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

On Mon, 25 Apr 2022 09:59:55 -0400
"Michael S. Tsirkin" <mst at redhat.com> wrote:
> On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote:
> > On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst at
redhat.com> 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 at linutronix.de>
> > >> Cc: Peter Zijlstra <peterz at infradead.org>
> > >> Cc: "Paul E. McKenney" <paulmck at
kernel.org>
> > >> Cc: Marc Zyngier <maz at kernel.org>
> > >> Cc: Halil Pasic <pasic at linux.ibm.com>
> > >> Cc: Cornelia Huck <cohuck at redhat.com>
> > >> Signed-off-by: Jason Wang <jasowang at redhat.com>  
> > >
> > >
> > > 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
> this patch serializes them on a spinlock.
>
Those could then pile up on the newly introduced spinlock.

Regards,
Halil

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 at redhat.com> wrote:
> 
> > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote:
> > > On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst at
redhat.com> 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 at linutronix.de>
> > > >> Cc: Peter Zijlstra <peterz at infradead.org>
> > > >> Cc: "Paul E. McKenney" <paulmck at
kernel.org>
> > > >> Cc: Marc Zyngier <maz at kernel.org>
> > > >> Cc: Halil Pasic <pasic at linux.ibm.com>
> > > >> Cc: Cornelia Huck <cohuck at redhat.com>
> > > >> Signed-off-by: Jason Wang <jasowang at redhat.com>
> > > >
> > > >
> > > > 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.

-- 
MST