Michael S. Tsirkin
2022-Sep-08 05:19 UTC
[PATCH net] virtio-net: add cond_resched() to the command waiting loop
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. > > ThanksI was referring to Paolo's idea of having a timeout. -- MST
Jason Wang
2022-Oct-09 05:58 UTC
[PATCH net] virtio-net: add cond_resched() to the command waiting loop
? 2022/9/8 13:19, Michael S. Tsirkin ??:> 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.Ok, I think we're fine with this patch. Any chance to merge this or do I need to resend? Thanks>