On Wed, Apr 27, 2022 at 2:30 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > On Wed, Apr 27, 2022 at 11:53:25AM +0800, Jason Wang wrote: > > On Tue, Apr 26, 2022 at 2:30 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > > > On Tue, Apr 26, 2022 at 12:07:39PM +0800, Jason Wang wrote: > > > > On Tue, Apr 26, 2022 at 11:55 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > > > > > > > On Mon, Apr 25, 2022 at 11:53:24PM -0400, Michael S. Tsirkin wrote: > > > > > > On Tue, Apr 26, 2022 at 11:42:45AM +0800, Jason Wang wrote: > > > > > > > > > > > > > > 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 > > > > > > > > > > > > > > > > > > > rwlock is still a shared cacheline bouncing between CPUs and > > > > > > a bunch of ordering instructions. > > > > > > > > Yes, but it should be faster than spinlocks anyhow. > > > > > > > > > > Maybe something per-cpu + some IPIs to run things on all CPUs instead? > > > > > > > > Is this something like a customized version of synchronzie_rcu_expedited()? > > > > > > With interrupts running in an RCU read size critical section? > > > > For vring_interrupt(), yes. > > > > > > > Quite possibly that is also an option. > > > This will need a bunch of documentation since this is not > > > a standard use of RCU, > > > > According to Documentation/RCU/requirements.rst, it looks like a legal case: > > > > " > > The Linux kernel has interrupts, and RCU read-side critical sections are > > legal within interrupt handlers and within interrupt-disabled regions of > > code, as are invocations of call_rcu(). > > " > > My problem is it is not clear what data is protected by rcu here. > Nothing is tagged with __rcu or uses rcu_dereference. It should be the vq->broken here. We can rcuify it. > We need at least an ack from rcu maintainers that this is > a valid use of rcu and not an undocumented side effect. Yes. Thanks > > > And as discussed, synchronize_rcu_expedited() is not friendly to real > > time workload. I think we can simply > > > > 1) protect vring_interrupt() with rcu_read_lock() > > 2) use synchronize_rcu() in synchronize_cbs for ccw > > > > And if we care about the long delay we can use per device srcu to reduce that? > > > > Thanks > > > > > and probably get a confirmation > > > from RCU maintainers that whatever assumptions we make > > > are guaranteed to hold down the road. > > > > > > > > > > > > > ... and I think classic and device interrupts are different enough > > > > > here ... > > > > > > > > Yes. > > > > > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > -- > > > > > > > > > MST > > > > > > > > > _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
