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

[Xuewen Yan <xuewen.yan94@gmail.com>]

Hi Qais

Sorry for the late reply.

I tested the patch on the "UNISOC T610" platform, and it looks fine.

According to the two cases:
case1:
without patch:
set task's clamp_min=614(60%), clamp_max=819(80%), the result as:
[  215.780435]c6  uclamp_test :
pid=138,req_min=614,req_max=819,eff_min=614,eff_max=819

after set tg's cpu.uclamp.max = 50%，the result:
[  420.580443]c6  uclamp_test :
pid=138,req_min=614,req_max=819,eff_min=614,eff_max=512

with patch:
set task's clamp_min=614(60%), clamp_max=819(80%), the result as:
[  333.533878]c7  uclamp_test :
pid=138,req_min=614,req_max=819,eff_min=614,eff_max=819

after set tg's cpu.uclamp.max = 50%，the result:
[  430.813789]c6  uclamp_test :
pid=138,req_min=614,req_max=819,eff_min=512,eff_max=512

case2:
without patch:
set task's clamp_min=614(60%), clamp_max=819(80%), the result as:
[  169.700544]c0  uclamp_test :
pid=137,req_min=0,req_max=209,eff_min=0,eff_max=209

after set tg's cpu.uclamp.min = 30%, tg's cpu.uclamp.max = 50%，the result:
[  246.500634]c7  uclamp_test :
pid=137,req_min=0,req_max=209,eff_min=307,eff_max=209

with patch:
set task's clamp_min=0(0%), clamp_max=209(20%), the result as:
[  169.700544]c0  uclamp_test :
pid=137,req_min=0,req_max=209,eff_min=0,eff_max=209

after set tg's cpu.uclamp.min = 30%, tg's cpu.uclamp.max = 50%，the result:
[  179.933868]c6  uclamp_test :
pid=137,req_min=0,req_max=209,eff_min=307,eff_max=307

---
Cheers

xuewen

On Fri, Jun 11, 2021 at 8:23 PM Qais Yousef <qais.yousef@arm.com> 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
> the taskgroup it's attached to (tg). Following table demonstrates
> 2 corner cases:
>
>                    |  p  |  tg  |  effective
>         -----------+-----+------+-----------
>         CASE 1
>         -----------+-----+------+-----------
>         uclamp_min | 60% | 0%   |  60%
>         -----------+-----+------+-----------
>         uclamp_max | 80% | 50%  |  50%
>         -----------+-----+------+-----------
>         CASE 2
>         -----------+-----+------+-----------
>         uclamp_min | 0%  | 30%  |  30%
>         -----------+-----+------+-----------
>         uclamp_max | 20% | 50%  |  20%
>         -----------+-----+------+-----------
>
> With this fix we get:
>
>                    |  p  |  tg  |  effective
>         -----------+-----+------+-----------
>         CASE 1
>         -----------+-----+------+-----------
>         uclamp_min | 60% | 0%   |  50%
>         -----------+-----+------+-----------
>         uclamp_max | 80% | 50%  |  50%
>         -----------+-----+------+-----------
>         CASE 2
>         -----------+-----+------+-----------
>         uclamp_min | 0%  | 30%  |  30%
>         -----------+-----+------+-----------
>         uclamp_max | 20% | 50%  |  30%
>         -----------+-----+------+-----------
>
> Additionally uclamp_update_active_tasks() must now unconditionally
> update both UCLAMP_MIN/MAX because changing the tg's UCLAMP_MAX for
> instance could have an impact on the effective UCLAMP_MIN of the tasks.
>
>                    |  p  |  tg  |  effective
>         -----------+-----+------+-----------
>         old
>         -----------+-----+------+-----------
>         uclamp_min | 60% | 0%   |  50%
>         -----------+-----+------+-----------
>         uclamp_max | 80% | 50%  |  50%
>         -----------+-----+------+-----------
>         *new*
>         -----------+-----+------+-----------
>         uclamp_min | 60% | 0%   | *60%*
>         -----------+-----+------+-----------
>         uclamp_max | 80% |*70%* | *70%*
>         -----------+-----+------+-----------
>
> [1] https://lore.kernel.org/lkml/CAB8ipk_a6VFNjiEnHRHkUMBKbA+qzPQvhtNjJ_YNzQhqV_o8Zw@mail.gmail.com/
>
> Reported-by: Xuewen Yan <xuewen.yan94@gmail.com>
> Fixes: 0c18f2ecfcc2 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
> Signed-off-by: Qais Yousef <qais.yousef@arm.com>
> ---
>
> Xuewen, Yun, Wei
>
> If you can give this a spin and provide Tested-by that would be much
> appreciated.
>
> Thanks!
>
>
>  kernel/sched/core.c | 43 +++++++++++++++----------------------------
>  1 file changed, 15 insertions(+), 28 deletions(-)
>
> 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;
>  }
>
>  /*
> @@ -1661,8 +1651,7 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
>
>  #ifdef CONFIG_UCLAMP_TASK_GROUP
>  static inline void
> -uclamp_update_active_tasks(struct cgroup_subsys_state *css,
> -                          unsigned int clamps)
> +uclamp_update_active_tasks(struct cgroup_subsys_state *css)
>  {
>         enum uclamp_id clamp_id;
>         struct css_task_iter it;
> @@ -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);
>         }
>  }
>
> --
> 2.25.1
>