netdev - Re: [PATCH net] virtio-net: add cond_resched() to the command waiting loop

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Date:   Thu, 8 Sep 2022 10:21:45 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Gautam Dawar <gautam.dawar@...inx.com>,
        davem <davem@...emloft.net>
Subject: Re: [PATCH net] virtio-net: add cond_resched() to the command waiting
 loop


在 2022/9/7 15:46, Michael S. Tsirkin 写道:
> On Wed, Sep 07, 2022 at 09:07:20AM +0200, Paolo Abeni wrote:
>> On Wed, 2022-09-07 at 10:09 +0800, Jason Wang wrote:
>>> On Tue, Sep 6, 2022 at 6:56 PM Paolo Abeni <pabeni@...hat.com> wrote:
>>>> On Mon, 2022-09-05 at 15:49 +0800, Jason Wang wrote:
>>>>> On Mon, Sep 5, 2022 at 3:15 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>>>>>> On Mon, Sep 05, 2022 at 12:53:41PM +0800, Jason Wang wrote:
>>>>>>> Adding cond_resched() to the command waiting loop for a better
>>>>>>> co-operation with the scheduler. This allows to give CPU a breath to
>>>>>>> run other task(workqueue) instead of busy looping when preemption is
>>>>>>> not allowed.
>>>>>>>
>>>>>>> What's more important. This is a must for some vDPA parent to work
>>>>>>> since control virtqueue is emulated via a workqueue for those parents.
>>>>>>>
>>>>>>> Fixes: bda324fd037a ("vdpasim: control virtqueue support")
>>>>>> That's a weird commit to fix. so it fixes the simulator?
>>>>> Yes, since the simulator is using a workqueue to handle control virtueue.
>>>> Uhmm... touching a driver for a simulator's sake looks a little weird.
>>> Simulator is not the only one that is using a workqueue (but should be
>>> the first).
>>>
>>> I can see  that the mlx5 vDPA driver is using a workqueue as well (see
>>> mlx5_vdpa_kick_vq()).
>>>
>>> And in the case of VDUSE, it needs to wait for the response from the
>>> userspace, this means cond_resched() is probably a must for the case
>>> like UP.
>>>
>>>> Additionally, if the bug is vdpasim, I think it's better to try to
>>>> solve it there, if possible.
>>>>
>>>> Looking at vdpasim_net_work() and vdpasim_blk_work() it looks like
>>>> neither needs a process context, so perhaps you could rework it to run
>>>> the work_fn() directly from vdpasim_kick_vq(), at least for the control
>>>> virtqueue?
>>> It's possible (but require some rework on the simulator core). But
>>> considering we have other similar use cases, it looks better to solve
>>> it in the virtio-net driver.
>> I see.
>>
>>> Additionally, this may have better behaviour when using for the buggy
>>> hardware (e.g the control virtqueue takes too long to respond). We may
>>> consider switching to use interrupt/sleep in the future (but not
>>> suitable for -net).
>> Agreed. Possibly a timeout could be useful, too.
>>
>> Cheers,
>>
>> Paolo
>
> Hmm timeouts are kind of arbitrary.
> regular drivers basically derive them from hardware
> behaviour but with a generic driver like virtio it's harder.
> I guess we could add timeout as a config field, have
> device make a promise to the driver.
>
> Making the wait interruptible seems more reasonable.


Yes, but I think we still need this patch for -net and -stable.

Thanks


>