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

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Message-ID: <20220425233434-mutt-send-email-mst@kernel.org>
Date:   Mon, 25 Apr 2022 23:35:37 -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 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.

-- 
MST