On Wed, Jul 13, 2022 at 11:07:57AM +0530, Kanchan Joshi wrote: > > > > > > The way I would do this that in nvme_ioucmd_failover_req (or in the > > > > > > retry driven from command retriable failure) I would do the above, > > > > > > requeue it and kick the requeue work, to go over the requeue_list and > > > > > > just execute them again. Not sure why you even need an explicit retry > > > > > > code. > > > > > During retry we need passthrough command. But passthrough command is not > > > > > stable (i.e. valid only during first submission). We can make it stable > > > > > either by: > > > > > (a) allocating in nvme (b) return -EAGAIN to io_uring, and > > > > > it will do allocate + deferral > > > > > Both add a cost. And since any command can potentially fail, that > > > > > means taking that cost for every IO that we issue on mpath node. Even if > > > > > no failure (initial or subsquent after IO) occcured. > > > > > > > > As mentioned, I think that if a driver consumes a command as queued, > > > > it needs a stable copy for a later reformation of the request for > > > > failover purposes. > > > > > > So what do you propose to make that stable? > > > As I mentioned earlier, stable copy requires allocating/copying in fast > > > path. And for a condition (failover) that may not even occur. > > > I really think currrent solution is much better as it does not try to make > > > it stable. Rather it assembles pieces of passthrough command if retry > > > (which is rare) happens. > > > > Well, I can understand that io_uring_cmd is space constrained, otherwise > > we wouldn't be having this discussion. > > Indeed. If we had space for keeping passthrough command stable for > retry, that would really have simplified the plumbing. Retry logic would > be same as first submission. > > > However io_kiocb is less > > constrained, and could be used as a context to hold such a space. > > > > Even if it is undesired to have io_kiocb be passed to uring_cmd(), it > > can still hold a driver specific space paired with a helper to obtain it > > (i.e. something like io_uring_cmd_to_driver_ctx(ioucmd) ). Then if the > > space is pre-allocated it is only a small memory copy for a stable copy > > that would allow a saner failover design. > > I am thinking along the same lines, but it's not about few bytes of > space rather we need 80 (72 to be precise). Will think more, but > these 72 bytes really stand tall in front of my optimism. > > Do you see anything is possible in nvme-side? > Now also passthrough command (although in a modified form) gets copied > into this preallocated space i.e. nvme_req(req)->cmd. This part - I understand it can't be allocated in nvme request which is freed during retry, and looks the extra space has to be bound with io_uring_cmd. thanks, Ming
