Virtualization - Sep 2022 - [PATCH net] virtio-net: add cond_resched() to the command waiting loop

? 2022/9/7 15:46, Michael S. Tsirkin ??:
> 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 at
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
at 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

>

On Thu, Sep 08, 2022 at 10:21:45AM +0800, Jason Wang
wrote:
> 
> ? 2022/9/7 15:46, Michael S. Tsirkin ??:
> > 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 at
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 at 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.

-- 
MST