Re: [PATCH v2] blk-ioprio: Introduce promote-to-rt policy

[Chaitanya Kulkarni]

On 2/20/2023 5:54 AM, Hou Tao wrote:
> From: Hou Tao <houtao1@xxxxxxxxxx>
> 
> Since commit a78418e6a04c ("block: Always initialize bio IO priority on
> submit"), bio->bi_ioprio will never be IOPRIO_CLASS_NONE when calling
> blkcg_set_ioprio(), so there will be no way to promote the io-priority
> of one cgroup to IOPRIO_CLASS_RT, because bi_ioprio will always be
> greater than or equals to IOPRIO_CLASS_RT.
> 
> It seems possible to call blkcg_set_ioprio() first then try to
> initialize bi_ioprio later in bio_set_ioprio(), but this doesn't work
> for bio in which bi_ioprio is already initialized (e.g., direct-io), so
> introduce a new ioprio policy to promote the iopriority of bio to
> IOPRIO_CLASS_RT if the ioprio is not already RT.
> 
> So introduce a new promote-to-rt policy to achieve this. For none-to-rt
> policy, although it doesn't work now, but considering that its purpose
> was also to override the io-priority to RT and allow for a smoother
> transition, just keep it and treat it as an alias of the promote-to-rt
> policy.
> 
> Signed-off-by: Hou Tao <houtao1@xxxxxxxxxx>


Thanks for updating documentation and adding meaningful comment.

Looks good.

Reviewed-by: Chaitanya Kulkarni <kch@xxxxxxxxxx>

-ck