Hi,
On 2/2/2023 1:33 AM, Bart Van Assche wrote:
> On 1/31/23 20:52, Hou Tao wrote:
>> /**
>> * enum prio_policy - I/O priority class policy.
>> * @POLICY_NO_CHANGE: (default) do not modify the I/O priority class.
>> @@ -27,21 +34,30 @@
>> * @POLICY_RESTRICT_TO_BE: modify IOPRIO_CLASS_NONE and IOPRIO_CLASS_RT into
>> * IOPRIO_CLASS_BE.
>> * @POLICY_ALL_TO_IDLE: change the I/O priority class into IOPRIO_CLASS_IDLE.
>> - *
>> + * @POLICY_PROMOTE_TO_RT: modify IOPRIO_CLASS_NONE and IOPRIO_CLASS_BE into
>> + * IOPRIO_CLASS_RT.
>> * See also <linux/ioprio.h>.
>> */
>> enum prio_policy {
>> - POLICY_NO_CHANGE = 0,
>> - POLICY_NONE_TO_RT = 1,
>> - POLICY_RESTRICT_TO_BE = 2,
>> - POLICY_ALL_TO_IDLE = 3,
>> + POLICY_NO_CHANGE = IOPRIO_CLASS_NONE,
>> + POLICY_NONE_TO_RT = IOPRIO_CLASS_RT,
>> + POLICY_RESTRICT_TO_BE = IOPRIO_CLASS_BE,
>> + POLICY_ALL_TO_IDLE = IOPRIO_CLASS_IDLE,
>> + POLICY_PROMOTE_TO_RT = IOPRIO_CLASS_RT | IOPRIO_POL_PROMOTION,
>> +};
>
> The above change complicates the ioprio code. Additionally, I'm concerned that
> it makes the ioprio code slower. Has it been considered to keep the numerical
> values for the existing policies, to assign the number 4 to
> POLICY_PROMOTE_TO_RT and to use a lookup-array in blkcg_set_ioprio() to
> convert the policy number into an IOPRIO_CLASS value?
For the slowness, do you meaning the extra dereference of blkcg->ioprio->policy
when policy is no-change or the handle of IOPRIO_POL_PROMOTION in
blkcg_set_ioprio()? It seems other functions (e.g., ioprio_show_prio_policy()
and ioprio_set_prio_policy()) are not on the hot path. Using a lookup array in
blkcg_set_ioprio() to do the conversion will also be OK, although it will
introduce an extra lookup each time when policy is not no-change. I don't have
strong preference. If you are OK with lookup array in blkcg_set_ioprio(), will
do it in v2.
>
> Thanks,
>
> Bart.
>
>
> .
