On Fri, Jul 26, 2019 at 09:36:18PM +0800, Jason Wang wrote: > > On 2019/7/26 下午8:53, Jason Wang wrote: > > > > On 2019/7/26 下午8:38, Michael S. Tsirkin wrote: > > > On Fri, Jul 26, 2019 at 08:00:58PM +0800, Jason Wang wrote: > > > > On 2019/7/26 下午7:49, Michael S. Tsirkin wrote: > > > > > On Thu, Jul 25, 2019 at 10:25:25PM +0800, Jason Wang wrote: > > > > > > On 2019/7/25 下午9:26, Michael S. Tsirkin wrote: > > > > > > > > Exactly, and that's the reason actually I use > > > > > > > > synchronize_rcu() there. > > > > > > > > > > > > > > > > So the concern is still the possible synchronize_expedited()? > > > > > > > I think synchronize_srcu_expedited. > > > > > > > > > > > > > > synchronize_expedited sends lots of IPI and is bad for realtime VMs. > > > > > > > > > > > > > > > Can I do this > > > > > > > > on through another series on top of the incoming V2? > > > > > > > > > > > > > > > > Thanks > > > > > > > > > > > > > > > The question is this: is this still a gain if we switch to the > > > > > > > more expensive srcu? If yes then we can keep the feature on, > > > > > > I think we only care about the cost on srcu_read_lock() > > > > > > which looks pretty > > > > > > tiny form my point of view. Which is basically a > > > > > > READ_ONCE() + WRITE_ONCE(). > > > > > > > > > > > > Of course I can benchmark to see the difference. > > > > > > > > > > > > > > > > > > > if not we'll put it off until next release and think > > > > > > > of better solutions. rcu->srcu is just a find and replace, > > > > > > > don't see why we need to defer that. can be a separate patch > > > > > > > for sure, but we need to know how well it works. > > > > > > I think I get here, let me try to do that in V2 and > > > > > > let's see the numbers. > > > > > > > > > > > > Thanks > > > > > > > > It looks to me for tree rcu, its srcu_read_lock() have a mb() > > > > which is too > > > > expensive for us. > > > I will try to ponder using vq lock in some way. > > > Maybe with trylock somehow ... > > > > > > Ok, let me retry if necessary (but I do remember I end up with deadlocks > > last try). > > > Ok, I play a little with this. And it works so far. Will do more testing > tomorrow. > > One reason could be I switch to use get_user_pages_fast() to > __get_user_pages_fast() which doesn't need mmap_sem. > > Thanks OK that sounds good. If we also set a flag to make vhost_exceeds_weight exit, then I think it will be all good. -- MST
