Re: [PATCH virtio 0/8] virtio_pci_modern: allow parallel admin queue commands execution

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 24, 2024 at 05:10:36PM +0200, Jiri Pirko wrote:
> Mon, Jun 24, 2024 at 01:23:01PM CEST, mst@redhat.com wrote:
> >On Mon, Jun 24, 2024 at 05:53:52PM +0800, Heng Qi wrote:
> >> On Mon, 24 Jun 2024 11:04:43 +0200, Jiri Pirko <jiri@resnulli.us> wrote:
> >> > From: Jiri Pirko <jiri@nvidia.com>
> >> > 
> >> > Currently the admin queue command execution is serialized by a lock.
> >> > This patchsets lifts this limitation allowing to execute admin queue
> >> > commands in parallel. To do that, admin queue processing needs to be
> >> > converted from polling to interrupt based completion.
> >> > 
> >> > Patches #1-#6 are preparations, making things a bit smoother as well.
> >> > Patch #7 implements interrupt based completion for admin queue.
> >> 
> >> Hi, Jiri
> >> 
> >> Before this set, I pushed the cvq irq set [1], and the discussion focused on the
> >> fact that the newly added irq vector may cause the IO queue to fall back to
> >> shared interrupt mode.
> >> But it is true that devices implemented according to the specification should
> >> not encounter this problem. So what do you think?
> >> 
> >> [1] https://lore.kernel.org/all/20240619171708-mutt-send-email-mst@kernel.org/
> >
> >It's true - this can cause guest to run out of vectors for a variety of
> >reasons.
> >
> >First we have guest irqs - I am guessing avq could use IRQF_SHARED ?
> >I am not sure why we don't allow IRQF_SHARED for the config
> >interrupt though. So I think addressing this part can be deferred.
> >
> >Second, we might not have enough msix vectors on the device. Here sharing
> >with e.g. cvq and further with config interrupt would make sense.
> >
> >Jiri do you think you can help Heng Qi hammer out a solution for cvq?
> >I feel this will work will then benefit in a similar way,
> >and having us poll aggressively for cvq but not admin commands
> >does not make much sense, right?
> 
> 
> How about to enhance existing fallback in vp_find_vqs() to something
> like this:
> 
>         /* Try MSI-X with one vector per queue. */
>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names, ONE_FOR_EACH_VQ, ctx, desc);
>         if (!err)
>                 return 0;
>         /* Try MSI-X with one vector per data queue, share one for the
>          * rest of the queues and config. */
>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names, ONE_FOR_EACH_DATA_VQ_ONE_FOR_REST, ctx, desc);
>         if (!err)
>                 return 0;
>         /* Fallback: MSI-X with one vector for config, one shared for queues. */
>         err = vp_find_vqs_msix(vdev, nvqs, vqs, callbacks, names, ONE_DATA_VQS_ONE_FOR_REST, ctx, desc);
>         if (!err)
>                 return 0;
> 
> I can implement this for admin queue, control queue can benefit from
> this eventually.
> 

OKay superficially except in attempt 3 I would still combine
adminq with config, for consistency.

> 
> >
> >> > Patch #8 finally removes the admin queue serialization lock.
> >> > 
> >> > Jiri Pirko (8):
> >> >   virtio_pci: push out single vq find code to vp_find_one_vq_msix()
> >> >   virtio_pci_modern: treat vp_dev->admin_vq.info.vq pointer as static
> >> >   virtio: push out code to vp_avq_index()
> >> >   virtio: create admin queues alongside other virtqueues
> >> >   virtio_pci_modern: create admin queue of queried size
> >> >   virtio_pci_modern: pass cmd as an identification token
> >> >   virtio_pci_modern: use completion instead of busy loop to wait on
> >> >     admin cmd result
> >> >   virtio_pci_modern: remove admin queue serialization lock
> >> > 
> >> >  drivers/virtio/virtio.c            |  28 +----
> >> >  drivers/virtio/virtio_pci_common.c | 109 ++++++++++++++------
> >> >  drivers/virtio/virtio_pci_common.h |   9 +-
> >> >  drivers/virtio/virtio_pci_modern.c | 160 ++++++++++++-----------------
> >> >  include/linux/virtio.h             |   2 +
> >> >  include/linux/virtio_config.h      |   2 -
> >> >  6 files changed, 150 insertions(+), 160 deletions(-)
> >> > 
> >> > -- 
> >> > 2.45.1
> >> > 
> >> > 
> >