On Mon, Apr 25 2022, "Michael S. Tsirkin" <m...@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: >> > >> > 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" <m...@redhat.com> wrote: >> > > > > >> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote: >> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" <m...@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 <t...@linutronix.de> >> > > > > > > > > Cc: Peter Zijlstra <pet...@infradead.org> >> > > > > > > > > Cc: "Paul E. McKenney" <paul...@kernel.org> >> > > > > > > > > Cc: Marc Zyngier <m...@kernel.org> >> > > > > > > > > Cc: Halil Pasic <pa...@linux.ibm.com> >> > > > > > > > > Cc: Cornelia Huck <coh...@redhat.com> >> > > > > > > > > Signed-off-by: Jason Wang <jasow...@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) - 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@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/virtualization
