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

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Jason Wang <jasowang@redhat.com>
Cc: Halil Pasic <pasic@linux.ibm.com>,
	Cornelia Huck <cohuck@redhat.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()
Date: Mon, 25 Apr 2022 23:53:19 -0400	[thread overview]
Message-ID: <20220425235134-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <ba0c3977-c471-3275-2327-c5910cdd506a@redhat.com>

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.
> > > > 
> > > > 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?

> > 
> > > -- 
> > > MST