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Date:   Sun, 9 Oct 2022 13:58:53 +0800
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Subject: Re: [PATCH net] virtio-net: add cond_resched() to the command waiting
 loop
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To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Jakub Kicinski <kuba@kernel.org>
Cc:     Paolo Abeni <pabeni@redhat.com>, netdev <netdev@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        virtualization <virtualization@lists.linux-foundation.org>,
        Eric Dumazet <edumazet@google.com>,
        Gautam Dawar <gautam.dawar@xilinx.com>,
        davem <davem@davemloft.net>
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From:   Jason Wang <jasowang@redhat.com>
In-Reply-To: <20220908011858-mutt-send-email-mst@kernel.org>
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在 2022/9/8 13:19, Michael S. Tsirkin 写道:
> On Thu, Sep 08, 2022 at 10:21:45AM +0800, Jason Wang wrote:
>> 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@redhat.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@redhat.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
> I was referring to Paolo's idea of having a timeout.


Ok, I think we're fine with this patch. Any chance to merge this or do I 
need to resend?

Thanks


>