On Sun, 24 Feb 2019 at 08:14, Michael S. Tsirkin <m...@redhat.com> wrote: > > On Sat, Feb 23, 2019 at 09:10:01PM +0800, Yongji Xie wrote: > > On Fri, 22 Feb 2019 at 22:54, Michael S. Tsirkin <m...@redhat.com> wrote: > > > > > > On Fri, Feb 22, 2019 at 03:05:23PM +0800, Yongji Xie wrote: > > > > On Fri, 22 Feb 2019 at 14:21, Michael S. Tsirkin <m...@redhat.com> > > > > wrote: > > > > > > > > > > On Fri, Feb 22, 2019 at 10:47:03AM +0800, Yongji Xie wrote: > > > > > > > > + > > > > > > > > +To track inflight I/O, the queue region should be processed as > > > > > > > > follows: > > > > > > > > + > > > > > > > > +When receiving available buffers from the driver: > > > > > > > > + > > > > > > > > + 1. Get the next available head-descriptor index from > > > > > > > > available ring, i > > > > > > > > + > > > > > > > > + 2. Set desc[i].inflight to 1 > > > > > > > > + > > > > > > > > +When supplying used buffers to the driver: > > > > > > > > + > > > > > > > > + 1. Get corresponding used head-descriptor index, i > > > > > > > > + > > > > > > > > + 2. Set desc[i].next to process_head > > > > > > > > + > > > > > > > > + 3. Set process_head to i > > > > > > > > + > > > > > > > > + 4. Steps 1,2,3 may be performed repeatedly if batching is > > > > > > > > possible > > > > > > > > + > > > > > > > > + 5. Increase the idx value of used ring by the size of the > > > > > > > > batch > > > > > > > > + > > > > > > > > + 6. Set the inflight field of each DescStateSplit entry in > > > > > > > > the batch to 0 > > > > > > > > + > > > > > > > > + 7. Set used_idx to the idx value of used ring > > > > > > > > + > > > > > > > > +When reconnecting: > > > > > > > > + > > > > > > > > + 1. If the value of used_idx does not match the idx value > > > > > > > > of used ring, > > > > > > > > + > > > > > > > > + (a) Subtract the value of used_idx from the idx value > > > > > > > > of used ring to get > > > > > > > > + the number of in-progress DescStateSplit entries > > > > > > > > + > > > > > > > > + (b) Set the inflight field of the in-progress > > > > > > > > DescStateSplit entries which > > > > > > > > + start from process_head to 0 > > > > > > > > + > > > > > > > > + (c) Set used_idx to the idx value of used ring > > > > > > > > + > > > > > > > > + 2. Resubmit each inflight DescStateSplit entry > > > > > > > > > > > > > > I re-read a couple of time and I still don't understand what it > > > > > > > says. > > > > > > > > > > > > > > For simplicity consider split ring. So we want a list of heads > > > > > > > that are > > > > > > > outstanding. Fair enough. Now device finishes a head. What now? I > > > > > > > needs > > > > > > > to drop head from the list. But list is unidirectional (just > > > > > > > next, no > > > > > > > prev). So how can you drop an entry from the middle? > > > > > > > > > > > > > > > > > > > The process_head is only used when slave crash between increasing > > > > > > the > > > > > > idx value of used ring and updating used_idx. We use it to find the > > > > > > in-progress DescStateSplit entries before the crash and complete > > > > > > them > > > > > > when reconnecting. Make sure guest and slave have the same view for > > > > > > inflight I/Os. > > > > > > > > > > > > > > > > But I don't understand how does the described process help do it? > > > > > > > > > > > > > For example, we need to submit descriptors A, B, C to driver in a batch. > > > > > > > > Firstly, we will link those descriptors like: > > > > > > > > process_head->A->B->C (A) > > > > > > > > Then, we need to update idx value of used vring to mark those > > > > descriptors as used: > > > > > > > > _vring.used->idx += 3 (B) > > > > > > > > At last, clear the inflight field of those descriptors and update > > > > used_idx field: > > > > > > > > A.inflight = 0; B.inflight = 0; C.inflight = 0; (C) > > > > > > > > used_idx = _vring.used->idx; (D) > > > > > > > > After (B), guest can consume the descriptors A,B,C. So we must make > > > > sure the inflight field of A,B,C is cleared when reconnecting to avoid > > > > re-submitting used descriptor. If slave crash during (C), the inflight > > > > field of A,B,C may be incorrect. To detect that case, we can see > > > > whether used_idx matches _vring.used->idx. And through process_head, > > > > we can get the in-progress descriptors A,B,C and clear their inflight > > > > field again when reconnecting. > > > > > > > > > > > > > > > In other case, the inflight field is enough to track inflight I/O. > > > > > > When reconnecting, we go through all DescStateSplit entries and > > > > > > re-submit the entry whose inflight field is equal to 1. > > > > > > > > > > What I don't understand is how do we know the order > > > > > in which they have to be resubmitted. Reordering > > > > > operations would be a big problem, won't it? > > > > > > > > > > > > > In previous patch, I record avail_idx for each DescStateSplit entry to > > > > preserve the order. Is it useful to fix this? > > > > > > > > > > > > > > Let's say I fetch descriptors A, B, C and start > > > > > processing. how does memory look? > > > > > > > > A.inflight = 1, C.inflight = 1, B.inflight = 1 > > > > > > > > > Now I finished B and marked it used. How does > > > > > memory look? > > > > > > > > > > > > > A.inflight = 1, C.inflight = 1, B.inflight = 0, process_head = B > > > > > > OK. And we have > > > > > > process_head->B->process_head > > > > > > ? > > > > > > Now if there is a reconnect, I want to submit A and then C, > > > correct? How do I know that from this picture? How do I > > > know to start with A? It's not on the list anymore... > > > > > > > We can go through all DescStateSplit entries (track all descriptors in > > Descriptor Table), then we can find A and C are inflight entry by its > > inflight field. And if we want to resubmit them in order (submit A and > > then C), we need to introduce a timestamp for each DescStateSplit > > entry to preserve the order when we fetch them from driver. Something > > like: > > > > When receiving available buffers from the driver: > > > > 1. Get the next available head-descriptor index from available ring, i > > > > 2. desc[i].timestamp = avail_idx++; > > > > 3. Set desc[i].inflight to 1 > > > > Thanks, > > Yongji > > OK I guess a 64 bit counter would be fine for that. > > In order seems critical for storage right? > Reordering write would seem to lead to data corruption. >
Actually I'm not sure. If we care about the order, we should not do access to one block until another access to the same block is completed? > But now I don't understand what does the next > field do. So it so you can maintain a freelist > within a statically allocated array? > Yes, we can use it to maintain a list. The head of the list is process_head. This list is only used when we want to submit descriptors in a batch. All descriptors in this batch are linked to the list. Then if we crash between marking those descriptors used and clearing their inflight field. We need to find those in-progress descriptors. The list will be helpful to achieve that. If no batch for submitting, the next field can be removed. Thanks, Yongji
