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Date: Mon, 5 Aug 2019 12:20:45 +0800 From: Jason Wang <jasowang@...hat.com> To: Jason Gunthorpe <jgg@...pe.ca> Cc: mst@...hat.com, kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [PATCH V2 7/9] vhost: do not use RCU to synchronize MMU notifier with worker On 2019/8/2 下午8:46, 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. Maybe, that's why I suggest to use vhost_work_flush() which is much lightweight can can achieve the same function. It can guarantee all previous work has been processed after vhost_work_flush() return. > >> 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 But the whole series is to speed up vhost. > >> 2) SRCU: full memory barrier requires on srcu_read_lock(), which still leads >> little performance improvement > >> 3) mutex: a possible issue is need to wait for the page to be swapped in (is >> this unacceptable ?), another issue is that we need hold vq lock during >> range overlap check. > I have a feeling that mmu notififers cannot safely become dependent on > progress of swap without causing deadlock. You probably should avoid > this. Yes, so that's why I try to synchronize the critical region by myself. >>> And, again, you can't re-invent a spinlock with open coding and get >>> something better. >> So the question is if waiting for swap is considered to be unsuitable for >> MMU notifiers. If not, it would simplify codes. If not, we still need to >> figure out a possible solution. >> >> Btw, I come up another idea, that is to disable preemption when vhost thread >> need to access the memory. Then register preempt notifier and if vhost >> thread is preempted, we're sure no one will access the memory and can do the >> cleanup. > I think you should use the spinlock so at least the code is obviously > functionally correct and worry about designing some properly justified > performance change after. > > Jason Spinlock is correct but make the whole series meaningless consider it won't bring any performance improvement. Thanks
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