Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Tue, 30 Jul 2019 15:44:47 +0800 |
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On 2019/7/29 下午10:44, Michael S. Tsirkin wrote: > On Mon, Jul 29, 2019 at 10:24:43PM +0800, Jason Wang wrote: >> On 2019/7/29 下午4:59, Michael S. Tsirkin wrote: >>> On Mon, Jul 29, 2019 at 01:54:49PM +0800, Jason Wang wrote: >>>> On 2019/7/26 下午9:49, Michael S. Tsirkin wrote: >>>>>>> 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. >>>> After some experiments, I came up two methods: >>>> >>>> 1) switch to use vq->mutex, then we must take the vq lock during range >>>> checking (but I don't see obvious slowdown for 16vcpus + 16queues). Setting >>>> flags during weight check should work but it still can't address the worst >>>> case: wait for the page to be swapped in. Is this acceptable? >>>> >>>> 2) using current RCU but replace synchronize_rcu() with vhost_work_flush(). >>>> The worst case is the same as 1) but we can check range without holding any >>>> locks. >>>> >>>> Which one did you prefer? >>>> >>>> Thanks >>> I would rather we start with 1 and switch to 2 after we >>> can show some gain. >>> >>> But the worst case needs to be addressed. >> >> Yes. >> >> >>> How about sending a signal to >>> the vhost thread? We will need to fix up error handling (I think that >>> at the moment it will error out in that case, handling this as EFAULT - >>> and we don't want to drop packets if we can help it, and surely not >>> enter any error states. In particular it might be especially tricky if >>> we wrote into userspace memory and are now trying to log the write. >>> I guess we can disable the optimization if log is enabled?). >> >> This may work but requires a lot of changes. > I agree. > >> And actually it's the price of >> using vq mutex. > Not sure what's meant here.
I mean if we use vq mutex, it means the critical section was increased and we need to deal with swapping then.
> >> Actually, the critical section should be rather small, e.g >> just inside memory accessors. > Also true. > >> I wonder whether or not just do synchronize our self like: >> >> static void inline vhost_inc_vq_ref(struct vhost_virtqueue *vq) >> { >> int ref = READ_ONCE(vq->ref); >> >> WRITE_ONCE(vq->ref, ref + 1); >> smp_rmb(); >> } >> >> static void inline vhost_dec_vq_ref(struct vhost_virtqueue *vq) >> { >> int ref = READ_ONCE(vq->ref); >> >> smp_wmb(); >> WRITE_ONCE(vq->ref, ref - 1); >> } >> >> static void inline vhost_wait_for_ref(struct vhost_virtqueue *vq) >> { >> while (READ_ONCE(vq->ref)); >> mb(); >> } > Looks good but I'd like to think of a strategy/existing lock that let us > block properly as opposed to spinning, that would be more friendly to > e.g. the realtime patch.
Does it make sense to disable preemption in the critical section? Then we don't need to block and we have a deterministic time spent on memory accssors?
> >> Or using smp_load_acquire()/smp_store_release() instead? >> >> Thanks > These are cheaper on x86, yes.
Will use this.
Thanks
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