On Sat, Mar 18, 2023 at 08:30:29AM +0800, Ming Lei wrote: > On Fri, Mar 17, 2023 at 10:41:28AM -0400, Stefan Hajnoczi wrote: > > On Fri, Mar 17, 2023 at 11:10:20AM +0800, Ming Lei wrote: > > > On Thu, Mar 02, 2023 at 10:09:25AM -0500, Stefan Hajnoczi wrote: > > > > On Thu, Mar 02, 2023 at 11:22:55AM +0800, Ming Lei wrote: > > > > > On Thu, Feb 23, 2023 at 03:18:19PM -0500, Stefan Hajnoczi wrote: > > > > > > On Thu, Feb 23, 2023 at 07:17:33AM +0800, Ming Lei wrote: > > > > > > > On Sat, Feb 18, 2023 at 01:38:08PM -0500, Stefan Hajnoczi wrote: > > > > > > > > On Sat, Feb 18, 2023 at 07:22:49PM +0800, Ming Lei wrote: > > > > > > > > > On Fri, Feb 17, 2023 at 11:39:58AM -0500, Stefan Hajnoczi wrote: > > > > > > > > > > On Fri, Feb 17, 2023 at 10:20:45AM +0800, Ming Lei wrote: > > > > > > > > > > > On Thu, Feb 16, 2023 at 12:21:32PM +0100, Andreas Hindborg wrote: > > > > > > > > > > > > > > > > > > > > > > > > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > > > > > > > > > > > > > > > > > > > > > > > On Thu, Feb 16, 2023 at 10:44:02AM +0100, Andreas Hindborg wrote: > > > > > > > > > > > > >> > > > > > > > > > > > > >> Hi Ming, > > > > > > > > > > > > >> > > > > > > > > > > > > >> Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > > > > > > > > > > >> > > > > > > > > > > > > >> > On Mon, Feb 13, 2023 at 02:13:59PM -0500, Stefan Hajnoczi wrote: > > > > > > > > > > > > >> >> On Mon, Feb 13, 2023 at 11:47:31AM +0800, Ming Lei wrote: > > > > > > > > > > > > >> >> > On Wed, Feb 08, 2023 at 07:17:10AM -0500, Stefan Hajnoczi wrote: > > > > > > > > > > > > >> >> > > On Wed, Feb 08, 2023 at 10:12:19AM +0800, Ming Lei wrote: > > > > > > > > > > > > >> >> > > > On Mon, Feb 06, 2023 at 03:27:09PM -0500, Stefan Hajnoczi wrote: > > > > > > > > > > > > >> >> > > > > On Mon, Feb 06, 2023 at 11:00:27PM +0800, Ming Lei wrote: > > > > > > > > > > > > >> >> > > > > > Hello, > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > > >> >> > > > > > So far UBLK is only used for implementing virtual block device from > > > > > > > > > > > > >> >> > > > > > userspace, such as loop, nbd, qcow2, ...[1]. > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > >> >> > > > > I won't be at LSF/MM so here are my thoughts: > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > >> >> > > > Thanks for the thoughts, :-) > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > > >> >> > > > > > It could be useful for UBLK to cover real storage hardware too: > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > > >> >> > > > > > - for fast prototype or performance evaluation > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > > >> >> > > > > > - some network storages are attached to host, such as iscsi and nvme-tcp, > > > > > > > > > > > > >> >> > > > > > the current UBLK interface doesn't support such devices, since it needs > > > > > > > > > > > > >> >> > > > > > all LUNs/Namespaces to share host resources(such as tag) > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > > >> >> > > > > Can you explain this in more detail? It seems like an iSCSI or > > > > > > > > > > > > >> >> > > > > NVMe-over-TCP initiator could be implemented as a ublk server today. > > > > > > > > > > > > >> >> > > > > What am I missing? > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > > >> >> > > > The current ublk can't do that yet, because the interface doesn't > > > > > > > > > > > > >> >> > > > support multiple ublk disks sharing single host, which is exactly > > > > > > > > > > > > >> >> > > > the case of scsi and nvme. > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > >> >> > > Can you give an example that shows exactly where a problem is hit? > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > >> >> > > I took a quick look at the ublk source code and didn't spot a place > > > > > > > > > > > > >> >> > > where it prevents a single ublk server process from handling multiple > > > > > > > > > > > > >> >> > > devices. > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > >> >> > > Regarding "host resources(such as tag)", can the ublk server deal with > > > > > > > > > > > > >> >> > > that in userspace? The Linux block layer doesn't have the concept of a > > > > > > > > > > > > >> >> > > "host", that would come in at the SCSI/NVMe level that's implemented in > > > > > > > > > > > > >> >> > > userspace. > > > > > > > > > > > > >> >> > > > > > > > > > > > > > > >> >> > > I don't understand yet... > > > > > > > > > > > > >> >> > > > > > > > > > > > > > >> >> > blk_mq_tag_set is embedded into driver host structure, and referred by queue > > > > > > > > > > > > >> >> > via q->tag_set, both scsi and nvme allocates tag in host/queue wide, > > > > > > > > > > > > >> >> > that said all LUNs/NSs share host/queue tags, current every ublk > > > > > > > > > > > > >> >> > device is independent, and can't shard tags. > > > > > > > > > > > > >> >> > > > > > > > > > > > > >> >> Does this actually prevent ublk servers with multiple ublk devices or is > > > > > > > > > > > > >> >> it just sub-optimal? > > > > > > > > > > > > >> > > > > > > > > > > > > > >> > It is former, ublk can't support multiple devices which share single host > > > > > > > > > > > > >> > because duplicated tag can be seen in host side, then io is failed. > > > > > > > > > > > > >> > > > > > > > > > > > > > >> > > > > > > > > > > > > >> I have trouble following this discussion. Why can we not handle multiple > > > > > > > > > > > > >> block devices in a single ublk user space process? > > > > > > > > > > > > >> > > > > > > > > > > > > >> From this conversation it seems that the limiting factor is allocation > > > > > > > > > > > > >> of the tag set of the virtual device in the kernel? But as far as I can > > > > > > > > > > > > >> tell, the tag sets are allocated per virtual block device in > > > > > > > > > > > > >> `ublk_ctrl_add_dev()`? > > > > > > > > > > > > >> > > > > > > > > > > > > >> It seems to me that a single ublk user space process shuld be able to > > > > > > > > > > > > >> connect to multiple storage devices (for instance nvme-of) and then > > > > > > > > > > > > >> create a ublk device for each namespace, all from a single ublk process. > > > > > > > > > > > > >> > > > > > > > > > > > > >> Could you elaborate on why this is not possible? > > > > > > > > > > > > > > > > > > > > > > > > > > If the multiple storages devices are independent, the current ublk can > > > > > > > > > > > > > handle them just fine. > > > > > > > > > > > > > > > > > > > > > > > > > > But if these storage devices(such as luns in iscsi, or NSs in nvme-tcp) > > > > > > > > > > > > > share single host, and use host-wide tagset, the current interface can't > > > > > > > > > > > > > work as expected, because tags is shared among all these devices. The > > > > > > > > > > > > > current ublk interface needs to be extended for covering this case. > > > > > > > > > > > > > > > > > > > > > > > > Thanks for clarifying, that is very helpful. > > > > > > > > > > > > > > > > > > > > > > > > Follow up question: What would the implications be if one tried to > > > > > > > > > > > > expose (through ublk) each nvme namespace of an nvme-of controller with > > > > > > > > > > > > an independent tag set? > > > > > > > > > > > > > > > > > > > > > > https://lore.kernel.org/linux-block/877cwhrgul.fsf@xxxxxxxxxxxx/T/#m57158db9f0108e529d8d62d1d56652c52e9e3e67 > > > > > > > > > > > > > > > > > > > > > > > What are the benefits of sharing a tagset across > > > > > > > > > > > > all namespaces of a controller? > > > > > > > > > > > > > > > > > > > > > > The userspace implementation can be simplified a lot since generic > > > > > > > > > > > shared tag allocation isn't needed, meantime with good performance > > > > > > > > > > > (shared tags allocation in SMP is one hard problem) > > > > > > > > > > > > > > > > > > > > In NVMe, tags are per Submission Queue. AFAIK there's no such thing as > > > > > > > > > > shared tags across multiple SQs in NVMe. So userspace doesn't need an > > > > > > > > > > > > > > > > > > In reality the max supported nr_queues of nvme is often much less than > > > > > > > > > nr_cpu_ids, for example, lots of nvme-pci devices just support at most > > > > > > > > > 32 queues, I remembered that Azure nvme supports less(just 8 queues). > > > > > > > > > That is because queue isn't free in both software and hardware, which > > > > > > > > > implementation is often tradeoff between performance and cost. > > > > > > > > > > > > > > > > I didn't say that the ublk server should have nr_cpu_ids threads. I > > > > > > > > thought the idea was the ublk server creates as many threads as it needs > > > > > > > > (e.g. max 8 if the Azure NVMe device only has 8 queues). > > > > > > > > > > > > > > > > Do you expect ublk servers to have nr_cpu_ids threads in all/most cases? > > > > > > > > > > > > > > No. > > > > > > > > > > > > > > In ublksrv project, each pthread maps to one unique hardware queue, so total > > > > > > > number of pthread is equal to nr_hw_queues. > > > > > > > > > > > > Good, I think we agree on that part. > > > > > > > > > > > > Here is a summary of the ublk server model I've been describing: > > > > > > 1. Each pthread has a separate io_uring context. > > > > > > 2. Each pthread has its own hardware submission queue (NVMe SQ, SCSI > > > > > > command queue, etc). > > > > > > 3. Each pthread has a distinct subrange of the tag space if the tag > > > > > > space is shared across hardware submission queues. > > > > > > 4. Each pthread allocates tags from its subrange without coordinating > > > > > > with other threads. This is cheap and simple. > > > > > > > > > > That is also not doable. > > > > > > > > > > The tag space can be pretty small, such as, usb-storage queue depth > > > > > is just 1, and usb card reader can support multi lun too. > > > > > > > > If the tag space is very limited, just create one pthread. > > > > > > What I meant is that sub-range isn't doable. > > > > > > And pthread is aligned with queue, that is nothing to do with nr_tags. > > > > > > > > > > > > That is just one extreme example, but there can be more low queue depth > > > > > scsi devices(sata : 32, ...), typical nvme/pci queue depth is 1023, but > > > > > there could be some implementation with less. > > > > > > > > NVMe PCI has per-sq tags so subranges aren't needed. Each pthread has > > > > its own independent tag space. That means NVMe devices with low queue > > > > depths work fine in the model I described. > > > > > > NVMe PCI isn't special, and it is covered by current ublk abstract, so one way > > > or another, we should not support both sub-range or non-sub-range for > > > avoiding unnecessary complexity. > > > > > > "Each pthread has its own independent tag space" may mean two things > > > > > > 1) each LUN/NS is implemented in standalone process space: > > > - so every queue of each LUN has its own space, but all the queues with > > > same ID share the whole queue tag space > > > - that matches with current ublksrv > > > - also easier to implement > > > > > > 2) all LUNs/NSs are implemented in single process space > > > - so each pthread handles one queue for all NSs/LUNs > > > > > > Yeah, if you mean 2), the tag allocation is cheap, but the existed ublk > > > char device has to handle multiple LUNs/NSs(disks), which still need > > > (big) ublk interface change. Also this way can't scale for single queue > > > devices. > > > > The model I described is neither 1) or 2). It's similar to 2) but I'm > > not sure why you say the ublk interface needs to be changed. I'm afraid > > I haven't explained it well, sorry. I'll try to describe it again with > > an NVMe PCI adapter being handled by userspace. > > > > There is a single ublk server process with an NVMe PCI device opened > > using VFIO. > > > > There are N pthreads and each pthread has 1 io_uring context and 1 NVMe > > PCI SQ/CQ pair. The size of the SQ and CQ rings is QD. > > > > The NVMe PCI device has M Namespaces. The ublk server creates M > > ublk_devices. Each ublk_device has N ublk_queues with queue_depth QD. > > > > The Linux block layer sees M block devices with N nr_hw_queues and QD > > queue_depth. The actual NVMe PCI device resources are less than what the > > Linux block layer sees because the each SQ/CQ pair is used for M > > ublk_devices. In other words, Linux thinks there can be M * N * QD > > requests in flight but in reality the NVMe PCI adapter only supports N * > > QD requests. > > Yeah, but it is really bad. > > Now QD is the host hard queue depth, which can be very big, and could be > more than thousands. > > ublk driver doesn't understand this kind of sharing(tag, io command buffer, io > buffers), M * M * QD requests are submitted to ublk server, and CPUs/memory > are wasted a lot. > > Every device has to allocate command buffers for holding QD io commands, and > command buffer is supposed to be per-host, instead of per-disk. Same with io > buffer pre-allocation in userspace side. I agree with you in cases with lots of LUNs (large M), block layer and ublk driver per-request memory is allocated that cannot be used simultaneously. > Userspace has to re-tag the requests for avoiding duplicated tag, and > requests have to be throttled in ublk server side. If you implement tag allocation > in userspace side, it is still one typical shared data issue in SMP, M pthreads > contends on single tags from multiple CPUs. Here I still disagree. There is no SMP contention with NVMe because tags are per SQ. For SCSI the tag namespace is shared but each pthread can trivially work with a sub-range to avoid SMP contention. If the tag namespace is too small for sub-ranges, then there should be fewer pthreads. > > > > Now I'll describe how userspace can take care of the mismatch between > > the Linux block layer and the NVMe PCI device without doing much work: > > > > Each pthread sets up QD UBLK_IO_COMMIT_AND_FETCH_REQ io_uring_cmds for > > each of the M Namespaces. > > > > When userspace receives a request from ublk, it cannot simply copy the > > struct ublksrv_io_cmd->tag field into the NVMe SQE Command Identifier > > (CID) field. There would be collisions between the tags used across the > > M ublk_queues that the pthread services. > > > > Userspace selects a free tag (e.g. from a bitmap with QD elements) and > > uses that as the NVMe Command Identifier. This is trivial because each > > pthread has its own bitmap and NVMe Command Identifiers are per-SQ. > > I believe I have explained, in reality, NVME SQ/CQ pair can be less( > or much less) than nr_cpu_ids, so the per-queue-tags can be allocated & freed > among CPUs of (nr_cpu_ids / nr_hw_queues). > > Not mention userspace is capable of overriding the pthread cpu affinity, > so it isn't trivial & cheap, M pthreads could be run from > more than (nr_cpu_ids / nr_hw_queues) CPUs and contend on the single hw queue tags. I don't understand your nr_cpu_ids concerns. In the model I have described, the number of pthreads is min(nr_cpu_ids, max_sq_cq_pairs) and the SQ/CQ pairs are per pthread. There is no sharing of SQ/CQ pairs across pthreads. On a limited NVMe controller nr_cpu_ids=128 and max_sq_cq_pairs=8, so there are only 8 pthreads. Each pthread has its own io_uring context through which it handles M ublk_queues. Even if a pthread runs from more than 1 CPU, its SQ Command Identifiers (tags) are only used by that pthread and there is no SMP contention. Can you explain where you see SMP contention for NVMe SQ Command Identifiers? > > > > If there are no free tags then the request is placed in the pthread's > > per Namespace overflow list. Whenever an NVMe command completes, the > > overflow lists are scanned. One pending request is submitted to the NVMe > > PCI adapter in a round-robin fashion until the lists are empty or there > > are no more free tags. > > > > That's it. No ublk API changes are necessary. The userspace code is not > > slow or complex (just a bitmap and overflow list). > > Fine, but I am not sure we need to support such mess & pool implementation. > > > > > The approach also works for SCSI or devices that only support 1 request > > in flight at a time, with small tweaks. > > > > Going back to the beginning of the discussion: I think it's possible to > > write a ublk server that handles multiple LUNs/NS today. > > It is possible, but it is poor in both performance and resource > utilization, meantime with complicated ublk server implementation. Okay. I wanted to make sure I wasn't missing a reason why it's fundamentally impossible. Performance, resource utilization, or complexity is debatable and I think I understand your position. I think you're looking for a general solution that works well even with a high number of LUNs, where the model I proposed wastes resources. > > > > > > Another thing is that io command buffer has to be shared among all LUNs/ > > > NSs. So interface change has to cover shared io command buffer. > > > > I think the main advantage of extending the ublk API to share io command > > buffers between ublk_devices is to reduce userspace memory consumption? > > > > It eliminates the need to over-provision I/O buffers for write requests > > (or use the slower UBLK_IO_NEED_GET_DATA approach). > > Not only avoiding memory and cpu waste, but also simplifying ublk > server. > > > > > > With zero copy support, io buffer sharing needn't to be considered, that > > > can be a bit easier. > > > > > > In short, the sharing of (tag, io command buffer, io buffer) needs to be > > > considered for shared host ublk disks. > > > > > > Actually I prefer to 1), which matches with current design, and we can > > > just add host concept into ublk, and implementation could be easier. > > > > > > BTW, ublk has been applied to implement iscsi alternative disk[1] for Longhorn[2], > > > and the performance improvement is pretty nice, so I think it is one reasonable > > > requirement to support "shared host" ublk disks for covering multi-lun or multi-ns. > > > > > > [1] https://github.com/ming1/ubdsrv/issues/49 > > > [2] https://github.com/longhorn/longhorn > > > > Nice performance improvement! > > > > I agree with you that the ublk API should have a way to declare the > > resource contraints for multi-LUN/NS servers (i.e. share the tag_set). I > > guess the simplest way to do that is by passing a reference to an > > existing device to UBLK_CMD_ADD_DEV so it can share the tag_set? Nothing > > else about the ublk API needs to change, at least for tags. > > Basically (tags, io command buffer, io buffers) need to move into > host/hw_queue wide from disk wide, so not so simple, but won't > be too complicated. > > > > > Solving I/O buffer over-provisioning sounds similar to io_uring's > > provided buffer mechanism :). > > blk-mq has built-in host/hw_queue wide tag allocation, which can provide > unique tag for ublk server from ublk driver side, so everything can be > simplified a lot if we move (tag, io command buffer, io buffers) into > host/hw_queue wide by telling ublk_driver that we are > BLK_MQ_F_TAG_QUEUE_SHARED. > > Not sure if io_uring's provided buffer is good here, cause we need to > discard io buffers after queue become idle. But it won't be one big > deal if zero copy can be supported. If the per-request ublk resources are shared like tags as you described, then that's a nice solution that also solves I/O buffer over-provisioning. Stefan
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