Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Fri, 26 Jul 2019 21:36:18 +0800 |
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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
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