Messages in this thread |  | | From | Pavan Kondeti <> | Date | Mon, 4 Sep 2017 20:29:27 +0530 | Subject | Re: [RFC 2/3] sched/fair: use util_est in LB |
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On Mon, Sep 4, 2017 at 7:48 PM, Patrick Bellasi <patrick.bellasi@arm.com> wrote: > On 29-Aug 10:15, Pavan Kondeti wrote: >> On Fri, Aug 25, 2017 at 3:50 PM, Patrick Bellasi >> <patrick.bellasi@arm.com> wrote: >> > When the scheduler looks at the CPU utlization, the current PELT value >> > for a CPU is returned straight away. In certain scenarios this can have >> > undesired side effects on task placement. >> > >> >> <snip> >> >> > +/** >> > + * cpu_util_est: estimated utilization for the specified CPU >> > + * @cpu: the CPU to get the estimated utilization for >> > + * >> > + * The estimated utilization of a CPU is defined to be the maximum between its >> > + * PELT's utilization and the sum of the estimated utilization of the tasks >> > + * currently RUNNABLE on that CPU. >> > + * >> > + * This allows to properly represent the expected utilization of a CPU which >> > + * has just got a big task running since a long sleep period. At the same time >> > + * however it preserves the benefits of the "blocked load" in describing the >> > + * potential for other tasks waking up on the same CPU. >> > + * >> > + * Return: the estimated utlization for the specified CPU >> > + */ >> > +static inline unsigned long cpu_util_est(int cpu) >> > +{ >> > + struct sched_avg *sa = &cpu_rq(cpu)->cfs.avg; >> > + unsigned long util = cpu_util(cpu); >> > + >> > + if (!sched_feat(UTIL_EST)) >> > + return util; >> > + >> > + return max(util, util_est(sa, UTIL_EST_LAST)); >> > +} >> > + >> > static inline int task_util(struct task_struct *p) >> > { >> > return p->se.avg.util_avg; >> > @@ -6007,11 +6033,19 @@ static int cpu_util_wake(int cpu, struct task_struct *p) >> > >> > /* Task has no contribution or is new */ >> > if (cpu != task_cpu(p) || !p->se.avg.last_update_time) >> > - return cpu_util(cpu); >> > + return cpu_util_est(cpu); >> > >> > capacity = capacity_orig_of(cpu); >> > util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0); >> > >> > + /* >> > + * Estimated utilization tracks only tasks already enqueued, but still >> > + * sometimes can return a bigger value than PELT, for example when the >> > + * blocked load is negligible wrt the estimated utilization of the >> > + * already enqueued tasks. >> > + */ >> > + util = max_t(long, util, cpu_util_est(cpu)); >> > + >> >> We are supposed to discount the task's util from its CPU. But the >> cpu_util_est() can potentially return cpu_util() which includes the >> task's utilization. > > You right, this instead should cover all the cases: > > ---8<--- > static int cpu_util_wake(int cpu, struct task_struct *p) > { > - unsigned long util, capacity; > + unsigned long util_est = cpu_util_est(cpu); > + unsigned long capacity; > > /* Task has no contribution or is new */ > if (cpu != task_cpu(p) || !p->se.avg.last_update_time) > - return cpu_util(cpu); > + return util_est; > > capacity = capacity_orig_of(cpu); > - util = max_t(long, cpu_rq(cpu)->cfs.avg.util_avg - task_util(p), 0); > + if (cpu_util(cpu) > util_est) > + util = max_t(long, cpu_util(cpu) - task_util(p), 0); > + else > + util = util_est; > > return (util >= capacity) ? capacity : util; > } > ---8<--- > > Indeed: > > - if *p is the only task sleeping on that CPU, then: > (cpu_util == task_util) > (cpu_util_est == 0) > and thus we return: > (cpu_util - task_util) == 0 > > - if other tasks are SLEEPING on the same CPU, which however is IDLE, then: > cpu_util > (cpu_util_est == 0) > and thus we discount *p's blocked load by returning: > (cpu_util - task_util) >= 0 > > - if other tasks are RUNNABLE on that CPU and > (cpu_util_est > cpu_util) > then we wanna use cpu_util_est since it returns a more restrictive > estimation of the spare capacity on that CPU, by just considering > the expected utilization of tasks already runnable on that CPU. > > What do you think? >
This looks good to me.
Thanks, Pavan
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