On 05/28/20 15:23, Peter Zijlstra wrote: > On Mon, May 11, 2020 at 04:40:52PM +0100, Qais Yousef wrote: > > +/* > > + * By default RT tasks run at the maximum performance point/capacity of the > > + * system. Uclamp enforces this by always setting UCLAMP_MIN of RT tasks to > > + * SCHED_CAPACITY_SCALE. > > + * > > + * This knob allows admins to change the default behavior when uclamp is being > > + * used. In battery powered devices, particularly, running at the maximum > > + * capacity and frequency will increase energy consumption and shorten the > > + * battery life. > > + * > > + * This knob only affects RT tasks that their uclamp_se->user_defined == false. > > + * > > + * This knob will not override the system default sched_util_clamp_min defined > > + * above. > > + * > > + * Any modification is applied lazily on the next attempt to calculate the > > + * effective value of the task. > > + */ > > +unsigned int sysctl_sched_uclamp_util_min_rt_default = SCHED_CAPACITY_SCALE; > > + > > /* All clamps are required to be less or equal than these values */ > > static struct uclamp_se uclamp_default[UCLAMP_CNT]; > > > > @@ -872,6 +892,28 @@ unsigned int uclamp_rq_max_value(struct rq *rq, enum uclamp_id clamp_id, > > return uclamp_idle_value(rq, clamp_id, clamp_value); > > } > > > > +static inline void uclamp_sync_util_min_rt_default(struct task_struct *p, > > + enum uclamp_id clamp_id) > > +{ > > + unsigned int default_util_min = sysctl_sched_uclamp_util_min_rt_default; > > + struct uclamp_se *uc_se; > > + > > + /* Only sync for UCLAMP_MIN and RT tasks */ > > + if (clamp_id != UCLAMP_MIN || !rt_task(p)) > > + return; > > + > > + uc_se = &p->uclamp_req[UCLAMP_MIN]; > > + > > + /* > > + * Only sync if user didn't override the default request and the sysctl > > + * knob has changed. > > + */ > > + if (uc_se->user_defined || uc_se->value == default_util_min) > > + return; > > + > > + uclamp_se_set(uc_se, default_util_min, false); > > +} > > So afaict this is directly added to the enqueue/dequeue path, and we've > recently already had complaints that uclamp is too slow. I wanted to keep this function simpler. > > Is there really no other way? There is my first attempt which performs the sync @ task_woken_rt(). https://lore.kernel.org/lkml/20191220164838.31619-1-qais.yousef@xxxxxxx/ I can revert the sync function to the simpler version defined in that patch too. I can potentially move this to uclamp_eff_value() too. Will need to think more if this is enough. If task_woken_rt() is good for you, I'd say that's more obviously correct and better to go with it. FWIW, I think you're referring to Mel's notice in OSPM regarding the overhead. Trying to see what goes on in there. Thanks! -- Qais Yousef