Re: [PATCH 2/2] Revert "mq-deadline: Fix request accounting"

Niklas Cassel
2021-09-07T16:28:53Z

On Tue, Sep 07, 2021 at 08:15:03AM -0700, Bart Van Assche wrote:
> On 9/7/21 7:21 AM, Niklas Cassel wrote:
> > blk-mq will no longer call the I/O scheduler .finish_request() callback
> > for requests that were never inserted to the I/O scheduler.
>
> I do not agree. Even with patch 1/2 from this series applied, finish_request()
> will still be called for requests inserted by blk_insert_cloned_request()
> although these requests are never inserted to the I/O scheduler.
>
> Bart.

Hello Bart,

Looking at blk_mq_free_request(),
e->type->ops.finish_request() will only be called if RQF_ELVPRIV
is set.

blk_insert_cloned_request() doesn't seem to allocate a request
itself, but instead takes an already cloned request.

So I guess it depends on how the supplied request was cloned.

I would assume if the original request doesn't have RQF_ELVPRIV set,
then neither will the cloned request?

I tried to look at blk_rq_prep_clone(), which seems to be a common
cloning function, but I don't see req->rq_flags being copied
(except for RQF_SPECIAL_PAYLOAD).

Anyway, I don't see how .finish_request() will be called in relation
to blk_insert_cloned_request(). Could you please help me out and
give me an example of a call chain where this can happen?

Kind regards,
Niklas