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From:   Cornelia Huck <cohuck@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Jason Wang <jasowang@redhat.com>,
        Halil Pasic <pasic@linux.ibm.com>,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, sgarzare@redhat.com,
        eperezma@redhat.com, lulu@redhat.com, tglx@linutronix.de,
        peterz@infradead.org, paulmck@kernel.org, maz@kernel.org
Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()
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Date:   Wed, 27 Apr 2022 11:27:03 +0200
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On Tue, Apr 26 2022, "Michael S. Tsirkin" <mst@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@redhat.com> wrote:
>>=20
>> > 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:
>> >> >=20
>> >> > =E5=9C=A8 2022/4/26 11:38, Michael S. Tsirkin =E5=86=99=E9=81=93:
>> >> > > On Mon, Apr 25, 2022 at 11:35:41PM -0400, Michael S. Tsirkin wrot=
e:
>> >> > > > 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@redhat.com> wrote:
>> >> > > > >=20
>> >> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wro=
te:
>> >> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst@redhat.com=
> wrote:
>> >> > > > > > > > On Mon, Apr 25, 2022 at 10:44:15AM +0800, Jason Wang wr=
ote:
>> >> > > > > > > > > This patch tries to implement the synchronize_cbs() f=
or ccw. For the
>> >> > > > > > > > > vring_interrupt() that is called via virtio_airq_hand=
ler(), the
>> >> > > > > > > > > synchronization is simply done via the airq_info's lo=
ck. For the
>> >> > > > > > > > > vring_interrupt() that is called via virtio_ccw_int_h=
andler(), a per
>> >> > > > > > > > > device spinlock for irq is introduced ans used in the=
 synchronization
>> >> > > > > > > > > method.
>> >> > > > > > > > >=20
>> >> > > > > > > > > Cc: Thomas Gleixner <tglx@linutronix.de>
>> >> > > > > > > > > Cc: Peter Zijlstra <peterz@infradead.org>
>> >> > > > > > > > > Cc: "Paul E. McKenney" <paulmck@kernel.org>
>> >> > > > > > > > > Cc: Marc Zyngier <maz@kernel.org>
>> >> > > > > > > > > Cc: Halil Pasic <pasic@linux.ibm.com>
>> >> > > > > > > > > Cc: Cornelia Huck <cohuck@redhat.com>
>> >> > > > > > > > > Signed-off-by: Jason Wang <jasowang@redhat.com>
>> >> > > > > > > >=20
>> >> > > > > > > > This is the only one that is giving me pause. Halil, Co=
rnelia,
>> >> > > > > > > > should we be concerned about the performance impact her=
e?
>> >> > > > > > > > Any chance it can be tested?
>> >> > > > > > > We can have a bunch of devices using the same airq struct=
ure, and the
>> >> > > > > > > sync cb creates a choke point, same as registering/unregi=
stering.
>> >> > > > > > BTW can callbacks for multiple VQs run on multiple CPUs at =
the moment?
>> >> > > > > I'm not sure I understand the question.
>> >> > > > >=20
>> >> > > > > 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. Conni=
e what do you think?
>> >> > > > >=20
>> >> > > > > On the other hand we could also end up serializing synchroniz=
e_cbs()
>> >> > > > > calls for different devices if they happen to use the same ai=
rq_info. But
>> >> > > > > this probably was not your question
>> >> > > >=20
>> >> > > > I am less concerned about  synchronize_cbs being slow and more =
about
>> >> > > > the slowdown in interrupt processing itself.
>> >> > > >=20
>> >> > > > > > this patch serializes them on a spinlock.
>> >> > > > > >=20
>> >> > > > > Those could then pile up on the newly introduced spinlock.
>>=20
>> 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 l=
ock
> 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)