On Tue, Apr 26, 2022 at 05:47:17PM +0200, Cornelia Huck wrote: > On Mon, Apr 25 2022, "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. > > How bad would that be in practice? IIUC, we hit on the spinlock when > - doing synchronize_cbs (should be rare) > - processing queue interrupts for devices using per-device indicators > (which is the non-preferred path, which I would basically only expect > when running on an ancient or non-standard hypervisor) this one is my concern. I am worried serializing everything on a single lock will drastically regress performance here. > - configuration change interrupts (should be rare) > - during setup, reset, etc. (should not be a concern) > > >> > > > > > >> > > > > 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. > >> Maybe something per-cpu + some IPIs to run things on all CPUs instead? > > > > ... and I think classic and device interrupts are different enough > > here ... > > You mean classic (per-device) and adapter interrupts, right? _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
