Re: [RFC PATCH 0/8] blk-iocost: BPF struct_ops cost model

[Tejun Heo]

Hello, Tao.

On Tue, 08 Sep 2026 18:01:35 +0800, Tao Cui wrote:

>  4. One model system-wide vs named models selectable per device (as
>     tcp-cc names are per-socket)?

The struct_ops instance should be per-device. Loading a model for a device
should switch it to BPF automatically. Switching back to the builtin model
should detach the struct_ops.

>  2. Per-cgroup state lifetime: iocg_id is the css id and is recycled
>     once the cgroup is removed, so models must treat it as a
>     transient key.  Does the interface need a release(iocg_id)
>     callback so models can clean up per-cgroup state instead of
>     detecting reuse, or is per-cgroup bpf local storage, which
>     carries its own lifetime, the better home for model state?

I'd pass blkcg as a kptr and add online/offline callbacks so the model can
manage its per-cgroup state.

> return value of 0 delegates the IO back to the builtin formula, so a
> model which only handles some IO types cannot make the rest free.

I don't think mixing BPF and builtin pricing makes sense, either per IO or
between charging and request sizing. It seems convoluted and error-prone.
Implementing the linear model in BPF is straightforward.

Please combine the interface, registration, dispatch and configuration
changes into one patch. These patches are small and implement one feature.

Thanks.

-- 
tejun