linux-kernel - Re: [PATCH V3 6/9] virtio-ccw: implement synchronize

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Date:   Tue, 26 Apr 2022 17:47:17 +0200
From:   Cornelia Huck <cohuck@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, sgarzare@...hat.com,
        eperezma@...hat.com, lulu@...hat.com, tglx@...utronix.de,
        peterz@...radead.org, paulmck@...nel.org, maz@...nel.org
Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()

On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst@...hat.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" <mst@...hat.com> wrote:
>> > > > > 
>> > > > > > On Mon, Apr 25, 2022 at 10:54:24AM +0200, Cornelia Huck wrote:
>> > > > > > > On Mon, Apr 25 2022, "Michael S. Tsirkin" <mst@...hat.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@...utronix.de>
>> > > > > > > > > Cc: Peter Zijlstra <peterz@...radead.org>
>> > > > > > > > > Cc: "Paul E. McKenney" <paulmck@...nel.org>
>> > > > > > > > > Cc: Marc Zyngier <maz@...nel.org>
>> > > > > > > > > Cc: Halil Pasic <pasic@...ux.ibm.com>
>> > > > > > > > > Cc: Cornelia Huck <cohuck@...hat.com>
>> > > > > > > > > Signed-off-by: Jason Wang <jasowang@...hat.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?