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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Wed, Jul 31, 2019 at 09:28:20PM +0800, Jason Wang wrote:
> 
> On 2019/7/31 下午8:39, Jason Gunthorpe wrote:
> > On Wed, Jul 31, 2019 at 04:46:53AM -0400, Jason Wang wrote:
> > > We used to use RCU to synchronize MMU notifier with worker. This leads
> > > calling synchronize_rcu() in invalidate_range_start(). But on a busy
> > > system, there would be many factors that may slow down the
> > > synchronize_rcu() which makes it unsuitable to be called in MMU
> > > notifier.
> > > 
> > > A solution is SRCU but its overhead is obvious with the expensive full
> > > memory barrier. Another choice is to use seqlock, but it doesn't
> > > provide a synchronization method between readers and writers. The last
> > > choice is to use vq mutex, but it need to deal with the worst case
> > > that MMU notifier must be blocked and wait for the finish of swap in.
> > > 
> > > So this patch switches use a counter to track whether or not the map
> > > was used. The counter was increased when vq try to start or finish
> > > uses the map. This means, when it was even, we're sure there's no
> > > readers and MMU notifier is synchronized. When it was odd, it means
> > > there's a reader we need to wait it to be even again then we are
> > > synchronized.
> > You just described a seqlock.
> 
> 
> Kind of, see my explanation below.
> 
> 
> > 
> > We've been talking about providing this as some core service from mmu
> > notifiers because nearly every use of this API needs it.
> 
> 
> That would be very helpful.
> 
> 
> > 
> > IMHO this gets the whole thing backwards, the common pattern is to
> > protect the 'shadow pte' data with a seqlock (usually open coded),
> > such that the mmu notififer side has the write side of that lock and
> > the read side is consumed by the thread accessing or updating the SPTE.
> 
> 
> Yes, I've considered something like that. But the problem is, mmu notifier
> (writer) need to wait for the vhost worker to finish the read before it can
> do things like setting dirty pages and unmapping page.  It looks to me
> seqlock doesn't provide things like this.  

The seqlock is usually used to prevent a 2nd thread from accessing the
VA while it is being changed by the mm. ie you use something seqlocky
instead of the ugly mmu_notifier_unregister/register cycle.

You are supposed to use something simple like a spinlock or mutex
inside the invalidate_range_start to serialized tear down of the SPTEs
with their accessors.

> write_seqcount_begin()
> 
> map = vq->map[X]
> 
> write or read through map->addr directly
> 
> write_seqcount_end()
> 
> 
> There's no rmb() in write_seqcount_begin(), so map could be read before
> write_seqcount_begin(), but it looks to me now that this doesn't harm at
> all, maybe we can try this way.

That is because it is a write side lock, not a read lock. IIRC
seqlocks have weaker barriers because the write side needs to be
serialized in some other way.

The requirement I see is you need invalidate_range_start to block
until another thread exits its critical section (ie stops accessing
the SPTEs). 

That is a spinlock/mutex.

You just can't invent a faster spinlock by open coding something with
barriers, it doesn't work.

Jason