idea is that if an irq is shared among several 
virtqueues, there's no need to check for more_used() there.

Yes, we can try sometime in the future. (Or e.g we have more than 128 
queue pairs).

Thanks


>
> Thanks for the review!
>

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