Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()

[Jason Wang]

在 2022/4/26 11:38, Michael S. Tsirkin 写道:
> On Mon, Apr 25, 2022 at 11:35:41PM -0400, Michael S. Tsirkin wrote:
> > On Tue, Apr 26, 2022 at 04:29:11AM +0200, Halil Pasic wrote:
> > > On Mon, 25 Apr 2022 09:59:55 -0400
> > > "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > >
> > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote:
> > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> > > > >    
> > > > > > On Mon, Apr 25, 2022 at 10:44:15AM +0800, Jason Wang wrote:
> > > > > > > This patch tries to implement the synchronize_cbs() for ccw. For the
> > > > > > > vring_interrupt() that is called via virtio_airq_handler(), the
> > > > > > > synchronization is simply done via the airq_info's lock. For the
> > > > > > > vring_interrupt() that is called via virtio_ccw_int_handler(), a per
> > > > > > > device spinlock for irq is introduced ans used in the synchronization
> > > > > > > method.
> > > > > > >
> > > > > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > > > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > > > > > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > > > > > > Cc: Marc Zyngier <maz@xxxxxxxxxx>
> > > > > > > Cc: Halil Pasic <pasic@xxxxxxxxxxxxx>
> > > > > > > Cc: Cornelia Huck <cohuck@xxxxxxxxxx>
> > > > > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > > > >
> > > > > > This is the only one that is giving me pause. Halil, Cornelia,
> > > > > > should we be concerned about the performance impact here?
> > > > > > Any chance it can be tested?
> > > > > We can have a bunch of devices using the same airq structure, and the
> > > > > sync cb creates a choke point, same as registering/unregistering.
> > > > BTW can callbacks for multiple VQs run on multiple CPUs at the moment?
> > > I'm not sure I understand the question.
> > >
> > > I do think we can have multiple CPUs that are executing some portion of
> > > virtio_ccw_int_handler(). So I guess the answer is yes. Connie what do you think?
> > >
> > > On the other hand we could also end up serializing synchronize_cbs()
> > > calls for different devices if they happen to use the same airq_info. But
> > > this probably was not your question
> >
> > I am less concerned about  synchronize_cbs being slow and more about
> > the slowdown in interrupt processing itself.
> >
> > > > this patch serializes them on a spinlock.
> > > Those could then pile up on the newly introduced spinlock.
> > >
> > > Regards,
> > > Halil
> > Hmm yea ... not good.
> Is there any other way to synchronize with all callbacks?


Maybe using rwlock as airq handler?

Thanks


> > --
> > MST