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

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
> >

On Thu, Apr 28, 2022 at 1:55 PM Michael S. Tsirkin <mst at redhat.com>
wrote:
>
> 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?
I think what you meant is using a dedicated rwlock instead of trying
to reuse one of the airq_info locks.

If this is true, it should be fine.

Thanks
>
> > >
> > > --
> > > MST
> > >
>