Messages in this thread | | | Subject | Re: WARNING in __mmdrop | From | Jason Wang <> | Date | Mon, 29 Jul 2019 22:24:43 +0800 |
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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. And actually it's the price of using vq mutex. Actually, the critical section should be rather small, e.g just inside memory accessors.
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(); }
Or using smp_load_acquire()/smp_store_release() instead?
Thanks
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