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From:   Jason Wang <jasowang@redhat.com>
Date:   Fri, 29 Apr 2022 10:02:09 +0800
Message-ID: <CACGkMEtAXGdWKTSh90DH-=0YHhoSFxR43jwA8eROHqxUxdrVYQ@mail.gmail.com>
Subject: Re: [PATCH V3 6/9] virtio-ccw: implement synchronize_cbs()
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Halil Pasic <pasic@linux.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        virtualization <virtualization@lists.linux-foundation.org>,
        Stefano Garzarella <sgarzare@redhat.com>,
        eperezma <eperezma@redhat.com>, Cindy Lu <lulu@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Marc Zyngier <maz@kernel.org>
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On Thu, Apr 28, 2022 at 3:42 PM Cornelia Huck <cohuck@redhat.com> wrote:
>
> On Thu, Apr 28 2022, Jason Wang <jasowang@redhat.com> wrote:
>
> > On Thu, Apr 28, 2022 at 1:55 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >>
> >> On Thu, Apr 28, 2022 at 01:51:59PM +0800, Jason Wang wrote:
> >> > On Thu, Apr 28, 2022 at 1:24 PM Michael S. Tsirkin <mst@redhat.com> wrote:
> >> > >
> >> > > On Thu, Apr 28, 2022 at 11:04:41AM +0800, Jason Wang wrote:
> >> > > > > But my guess is that rwlock + some testing for the legacy indicator case
> >> > > > > just to double check if there is a heavy regression despite of our
> >> > > > > expectations to see none should do the trick.
> >> > > >
> >> > > > I suggest this, rwlock (for not airq) seems better than spinlock, but
> >> > > > at worst case it will cause cache line bouncing. But I wonder if it's
> >> > > > noticeable (anyhow it has been used for airq).
> >> > > >
> >> > > > Thanks
> >> > >
> >> > > Which existing rwlock does airq use right now? Can we take it to sync?
> >> >
> >> > It's the rwlock in airq_info, it has already been used in this patch.
> >> >
> >> >                 write_lock(&info->lock);
> >> >                 write_unlock(&info->lock);
> >> >
> >> > But the problem is, it looks to me there could be a case that airq is
> >> > not used, (virtio_ccw_int_hander()). That's why the patch use a
> >> > spinlock, it could be optimized with using a rwlock as well.
> >> >
> >> > Thanks
> >>
> >> Ah, right. So let's take that on the legacy path too and Halil promises
> >> to test to make sure performance isn't impacted too badly?
> >
> > I think what you meant is using a dedicated rwlock instead of trying
> > to reuse one of the airq_info locks.
> >
> > If this is true, it should be fine.
>
> FWIW, that approach makes sense to me as well.
>

Good to know that. Let me post a new version.

Thanks