Re: [RFC PATCH v10 1/4] blk-iocost: add BPF struct_ops cost model support

[bot+bpf-ci]

> blk-iocost: add BPF struct_ops cost model support

TL;DR: Documentation issue only: cgroup-v2.rst isn't updated for io.cost.model
accepting "model=bpf", and writing "model=linear" now switches a device off an
attached BPF model, which may surprise existing tools.

> diff --git a/block/blk-iocost.c b/block/blk-iocost.c
> index 2745bffcd5ee..509ae36f99ed 100644
> --- a/block/blk-iocost.c
> +++ b/block/blk-iocost.c

[ ... ]

> @@ -3536,9 +3961,25 @@ static ssize_t ioc_cost_model_write(struct kernfs_open_file *of, char *input,
>  			continue;
>  		case COST_MODEL:
>  			match_strlcpy(buf, &args[0], sizeof(buf));
> -			if (strcmp(buf, "linear"))
> -				goto unlock;
> -			continue;
> +			if (!strcmp(buf, "linear")) {
> +				/* staged and committed below, so a parse
> +				 * error later in the same write leaves
> +				 * the model selection untouched
> +				 */
> +				new_model = NULL;
> +				model_write = true;
> +				continue;
> +			}
> +			if (!strcmp(buf, "bpf")) {
> +				new_model = rcu_dereference_protected(
> +						ioc->attached,
> +						lockdep_is_held(&ioc->lock));
> +				if (!new_model)
> +					goto unlock;
> +				model_write = true;
> +				continue;
> +			}
> +			goto unlock;
>  		}

Does this change to the io.cost.model interface need a matching update to
Documentation/admin-guide/cgroup-v2.rst?

ioc_cost_model_write() now accepts "model=bpf", and "model=linear" is no
longer only a value check.  It now clears ioc->model and moves the device
off an attached BPF model.  ioc_cost_model_prfill() can also print
"model=bpf".

The documentation still describes the key as:

  model		The cost model in use - "linear"

and says nothing about "bpf" or the new effect of writing "linear".

Existing tools that write lines like "MAJ:MIN ctrl=user model=linear
rbps=..." will now silently switch a device away from an attached BPF
model.  Could cgroup-v2.rst be updated to cover the new value and the
changed semantics?


---
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See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/37124306614