Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()

[Cornelia Huck]

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)