Re: [PATCH] sched/uclamp: Fix uclamp_tg_restrict()

From: Dietmar Eggemann
Date: Wed Jun 16 2021 - 13:09:43 EST


On 11/06/2021 14:22, Qais Yousef wrote:
> Now cpu.uclamp.min acts as a protection, we need to make sure that the
> uclamp request of the task is within the allowed range of the cgroup,
> that is it is clamp()'ed correctly by tg->uclamp[UCLAMP_MIN] and
> tg->uclamp[UCLAMP_MAX].
>
> As reported by Xuewen [1] we can have some corner cases where there's
> inverstion between uclamp requested by task (p) and the uclamp values of

s/inverstion/inversion

[...]

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9e9a5be35cde..0318b00baa97 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1403,38 +1403,28 @@ static void uclamp_sync_util_min_rt_default(void)
> static inline struct uclamp_se
> uclamp_tg_restrict(struct task_struct *p, enum uclamp_id clamp_id)
> {
> - struct uclamp_se uc_req = p->uclamp_req[clamp_id];
> + /* Copy by value as we could modify it */
> + struct uclamp_se uc_eff = p->uclamp_req[clamp_id];
> #ifdef CONFIG_UCLAMP_TASK_GROUP
> + unsigned int tg_min, tg_max, value;
>
> /*
> * Tasks in autogroups or root task group will be
> * restricted by system defaults.
> */
> if (task_group_is_autogroup(task_group(p)))
> - return uc_req;
> + return uc_eff;
> if (task_group(p) == &root_task_group)
> - return uc_req;
> + return uc_eff;
>
> - switch (clamp_id) {
> - case UCLAMP_MIN: {
> - struct uclamp_se uc_min = task_group(p)->uclamp[clamp_id];
> - if (uc_req.value < uc_min.value)
> - return uc_min;
> - break;
> - }
> - case UCLAMP_MAX: {
> - struct uclamp_se uc_max = task_group(p)->uclamp[clamp_id];
> - if (uc_req.value > uc_max.value)
> - return uc_max;
> - break;
> - }
> - default:
> - WARN_ON_ONCE(1);
> - break;
> - }
> + tg_min = task_group(p)->uclamp[UCLAMP_MIN].value;
> + tg_max = task_group(p)->uclamp[UCLAMP_MAX].value;
> + value = uc_eff.value;
> + value = clamp(value, tg_min, tg_max);
> + uclamp_se_set(&uc_eff, value, false);
> #endif
>
> - return uc_req;
> + return uc_eff;
> }

I got confused by the renaming uc_req -> uc_eff.

We have:

uclamp_eff_value() (1)

uclamp_se uc_eff = uclamp_eff_get(p, clamp_id); (2)

uclamp_se uc_req = uclamp_tg_restrict(p, clamp_id) (3)

struct uclamp_se uc_eff = p->uclamp_req[clamp_id];
....

(3) is now calling it uc_eff where (2) still uses uc_req for the return
of (3). IMHO uc_*eff* was used after the system level (
uclamp_default) have been applied.

[...]

> @@ -1670,10 +1659,8 @@ uclamp_update_active_tasks(struct cgroup_subsys_state *css,
>
> css_task_iter_start(css, 0, &it);
> while ((p = css_task_iter_next(&it))) {
> - for_each_clamp_id(clamp_id) {
> - if ((0x1 << clamp_id) & clamps)
> - uclamp_update_active(p, clamp_id);
> - }
> + for_each_clamp_id(clamp_id)
> + uclamp_update_active(p, clamp_id);
> }
> css_task_iter_end(&it);
> }
> @@ -9626,7 +9613,7 @@ static void cpu_util_update_eff(struct cgroup_subsys_state *css)
> }
>
> /* Immediately update descendants RUNNABLE tasks */
> - uclamp_update_active_tasks(css, clamps);
> + uclamp_update_active_tasks(css);

Since we now always have to update both clamp_id's, can you not update
both under the same task_rq_lock() (in uclamp_update_active())?