Re: [RFC PATCH v5 1/5] blk-iocost: add BPF struct_ops cost model support

[Tao Cui]

Hi Alexei,

在 2026/9/18 23:43, Alexei Starovoitov 写道:
> On Fri, Sep 18, 2026 at 11:17 AM Tao Cui <cui.tao@xxxxxxxxx> wrote:
>> The registration and binding model follows the TCP congestion
>> model registration pattern: registering a struct_ops makes the model
>> available by its name (char name[IOCOST_MODEL_NAME_LEN], validated at
>> init_member), while
>> io.cost.model binds one registered model to a device with
>> "model=<name>" and unbinds with "model=linear" or "ctrl=auto/user".
>> Unregistering a model removes it from the registry, but its name
> 
> I don't think it's what Tejun asked for in v1. He asked for a per-device
> struct_ops instance where attaching switches the device to bpf and
> detaching switches it back. Instead there is a global name registry,
> a lifecycle list, an "unregistered but still bound" state and a
> refcount_t on top of bpf_struct_ops_get(), and every version since v3
> had a lifetime bug in exactly that code.
> Do it like hid_bpf_ops does with hid_id: device in the struct_ops,
> bind in .reg, unbind in .unreg, and le
> t struct_ops own the lifetime.
> 

Thanks for pointing this out.

You are right. I misunderstood Tejun's comment from v1. I took
"per-device" as selecting a registered model by name for each device,
which led to the current registry/lifecycle design.

My original implementation followed the tcp-cc style named model
approach, but that was not what Tejun was suggesting. The intended
model should be a per-device struct_ops instance, with attach/detach
controlling the switch between the BPF model and the builtin model,
and the struct_ops core owning the lifetime.

> pls wait for Tejun's feedback before respinning.
> 

I'll rework the design in that direction and wait for Tejun's feedback
before sending another revision.

Thanks,
Tao

> pw-bot: cr