Re: [RFC PATCH v10 0/4] blk-iocost: add BPF struct_ops cost model support

[Tejun Heo <tj@kernel.org>]

Hello, Tao.

On Thu, 15 Oct 2026 20:00:20 +0800, Tao Cui wrote:
> When you have a chance, could you take a look at this series? I'd
> appreciate your thoughts on whether the implementation still matches
> what we discussed, or whether it has drifted somewhere along the way.

The behavior matches what was discussed, but the implementation needs a
lot of cleanup, and there are bugs in the detach path and the
iocg_init()/iocg_free() pairing. Details are in the replies to the
patches.

> Sashiko flagged a possible ABBA between the BPF paths and
> ioc_cost_model_write(): attach/detach take the queue freeze before
> q->rq_qos_mutex, while the cost-model write holds rq_qos_mutex across
> its freeze. That is indeed the current ordering, but I don't think this
> can deadlock because the queue freeze is a depth counter

The attach and detach order matches ioc_qos_write(). The reverse order in
ioc_cost_model_write() predates this series and is better fixed
separately. An earlier attempt to convert it ran into blk_iocost_init()'s
percpu allocation under the freeze:

  https://lore.kernel.org/all/aV5OwAvw1sRF1IW1@fedora/

Thanks.

-- 
tejun