On Tue, Apr 26 2022, "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote: > 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. Yeah, that case could get much worse. OTOH, how likely is it that any setup that runs a recent kernel will actually end up with devices using per-device indicators? Anything running under a QEMU released in the last couple of years is unlikely to not use airqs, I think. Halil, do you think that the classic indicator setup would be more common on any non-QEMU hypervisors? IOW, how much effort is it worth spending on optimizing this case? We certainly should explore any simple solutions, but I don't think we need to twist ourselves into pretzels to solve it. > > >> - configuration change interrupts (should be rare) >> - during setup, reset, etc. (should not be a concern) _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
