Messages in this thread |  | | Date | Wed, 18 Apr 2018 09:13:39 +0100 | From | Quentin Perret <> | Subject | Re: [RFC PATCH v2 4/6] sched/fair: Introduce an energy estimation helper function |
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On Tuesday 17 Apr 2018 at 23:22:13 (+0800), Leo Yan wrote: > On Fri, Apr 06, 2018 at 04:36:05PM +0100, Dietmar Eggemann wrote: > > From: Quentin Perret <quentin.perret@arm.com> > > > > In preparation for the definition of an energy-aware wakeup path, a > > helper function is provided to estimate the consequence on system energy > > when a specific task wakes-up on a specific CPU. compute_energy() > > estimates the OPPs to be reached by all frequency domains and estimates > > the consumption of each online CPU according to its energy model and its > > percentage of busy time. > > > > Cc: Ingo Molnar <mingo@redhat.com> > > Cc: Peter Zijlstra <peterz@infradead.org> > > Signed-off-by: Quentin Perret <quentin.perret@arm.com> > > Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com> > > --- > > include/linux/sched/energy.h | 20 +++++++++++++ > > kernel/sched/fair.c | 68 ++++++++++++++++++++++++++++++++++++++++++++ > > kernel/sched/sched.h | 2 +- > > 3 files changed, 89 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/sched/energy.h b/include/linux/sched/energy.h > > index 941071eec013..b4110b145228 100644 > > --- a/include/linux/sched/energy.h > > +++ b/include/linux/sched/energy.h > > @@ -27,6 +27,24 @@ static inline bool sched_energy_enabled(void) > > return static_branch_unlikely(&sched_energy_present); > > } > > > > +static inline > > +struct capacity_state *find_cap_state(int cpu, unsigned long util) > > +{ > > + struct sched_energy_model *em = *per_cpu_ptr(energy_model, cpu); > > + struct capacity_state *cs = NULL; > > + int i; > > + > > + util += util >> 2; > > + > > + for (i = 0; i < em->nr_cap_states; i++) { > > + cs = &em->cap_states[i]; > > + if (cs->cap >= util) > > + break; > > + } > > + > > + return cs; > > 'cs' is possible to return NULL.
Only if em-nr_cap_states==0, and that shouldn't be possible if sched_energy_present==True, so this code should be safe :-)
> > > +} > > + > > static inline struct cpumask *freq_domain_span(struct freq_domain *fd) > > { > > return &fd->span; > > @@ -42,6 +60,8 @@ struct freq_domain; > > static inline bool sched_energy_enabled(void) { return false; } > > static inline struct cpumask > > *freq_domain_span(struct freq_domain *fd) { return NULL; } > > +static inline struct capacity_state > > +*find_cap_state(int cpu, unsigned long util) { return NULL; } > > static inline void init_sched_energy(void) { } > > #define for_each_freq_domain(fdom) for (; fdom; fdom = NULL) > > #endif > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 6960e5ef3c14..8cb9fb04fff2 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6633,6 +6633,74 @@ static int wake_cap(struct task_struct *p, int cpu, int prev_cpu) > > } > > > > /* > > + * Returns the util of "cpu" if "p" wakes up on "dst_cpu". > > + */ > > +static unsigned long cpu_util_next(int cpu, struct task_struct *p, int dst_cpu) > > +{ > > + unsigned long util, util_est; > > + struct cfs_rq *cfs_rq; > > + > > + /* Task is where it should be, or has no impact on cpu */ > > + if ((task_cpu(p) == dst_cpu) || (cpu != task_cpu(p) && cpu != dst_cpu)) > > + return cpu_util(cpu); > > + > > + cfs_rq = &cpu_rq(cpu)->cfs; > > + util = READ_ONCE(cfs_rq->avg.util_avg); > > + > > + if (dst_cpu == cpu) > > + util += task_util(p); > > + else > > + util = max_t(long, util - task_util(p), 0); > > I tried to understand the logic at here, below code is more clear for > myself: > > int prev_cpu = task_cpu(p); > > cfs_rq = &cpu_rq(cpu)->cfs; > util = READ_ONCE(cfs_rq->avg.util_avg); > > /* Bail out if src and dst CPUs are the same one */ > if (prev_cpu == cpu && dst_cpu == cpu) > return util; > > /* Remove task utilization for src CPU */ > if (cpu == prev_cpu) > util = max_t(long, util - task_util(p), 0); > > /* Add task utilization for dst CPU */ > if (dst_cpu == cpu) > util += task_util(p); > > BTW, CPU utilization is decayed value and task_util() is not decayed > value, so 'util - task_util(p)' calculates a smaller value than the > prev CPU pure utilization, right?
task_util() is the raw PELT signal, without UTIL_EST, so I think it's fine to do `util - task_util()`.
> > Another question is can we reuse the function cpu_util_wake() and > just compenstate task util for dst cpu?
Well it's not that simple. cpu_util_wake() will give you the max between the util_avg and the util_est value, so which task_util() should you add to it ? The util_avg or the uti_est value ?
Here we are trying to predict what will be the cpu_util signal in the future, so the only always-correct implementation of this function has to predict what will be the CPU util_avg and util_est signals in parallel and take the max of the two.
> > > + if (sched_feat(UTIL_EST)) { > > + util_est = READ_ONCE(cfs_rq->avg.util_est.enqueued); > > + if (dst_cpu == cpu) > > + util_est += _task_util_est(p); > > + else > > + util_est = max_t(long, util_est - _task_util_est(p), 0); > > + util = max(util, util_est); > > + } > > + > > + return min_t(unsigned long, util, capacity_orig_of(cpu)); > > +} > > + > > +/* > > + * Estimates the system level energy assuming that p wakes-up on dst_cpu. > > + * > > + * compute_energy() is safe to call only if an energy model is available for > > + * the platform, which is when sched_energy_enabled() is true. > > + */ > > +static unsigned long compute_energy(struct task_struct *p, int dst_cpu) > > +{ > > + unsigned long util, max_util, sum_util; > > + struct capacity_state *cs; > > + unsigned long energy = 0; > > + struct freq_domain *fd; > > + int cpu; > > + > > + for_each_freq_domain(fd) { > > + max_util = sum_util = 0; > > + for_each_cpu_and(cpu, freq_domain_span(fd), cpu_online_mask) { > > + util = cpu_util_next(cpu, p, dst_cpu); > > + util += cpu_util_dl(cpu_rq(cpu)); > > + max_util = max(util, max_util); > > + sum_util += util; > > + } > > + > > + /* > > + * Here we assume that the capacity states of CPUs belonging to > > + * the same frequency domains are shared. Hence, we look at the > > + * capacity state of the first CPU and re-use it for all. > > + */ > > + cpu = cpumask_first(freq_domain_span(fd)); > > + cs = find_cap_state(cpu, max_util); > > + energy += cs->power * sum_util / cs->cap; > > + } > > This means all CPUs will be iterated for calculation, the complexity is > O(n)... > > > + return energy; > > +} > > + > > +/* > > * select_task_rq_fair: Select target runqueue for the waking task in domains > > * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE, > > * SD_BALANCE_FORK, or SD_BALANCE_EXEC. > > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > > index 5d552c0d7109..6eb38f41d5d9 100644 > > --- a/kernel/sched/sched.h > > +++ b/kernel/sched/sched.h > > @@ -2156,7 +2156,7 @@ static inline void cpufreq_update_util(struct rq *rq, unsigned int flags) {} > > # define arch_scale_freq_invariant() false > > #endif > > > > -#ifdef CONFIG_CPU_FREQ_GOV_SCHEDUTIL > > +#ifdef CONFIG_SMP > > static inline unsigned long cpu_util_dl(struct rq *rq) > > { > > return (rq->dl.running_bw * SCHED_CAPACITY_SCALE) >> BW_SHIFT; > > -- > > 2.11.0 > >
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