Re: [PATCH] blk-ioprio: Introduce promote-to-rt policy

Bart Van Assche
2023-02-03T19:51:52Z

On 1/31/23 20:52, Hou Tao wrote:
> diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
> index c8ae7c897f14..e0b9f73ef62a 100644
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -2038,17 +2038,27 @@ that attribute:
> Change the I/O priority class of all requests into IDLE, the lowest
> I/O priority class.
>
> + promote-to-rt
> + For requests that have I/O priority class BE or that have I/O priority
> + class IDLE, change it into RT. Do not modify the I/O priority class
> + of requests that have priority class RT.

Please document whether or not this policy modifies the I/O priority
(IOPRIO_PRIO_DATA()). Do you agree that the I/O priority should be preserved
when promoting from BE to RT and that only the I/O priority class should be
modified for such promotions?

> The following numerical values are associated with the I/O priority policies:
>
> -+-------------+---+
> -| no-change | 0 |
> -+-------------+---+
> -| none-to-rt | 1 |
> -+-------------+---+
> -| rt-to-be | 2 |
> -+-------------+---+
> -| all-to-idle | 3 |
> -+-------------+---+
> +
> ++---------------+---------+-----+
> +| policy | inst | num |
> ++---------------+---------+-----+
> +| no-change | demote | 0 |
> ++---------------+---------+-----+
> +| none-to-rt | demote | 1 |
> ++---------------+---------+-----+
> +| rt-to-be | demote | 2 |
> ++---------------+---------+-----+
> +| idle | demote | 3 |
> ++---------------+---------+-----+
> +| promote-to-rt | promote | 1 |
> ++---------------+---------+-----+

I prefer that this table is not modified. The numerical values associated with
policies only matters for none-to-rt, rt-to-be and all-to-idle but not for
promote-to-rt. So I don't think that it is necessary to mention a numerical
value for the promote-to-rt policy. Additionally, "none-to-rt" is not a policy
that demotes the I/O priority but a policy that may promote the I/O priority.

> +-- If the instruction is promotion, change the request I/O priority class
> +- into the minimum of the I/O priority class policy number and the numerical
> +- I/O priority class.

Using the minimum value seems wrong to me because that will change
IOPRIO_VALUE(IOPRIO_CLASS_RT, 1) into IOPRIO_VALUE(IOPRIO_CLASS_RT, 0).

Thanks,

Bart.