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

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Date:   Mon, 25 Apr 2022 23:38:00 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        Jason Wang <jasowang@...hat.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 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.
> > 
> > Regards,
> > Halil
> 
> Hmm yea ... not good.

Is there any other way to synchronize with all callbacks?

> -- 
> MST