Re: [PATCH] Delay the initialization of rnbd_server module to late_initcall level

[Haris Iqbal <haris.iqbal@cloud.ionos.com>]

On Tue, Jun 23, 2020 at 7:54 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Jun 23, 2020 at 07:15:03PM +0530, Haris Iqbal wrote:
> > On Tue, Jun 23, 2020 at 5:47 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
> > >
> > > On Tue, Jun 23, 2020 at 03:20:27PM +0530, Haris Iqbal wrote:
> > > > Hi Jason and Leon,
> > > >
> > > > Did you get a chance to look into my previous email?
> > >
> > > Was there a question?
> >
> > Multiple actually :)
> >
> > >
> > > Jason
> >
> > In response to your emails,
> >
> > > Somehow nvme-rdma works:
> >
> > I think that's because the callchain during the nvme_rdma_init_module
> > initialization stops at "nvmf_register_transport()". Here only the
> > "struct nvmf_transport_ops nvme_rdma_transport" is registered, which
> > contains the function "nvme_rdma_create_ctrl()". I tested this in my
> > local setup and during kernel boot, that's the extent of the
> > callchain.
> > The ".create_ctrl"; which now points to "nvme_rdma_create_ctrl()" is
> > called later from "nvmf_dev_write()". I am not sure when this is
> > called, probably when the "discover" happens from the client side or
> > during the server config.
> >
> > It seems that the "rdma_bind_addr()" is called by the nvme rdma
> > module; but during the following events
> > 1) When a discover happens from the client side. Call trace for that looks like,
> > [ 1098.409398] nvmf_dev_write
> > [ 1098.409403] nvmf_create_ctrl
> > [ 1098.414568] nvme_rdma_create_ctrl
> > [ 1098.415009] nvme_rdma_setup_ctrl
> > [ 1098.415010] nvme_rdma_configure_admin_queue
> > [ 1098.415010] nvme_rdma_alloc_queue
> > [ 1098.415032] rdma_resolve_addr
> > [ 1098.415032] cma_bind_addr
> > [ 1098.415033] rdma_bind_addr
> >
> > 2) When a connect happens from the client side. Call trace is the same
> > as above, plus "nvme_rdma_alloc_queue()" is called n number of times;
> > n being the number of IO queues being created.
> >
> > On the server side, when an nvmf port is enabled, that also triggers a
> > call to "rdma_bind_addr()", but that is not from the nvme rdma module.
> > may be nvme target rdma? (not sure).
> >
> > Does this make sense or am I missing something here?
>
> It make sense, delaying creating and CM ID's until user space starts
> will solve this init time problme

Right, and the patch is trying to achieve the delay by changing the
init level to "late_initcall()"

>
> >
> > > If the rdma_create_id() is not on a callchain from module_init then you don't have a problem.
> >
> > I am a little confused. I thought the problem occurs from a call to
> > either "rdma_resolve_addr()" which calls "rdma_bind_addr()",
> > or a direct call to "rdma_bind_addr()" as in rtrs case.
> > In both the cases, a call to "rdma_create_id()" is needed before this.
>
> Right rdma_create_id() must precede anything that has problems, and it
> should not be done from module_init.

I understand this, but I am not sure why that is; as in why it should
not be done from module_init?

Also, about one of your previous statements,

> It is not OK to create RDMA CM IDs outside
> a client - CM IDs are supposed to be cleaned up when the client is
> removed.
>
> Similarly they are supposed to be created from the client attachment.

This again is a little confusing to me, since what I've observed in
nvmt is, when a server port is created, the "rdma_bind_addr()"
function is called.
And this goes well with the server/target and client/initiator model,
where the server has to get ready and start listening before a client
can initiate a connection.
What am I missing here?

>
> Jason

-- 

Regards
-Haris