netdev - Re: [PATCH net] virtio-net: suppress bad irq warning for tx napi

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Message-ID: <9b077d6c-aeca-8266-4579-fae02c8b31de@redhat.com>
Date:   Thu, 18 Feb 2021 13:39:19 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>, Wei Wang <weiwan@...gle.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.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 2021/2/10 下午5:14, Michael S. Tsirkin wrote:
> On Tue, Feb 09, 2021 at 10:00:22AM -0800, Wei Wang wrote:
>> On Tue, Feb 9, 2021 at 6:58 AM Willem de Bruijn
>> <willemdebruijn.kernel@...il.com> wrote:
>>>>>>> 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.
>>>
>>> The approach to schedule tx-napi from virtnet_poll_cleantx instead of
>>> cleaning directly in this rx-napi function was not effective at
>>> suppressing the warning, I understand.
>> Correct. I tried the approach to schedule tx napi instead of directly
>> do free_old_xmit_skbs() in virtnet_poll_cleantx(). But the warning
>> still happens.
> Two questions here: is the device using packed or split vqs?
> And is event index enabled?
>
> I think one issue is that at the moment with split and event index we
> don't actually disable events at all.


Do we really have a way to disable that? (We don't have a flag like 
packed virtqueue)

Or you mean the trick [1] when I post tx interrupt RFC?

Thanks

[1] https://lkml.org/lkml/2015/2/9/113


>
> static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> {
>          struct vring_virtqueue *vq = to_vvq(_vq);
>
>          if (!(vq->split.avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
>                  vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
>                  if (!vq->event)
>                          vq->split.vring.avail->flags =
>                                  cpu_to_virtio16(_vq->vdev,
>                                                  vq->split.avail_flags_shadow);
>          }
> }
>
> Can you try your napi patch + disable event index?
>
>