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SubjectRe: [PATCH v2] sched/uclamp: Allow to reset a task uclamp constraint value
Hi Dietmar,
On Fri, Nov 13, 2020 at 12:34:54PM +0100, Dietmar Eggemann wrote:
> In case the user wants to stop controlling a uclamp constraint value
> for a task, use the magic value -1 in sched_util_{min,max} with the
> appropriate sched_flags (SCHED_FLAG_UTIL_CLAMP_{MIN,MAX}) to indicate
> the reset.
>
> The advantage over the 'additional flag' approach (i.e. introducing
> SCHED_FLAG_UTIL_CLAMP_RESET) is that no additional flag has to be
> exported via uapi. This avoids the need to document how this new flag
> has be used in conjunction with the existing uclamp related flags.
>
> The following subtle issue is fixed as well. When a uclamp constraint
> value is set on a !user_defined uclamp_se it is currently first reset
> and then set.
> Fix this by AND'ing !user_defined with !SCHED_FLAG_UTIL_CLAMP which
> stands for the 'sched class change' case.
> The related condition 'if (uc_se->user_defined)' moved from
> __setscheduler_uclamp() into uclamp_reset().

I think this is great, thanks!
Reviewed-by: Yun Hsiang <hsiang023167@gmail.com>

>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>
> v1 [1] -> v2:
>
> 1) Removed struct sched_attr (UAPI) change.
>
> 2) Use single branch range check in uclamp_validate().
>
> [1] https://lkml.kernel.org/r/f3b59aad-3d5d-039b-205d-024308b609a1@arm.com
>
> include/uapi/linux/sched/types.h | 2 +
> kernel/sched/core.c | 70 +++++++++++++++++++++++---------
> 2 files changed, 53 insertions(+), 19 deletions(-)
>
> diff --git a/include/uapi/linux/sched/types.h b/include/uapi/linux/sched/types.h
> index c852153ddb0d..f2c4589d4dbf 100644
> --- a/include/uapi/linux/sched/types.h
> +++ b/include/uapi/linux/sched/types.h
> @@ -96,6 +96,8 @@ struct sched_param {
> * on a CPU with a capacity big enough to fit the specified value.
> * A task with a max utilization value smaller than 1024 is more likely
> * scheduled on a CPU with no more capacity than the specified value.
> + *
> + * A task utilization boundary can be reset by setting the attribute to -1.
> */
> struct sched_attr {
> __u32 size;
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 3dc415f58bd7..a4805747b304 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1413,17 +1413,24 @@ int sysctl_sched_uclamp_handler(struct ctl_table *table, int write,
> static int uclamp_validate(struct task_struct *p,
> const struct sched_attr *attr)
> {
> - unsigned int lower_bound = p->uclamp_req[UCLAMP_MIN].value;
> - unsigned int upper_bound = p->uclamp_req[UCLAMP_MAX].value;
> + int util_min = p->uclamp_req[UCLAMP_MIN].value;
> + int util_max = p->uclamp_req[UCLAMP_MAX].value;
>
> - if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN)
> - lower_bound = attr->sched_util_min;
> - if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX)
> - upper_bound = attr->sched_util_max;
> + if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> + util_min = attr->sched_util_min;
>
> - if (lower_bound > upper_bound)
> - return -EINVAL;
> - if (upper_bound > SCHED_CAPACITY_SCALE)
> + if (util_min + 1 > SCHED_CAPACITY_SCALE + 1)
> + return -EINVAL;
> + }
> +
> + if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> + util_max = attr->sched_util_max;
> +
> + if (util_max + 1 > SCHED_CAPACITY_SCALE + 1)
> + return -EINVAL;
> + }
> +
> + if (util_min != -1 && util_max != -1 && util_min > util_max)
> return -EINVAL;
>
> /*
> @@ -1438,20 +1445,41 @@ static int uclamp_validate(struct task_struct *p,
> return 0;
> }
>
> +static bool uclamp_reset(const struct sched_attr *attr,
> + enum uclamp_id clamp_id,
> + struct uclamp_se *uc_se)
> +{
> + /* Reset on sched class change for a non user-defined clamp value. */
> + if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)) &&
> + !uc_se->user_defined)
> + return true;
> +
> + /* Reset on sched_util_{min,max} == -1. */
> + if (clamp_id == UCLAMP_MIN &&
> + attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN &&
> + attr->sched_util_min == -1) {
> + return true;
> + }
> +
> + if (clamp_id == UCLAMP_MAX &&
> + attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX &&
> + attr->sched_util_max == -1) {
> + return true;
> + }
> +
> + return false;
> +}
> +
> static void __setscheduler_uclamp(struct task_struct *p,
> const struct sched_attr *attr)
> {
> enum uclamp_id clamp_id;
>
> - /*
> - * On scheduling class change, reset to default clamps for tasks
> - * without a task-specific value.
> - */
> for_each_clamp_id(clamp_id) {
> struct uclamp_se *uc_se = &p->uclamp_req[clamp_id];
> + unsigned int value;
>
> - /* Keep using defined clamps across class changes */
> - if (uc_se->user_defined)
> + if (!uclamp_reset(attr, clamp_id, uc_se))
> continue;
>
> /*
> @@ -1459,21 +1487,25 @@ static void __setscheduler_uclamp(struct task_struct *p,
> * at runtime.
> */
> if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN))
> - __uclamp_update_util_min_rt_default(p);
> + value = sysctl_sched_uclamp_util_min_rt_default;
> else
> - uclamp_se_set(uc_se, uclamp_none(clamp_id), false);
> + value = uclamp_none(clamp_id);
> +
> + uclamp_se_set(uc_se, value, false);
>
> }
>
> if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP)))
> return;
>
> - if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN) {
> + if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MIN &&
> + attr->sched_util_min != -1) {
> uclamp_se_set(&p->uclamp_req[UCLAMP_MIN],
> attr->sched_util_min, true);
> }
>
> - if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX) {
> + if (attr->sched_flags & SCHED_FLAG_UTIL_CLAMP_MAX &&
> + attr->sched_util_max != -1) {
> uclamp_se_set(&p->uclamp_req[UCLAMP_MAX],
> attr->sched_util_max, true);
> }
> --
> 2.17.1
>

Best Regards,
Yun

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