Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 05, 2019 at 12:41:45PM +0800, Jason Wang wrote:
> 
> On 2019/8/5 下午12:36, Jason Wang wrote:
> > 
> > On 2019/8/2 下午10:27, Michael S. Tsirkin wrote:
> > > On Fri, Aug 02, 2019 at 09:46:13AM -0300, Jason Gunthorpe wrote:
> > > > On Fri, Aug 02, 2019 at 05:40:07PM +0800, Jason Wang wrote:
> > > > > > This must be a proper barrier, like a spinlock, mutex, or
> > > > > > synchronize_rcu.
> > > > > 
> > > > > I start with synchronize_rcu() but both you and Michael raise some
> > > > > concern.
> > > > I've also idly wondered if calling synchronize_rcu() under the various
> > > > mm locks is a deadlock situation.
> > > > 
> > > > > Then I try spinlock and mutex:
> > > > > 
> > > > > 1) spinlock: add lots of overhead on datapath, this leads 0
> > > > > performance
> > > > > improvement.
> > > > I think the topic here is correctness not performance improvement
> > > The topic is whether we should revert
> > > commit 7f466032dc9 ("vhost: access vq metadata through kernel
> > > virtual address")
> > > 
> > > or keep it in. The only reason to keep it is performance.
> > 
> > 
> > Maybe it's time to introduce the config option?
> 
> 
> Or does it make sense if I post a V3 with:
> 
> - introduce config option and disable the optimization by default
> 
> - switch from synchronize_rcu() to vhost_flush_work(), but the rest are the
> same
> 
> This can give us some breath to decide which way should go for next release?
> 
> Thanks

As is, with preempt enabled?  Nope I don't think blocking an invalidator
on swap IO is ok, so I don't believe this stuff is going into this
release at this point.

So it's more a question of whether it's better to revert and apply a clean
patch on top, or just keep the code around but disabled with an ifdef as is.
I'm open to both options, and would like your opinion on this.

> 
> > 
> > 
> > > 
> > > Now as long as all this code is disabled anyway, we can experiment a
> > > bit.
> > > 
> > > I personally feel we would be best served by having two code paths:
> > > 
> > > - Access to VM memory directly mapped into kernel
> > > - Access to userspace
> > > 
> > > 
> > > Having it all cleanly split will allow a bunch of optimizations, for
> > > example for years now we planned to be able to process an incoming short
> > > packet directly on softirq path, or an outgoing on directly within
> > > eventfd.
> > 
> > 
> > It's not hard consider we've already had our own accssors. But the
> > question is (as asked in another thread), do you want permanent GUP or
> > still use MMU notifiers.
> > 
> > Thanks
> > 
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