Michael S. Tsirkin
2022-Apr-25 13:59 UTC
[PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()
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? this patch serializes them on a spinlock.> If > invoking the sync cb is a rare operation (same as (un)registering), it > should not affect interrupt processing for other devices too much, but > it really should be rare. > > For testing, you would probably want to use a setup with many devices > that share the same airq area (you can fit a lot of devices if they have > few queues), generate traffic on the queues, and then do something that > triggers the callback (adding/removing a new device in a loop?) > > I currently don't have such a setup handy; Halil, would you be able to > test that? > > > > >> --- > >> drivers/s390/virtio/virtio_ccw.c | 27 +++++++++++++++++++++++++++ > >> 1 file changed, 27 insertions(+) > >> > >> diff --git a/drivers/s390/virtio/virtio_ccw.c b/drivers/s390/virtio/virtio_ccw.c > >> index d35e7a3f7067..c19f07a82d62 100644 > >> --- a/drivers/s390/virtio/virtio_ccw.c > >> +++ b/drivers/s390/virtio/virtio_ccw.c > >> @@ -62,6 +62,7 @@ struct virtio_ccw_device { > >> unsigned int revision; /* Transport revision */ > >> wait_queue_head_t wait_q; > >> spinlock_t lock; > >> + spinlock_t irq_lock; > >> struct mutex io_lock; /* Serializes I/O requests */ > >> struct list_head virtqueues; > >> bool is_thinint; > >> @@ -984,6 +985,27 @@ static const char *virtio_ccw_bus_name(struct virtio_device *vdev) > >> return dev_name(&vcdev->cdev->dev); > >> } > >> > >> +static void virtio_ccw_synchronize_cbs(struct virtio_device *vdev) > >> +{ > >> + struct virtio_ccw_device *vcdev = to_vc_device(vdev); > >> + struct airq_info *info = vcdev->airq_info; > >> + > >> + /* > >> + * Synchronize with the vring_interrupt() called by > >> + * virtio_ccw_int_handler(). > >> + */ > >> + spin_lock(&vcdev->irq_lock); > >> + spin_unlock(&vcdev->irq_lock); > >> + > >> + if (info) { > >> + /* > >> + * Synchronize with the vring_interrupt() with airq indicator > >> + */ > >> + write_lock(&info->lock); > >> + write_unlock(&info->lock); > >> + } > > I think we can make this an either/or operation (devices will either use > classic interrupts or adapter interrupts)? > > >> +} > >> + > >> static const struct virtio_config_ops virtio_ccw_config_ops = { > >> .get_features = virtio_ccw_get_features, > >> .finalize_features = virtio_ccw_finalize_features,
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