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Date:   Tue, 26 Apr 2022 12:43:36 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Cornelia Huck <cohuck@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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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:
> 
> > 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@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@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@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>
> >> > > > > > > > 
> >> > > > > > > > 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?