Virtualization - Apr 2022 - [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()

On Thu, Apr 28, 2022 at 1:24 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> On Thu, Apr 28, 2022 at 11:04:41AM +0800, Jason Wang wrote:
> > > But my guess is that rwlock + some testing for the legacy
indicator case
> > > just to double check if there is a heavy regression despite of
our
> > > expectations to see none should do the trick.
> >
> > I suggest this, rwlock (for not airq) seems better than spinlock, but
> > at worst case it will cause cache line bouncing. But I wonder if
it's
> > noticeable (anyhow it has been used for airq).
> >
> > Thanks
>
> Which existing rwlock does airq use right now? Can we take it to sync?
It's the rwlock in airq_info, it has already been used in this patch.

                write_lock(&info->lock);
                write_unlock(&info->lock);

But the problem is, it looks to me there could be a case that airq is
not used, (virtio_ccw_int_hander()). That's why the patch use a
spinlock, it could be optimized with using a rwlock as well.

Thanks
>
> --
> MST
>

On Thu, Apr 28, 2022 at 01:51:59PM +0800, Jason Wang
wrote:
> On Thu, Apr 28, 2022 at 1:24 PM Michael S. Tsirkin <mst at
redhat.com> wrote:
> >
> > On Thu, Apr 28, 2022 at 11:04:41AM +0800, Jason Wang wrote:
> > > > But my guess is that rwlock + some testing for the legacy
indicator case
> > > > just to double check if there is a heavy regression despite
of our
> > > > expectations to see none should do the trick.
> > >
> > > I suggest this, rwlock (for not airq) seems better than spinlock,
but
> > > at worst case it will cause cache line bouncing. But I wonder if
it's
> > > noticeable (anyhow it has been used for airq).
> > >
> > > Thanks
> >
> > Which existing rwlock does airq use right now? Can we take it to sync?
> 
> It's the rwlock in airq_info, it has already been used in this patch.
> 
>                 write_lock(&info->lock);
>                 write_unlock(&info->lock);
> 
> But the problem is, it looks to me there could be a case that airq is
> not used, (virtio_ccw_int_hander()). That's why the patch use a
> spinlock, it could be optimized with using a rwlock as well.
> 
> Thanks
Ah, right. So let's take that on the legacy path too and Halil promises
to test to make sure performance isn't impacted too badly?
> >
> > --
> > MST
> >