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

[Qais Yousef]

On 06/16/21 19:09, Dietmar Eggemann wrote:
> 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

Fixed.

> 
> [...]
> 
> > 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.

Renamed it back to uc_req.

> 
> [...]
> 
> > @@ -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())?

Good idea. Done this

--->8---

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b4e856a4335d..fdb9a109fd68 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1620,8 +1620,9 @@ static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
 }

 static inline void
-uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
+uclamp_update_active(struct task_struct *p)
 {
+       enum uclamp_id clamp_id;
        struct rq_flags rf;
        struct rq *rq;

@@ -1641,9 +1642,11 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
         * affecting a valid clamp bucket, the next time it's enqueued,
         * it will already see the updated clamp bucket value.
         */
-       if (p->uclamp[clamp_id].active) {
-               uclamp_rq_dec_id(rq, p, clamp_id);
-               uclamp_rq_inc_id(rq, p, clamp_id);
+       for_each_clamp_id(clamp_id) {
+               if (p->uclamp[clamp_id].active) {
+                       uclamp_rq_dec_id(rq, p, clamp_id);
+                       uclamp_rq_inc_id(rq, p, clamp_id);
+               }
        }

        task_rq_unlock(rq, p, &rf);
@@ -1653,15 +1656,12 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
 static inline void
 uclamp_update_active_tasks(struct cgroup_subsys_state *css)
 {
-       enum uclamp_id clamp_id;
        struct css_task_iter it;
        struct task_struct *p;

        css_task_iter_start(css, 0, &it);
-       while ((p = css_task_iter_next(&it))) {
-               for_each_clamp_id(clamp_id)
-                       uclamp_update_active(p, clamp_id);
-       }
+       while ((p = css_task_iter_next(&it)))
+               uclamp_update_active(p);
        css_task_iter_end(&it);
 }

--->8---

Thanks!

--
Qais Yousef