Michael S. Tsirkin
2022-Apr-26 16:43 UTC
[PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()
On Tue, Apr 26, 2022 at 05:47:17PM +0200, Cornelia Huck wrote:> On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst at redhat.com> 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: > >> > > >> > ? 2022/4/26 11:38, Michael S. Tsirkin ??: > >> > > 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 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. > > 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?
Cornelia Huck
2022-Apr-27 09:27 UTC
[PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()
On Tue, Apr 26 2022, "Michael S. Tsirkin" <mst at redhat.com> wrote:> On Tue, Apr 26, 2022 at 05:47:17PM +0200, Cornelia Huck wrote: >> On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst at redhat.com> 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: >> >> > >> >> > ? 2022/4/26 11:38, Michael S. Tsirkin ??: >> >> > > 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 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. >> >> 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)