| Message ID | 20180912091309.7551-13-quentin.perret@arm.com (mailing list archive) |
|---|---|
| State | Changes Requested, archived |
| Headers | show |
| Series | Energy Aware Scheduling | expand |
On Wed, Sep 12, 2018 at 10:13:07AM +0100, Quentin Perret wrote: > + while (pd) { > + unsigned long cur_energy, spare_cap, max_spare_cap = 0; > + int max_spare_cap_cpu = -1; > + > + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) { Which of the two masks do we expect to be the smallest? > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) > + continue; > + > + /* Skip CPUs that will be overutilized. */ > + util = cpu_util_next(cpu, p, cpu); > + cpu_cap = capacity_of(cpu); > + if (cpu_cap * 1024 < util * capacity_margin) > + continue; > + > + /* Always use prev_cpu as a candidate. */ > + if (cpu == prev_cpu) { > + prev_energy = compute_energy(p, prev_cpu, head); > + if (prev_energy < best_energy) > + best_energy = prev_energy; best_energy = min(best_energy, prev_energy); That's both shorter and clearer. > + continue; > + } > + > + /* > + * Find the CPU with the maximum spare capacity in > + * the performance domain > + */ > + spare_cap = cpu_cap - util; > + if (spare_cap > max_spare_cap) { > + max_spare_cap = spare_cap; > + max_spare_cap_cpu = cpu; > + } Sometimes I wonder if something like: #define min_filter(varp, val) \ ({ \ typeof(varp) _varp = (varp); \ typeof(val) _val = (val); \ bool f = false; \ \ if (_val < *_varp) { \ *_varp = _val; \ f = true; \ } \ \ f; \ }) and the corresponding max_filter() are worth the trouble; it would allow writing: if (max_filter(&max_spare_cap, spare_cap)) max_spare_cap_cpu = cpu; and: > + } > + > + /* Evaluate the energy impact of using this CPU. */ > + if (max_spare_cap_cpu >= 0) { > + cur_energy = compute_energy(p, max_spare_cap_cpu, head); > + if (cur_energy < best_energy) { > + best_energy = cur_energy; > + best_energy_cpu = max_spare_cap_cpu; > + } if (min_filter(&best_energy, cur_energy)) best_energy_cpu = max_spare_cap_cpu; But then I figure, it is not... dunno. We do lots of this stuff. > + } > + pd = pd->next; > + } > + > + /* > + * Pick the best CPU if prev_cpu cannot be used, or if it saves at > + * least 6% of the energy used by prev_cpu. > + */ > + if (prev_energy == ULONG_MAX || > + (prev_energy - best_energy) > (prev_energy >> 4)) > + return best_energy_cpu; Does that become more readable if we split that into two conditions? if (prev_energy == ULONG_MAX) return best_energy_cpu; if ((prev_energy - best_energy) > (prev_energy >> 4)) return best_energy_cpu; > + return prev_cpu; > +} > + > /* > * 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, > @@ -6360,13 +6468,37 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > int want_affine = 0; > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); > > + rcu_read_lock(); > if (sd_flag & SD_BALANCE_WAKE) { > record_wakee(p); > + > + /* > + * Forkees are not accepted in the energy-aware wake-up path > + * because they don't have any useful utilization data yet and > + * it's not possible to forecast their impact on energy > + * consumption. Consequently, they will be placed by > + * find_idlest_cpu() on the least loaded CPU, which might turn > + * out to be energy-inefficient in some use-cases. The > + * alternative would be to bias new tasks towards specific types > + * of CPUs first, or to try to infer their util_avg from the > + * parent task, but those heuristics could hurt other use-cases > + * too. So, until someone finds a better way to solve this, > + * let's keep things simple by re-using the existing slow path. > + */ > + if (sched_feat(ENERGY_AWARE)) { > + struct root_domain *rd = cpu_rq(cpu)->rd; > + struct perf_domain *pd = rcu_dereference(rd->pd); > + > + if (pd && !READ_ONCE(rd->overutilized)) { > + new_cpu = find_energy_efficient_cpu(p, prev_cpu, pd); > + goto unlock; > + } > + } > + > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) && > + cpumask_test_cpu(cpu, &p->cpus_allowed); > } I would much prefer this to be something like: diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index a8f601edd958..5475a885ec9f 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6299,12 +6299,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f { struct sched_domain *tmp, *sd = NULL; int cpu = smp_processor_id(); - int new_cpu = prev_cpu; + unsigned int new_cpu = prev_cpu; int want_affine = 0; int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); if (sd_flag & SD_BALANCE_WAKE) { record_wakee(p); + + if (static_branch_unlikely(sched_eas_balance)) { + new_cpu = select_task_rq_eas(p, prev_cpu, sd_flags, wake_flags); + if (new_cpu < nr_cpu_ids) + return new_cpu; + } + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) && cpumask_test_cpu(cpu, &p->cpus_allowed); } and then hide everything (including that giant comment) in select_task_rq_eas().
On Thursday 04 Oct 2018 at 11:44:12 (+0200), Peter Zijlstra wrote: > On Wed, Sep 12, 2018 at 10:13:07AM +0100, Quentin Perret wrote: > > + while (pd) { > > + unsigned long cur_energy, spare_cap, max_spare_cap = 0; > > + int max_spare_cap_cpu = -1; > > + > > + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) { > > Which of the two masks do we expect to be the smallest? Typically, perf_domain_span is smaller. > > + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) > > + continue; > > + > > + /* Skip CPUs that will be overutilized. */ > > + util = cpu_util_next(cpu, p, cpu); > > + cpu_cap = capacity_of(cpu); > > + if (cpu_cap * 1024 < util * capacity_margin) > > + continue; > > + > > + /* Always use prev_cpu as a candidate. */ > > + if (cpu == prev_cpu) { > > + prev_energy = compute_energy(p, prev_cpu, head); > > + if (prev_energy < best_energy) > > + best_energy = prev_energy; > > best_energy = min(best_energy, prev_energy); > > That's both shorter and clearer. OK. > > + continue; > > + } > > + > > + /* > > + * Find the CPU with the maximum spare capacity in > > + * the performance domain > > + */ > > + spare_cap = cpu_cap - util; > > + if (spare_cap > max_spare_cap) { > > + max_spare_cap = spare_cap; > > + max_spare_cap_cpu = cpu; > > + } > > Sometimes I wonder if something like: > > #define min_filter(varp, val) \ > ({ \ > typeof(varp) _varp = (varp); \ > typeof(val) _val = (val); \ > bool f = false; \ > \ > if (_val < *_varp) { \ > *_varp = _val; \ > f = true; \ > } \ > \ > f; \ > }) > > and the corresponding max_filter() are worth the trouble; it would allow > writing: > > if (max_filter(&max_spare_cap, spare_cap)) > max_spare_cap_cpu = cpu; > > and: > > > + } > > + > > + /* Evaluate the energy impact of using this CPU. */ > > + if (max_spare_cap_cpu >= 0) { > > + cur_energy = compute_energy(p, max_spare_cap_cpu, head); > > + if (cur_energy < best_energy) { > > + best_energy = cur_energy; > > + best_energy_cpu = max_spare_cap_cpu; > > + } > > if (min_filter(&best_energy, cur_energy)) > best_energy_cpu = max_spare_cap_cpu; > > But then I figure, it is not... dunno. We do lots of this stuff. If there are occurrences of this stuff all over the place, we could do that in a separate clean-up patch that does just that, for the entire file. Or maybe more ? > > + } > > + pd = pd->next; > > + } > > + > > + /* > > + * Pick the best CPU if prev_cpu cannot be used, or if it saves at > > + * least 6% of the energy used by prev_cpu. > > + */ > > + if (prev_energy == ULONG_MAX || > > + (prev_energy - best_energy) > (prev_energy >> 4)) > > + return best_energy_cpu; > > Does that become more readable if we split that into two conditions? > > if (prev_energy == ULONG_MAX) > return best_energy_cpu; > > if ((prev_energy - best_energy) > (prev_energy >> 4)) > return best_energy_cpu; Yeah, why not :-) > > + return prev_cpu; > > +} > > + > > /* > > * 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, > > @@ -6360,13 +6468,37 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > > int want_affine = 0; > > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); > > > > + rcu_read_lock(); > > if (sd_flag & SD_BALANCE_WAKE) { > > record_wakee(p); > > + > > + /* > > + * Forkees are not accepted in the energy-aware wake-up path > > + * because they don't have any useful utilization data yet and > > + * it's not possible to forecast their impact on energy > > + * consumption. Consequently, they will be placed by > > + * find_idlest_cpu() on the least loaded CPU, which might turn > > + * out to be energy-inefficient in some use-cases. The > > + * alternative would be to bias new tasks towards specific types > > + * of CPUs first, or to try to infer their util_avg from the > > + * parent task, but those heuristics could hurt other use-cases > > + * too. So, until someone finds a better way to solve this, > > + * let's keep things simple by re-using the existing slow path. > > + */ > > + if (sched_feat(ENERGY_AWARE)) { > > + struct root_domain *rd = cpu_rq(cpu)->rd; > > + struct perf_domain *pd = rcu_dereference(rd->pd); > > + > > + if (pd && !READ_ONCE(rd->overutilized)) { > > + new_cpu = find_energy_efficient_cpu(p, prev_cpu, pd); > > + goto unlock; > > + } > > + } > > + > > + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) && > > + cpumask_test_cpu(cpu, &p->cpus_allowed); > > } > > I would much prefer this to be something like: > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index a8f601edd958..5475a885ec9f 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6299,12 +6299,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > { > struct sched_domain *tmp, *sd = NULL; > int cpu = smp_processor_id(); > - int new_cpu = prev_cpu; > + unsigned int new_cpu = prev_cpu; > int want_affine = 0; > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); > > if (sd_flag & SD_BALANCE_WAKE) { > record_wakee(p); > + > + if (static_branch_unlikely(sched_eas_balance)) { > + new_cpu = select_task_rq_eas(p, prev_cpu, sd_flags, wake_flags); > + if (new_cpu < nr_cpu_ids) > + return new_cpu; > + } > + > want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) > && cpumask_test_cpu(cpu, &p->cpus_allowed); > } > and then hide everything (including that giant comment) in > select_task_rq_eas(). So you think we should rename find_energy_efficient_cpu and put all the checks in there ? Or should select_task_rq_eas do the checks and then call find_energy_efficient_cpu ? Not a huge deal, but that'll save some time if we agree on that one upfront. Thanks
On Thu, Oct 04, 2018 at 11:27:22AM +0100, Quentin Perret wrote: > > > + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) { > > > > Which of the two masks do we expect to be the smallest? > > Typically, perf_domain_span is smaller. OK, then the above expression is in the right order :-) > > > + if (spare_cap > max_spare_cap) { > > > + max_spare_cap = spare_cap; > > > + max_spare_cap_cpu = cpu; > > > + } > > > > Sometimes I wonder if something like: > > > > #define min_filter(varp, val) \ > > ({ \ > > typeof(varp) _varp = (varp); \ > > typeof(val) _val = (val); \ > > bool f = false; \ > > \ > > if (_val < *_varp) { \ > > *_varp = _val; \ > > f = true; \ > > } \ > > \ > > f; \ > > }) > > > > and the corresponding max_filter() are worth the trouble; it would allow > > writing: > > > > if (max_filter(&max_spare_cap, spare_cap)) > > max_spare_cap_cpu = cpu; > > > > and: > > > > > + } > > > + > > > + /* Evaluate the energy impact of using this CPU. */ > > > + if (max_spare_cap_cpu >= 0) { > > > + cur_energy = compute_energy(p, max_spare_cap_cpu, head); > > > + if (cur_energy < best_energy) { > > > + best_energy = cur_energy; > > > + best_energy_cpu = max_spare_cap_cpu; > > > + } > > > > if (min_filter(&best_energy, cur_energy)) > > best_energy_cpu = max_spare_cap_cpu; > > > > But then I figure, it is not... dunno. We do lots of this stuff. > > If there are occurrences of this stuff all over the place, we could do > that in a separate clean-up patch that does just that, for the entire > file. Or maybe more ? Sure, not something that needs done now. I just always think of this when I see this pattern repeated, but never seem to get around to doing anything about it. I figured I'd mention it ;-) > > I would much prefer this to be something like: > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index a8f601edd958..5475a885ec9f 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -6299,12 +6299,19 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f > > { > > struct sched_domain *tmp, *sd = NULL; > > int cpu = smp_processor_id(); > > - int new_cpu = prev_cpu; > > + unsigned int new_cpu = prev_cpu; > > int want_affine = 0; > > int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); > > > > if (sd_flag & SD_BALANCE_WAKE) { > > record_wakee(p); > > + > > + if (static_branch_unlikely(sched_eas_balance)) { > > + new_cpu = select_task_rq_eas(p, prev_cpu, sd_flags, wake_flags); > > + if (new_cpu < nr_cpu_ids) > > + return new_cpu; > > + } > > + > > want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) > > && cpumask_test_cpu(cpu, &p->cpus_allowed); > > } > > and then hide everything (including that giant comment) in > > select_task_rq_eas(). > > So you think we should rename find_energy_efficient_cpu and put all the > checks in there ? Or should select_task_rq_eas do the checks and then > call find_energy_efficient_cpu ? > > Not a huge deal, but that'll save some time if we agree on that one > upfront. Not sure, see what it looks like ;-) My main concern here was to get rid of that giant blob in select_task_rq_fair().
On Thursday 04 Oct 2018 at 12:41:07 (+0200), Peter Zijlstra wrote: > Not sure, see what it looks like ;-) My main concern here was to get rid > of that giant blob in select_task_rq_fair(). OK, got it. I'll probably just move the checks into the function and merge that large comment into the already massive comment above find_energy_efficient_cpu() or something. We'll see :-) And do you actually care about the function name ? We've been using find_energy_efficient_cpu() for some time in Android now, so that's just slightly easier for us if that can be re-used. Not a big deal either, though. Thanks, Quentin
On Thu, Oct 04, 2018 at 11:55:09AM +0100, Quentin Perret wrote: > On Thursday 04 Oct 2018 at 12:41:07 (+0200), Peter Zijlstra wrote: > > Not sure, see what it looks like ;-) My main concern here was to get rid > > of that giant blob in select_task_rq_fair(). > > OK, got it. I'll probably just move the checks into the function and > merge that large comment into the already massive comment above > find_energy_efficient_cpu() or something. We'll see :-) > > And do you actually care about the function name ? We've been using > find_energy_efficient_cpu() for some time in Android now, so that's just > slightly easier for us if that can be re-used. Not a big deal either, > though. Sure, that's fine I suppose.
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 7ee3e43cdaf2..6e988a01011c 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -6339,6 +6339,114 @@ static long compute_energy(struct task_struct *p, int dst_cpu, return energy; } +/* + * find_energy_efficient_cpu(): Find most energy-efficient target CPU for the + * waking task. find_energy_efficient_cpu() looks for the CPU with maximum + * spare capacity in each performance domain and uses it as a potential + * candidate to execute the task. Then, it uses the Energy Model to figure + * out which of the CPU candidates is the most energy-efficient. + * + * The rationale for this heuristic is as follows. In a performance domain, + * all the most energy efficient CPU candidates (according to the Energy + * Model) are those for which we'll request a low frequency. When there are + * several CPUs for which the frequency request will be the same, we don't + * have enough data to break the tie between them, because the Energy Model + * only includes active power costs. With this model, if we assume that + * frequency requests follow utilization (e.g. using schedutil), the CPU with + * the maximum spare capacity in a performance domain is guaranteed to be among + * the best candidates of the performance domain. + * + * In practice, it could be preferable from an energy standpoint to pack + * small tasks on a CPU in order to let other CPUs go in deeper idle states, + * but that could also hurt our chances to go cluster idle, and we have no + * ways to tell with the current Energy Model if this is actually a good + * idea or not. So, find_energy_efficient_cpu() basically favors + * cluster-packing, and spreading inside a cluster. That should at least be + * a good thing for latency, and this is consistent with the idea that most + * of the energy savings of EAS come from the asymmetry of the system, and + * not so much from breaking the tie between identical CPUs. That's also the + * reason why EAS is enabled in the topology code only for systems where + * SD_ASYM_CPUCAPACITY is set. + */ +static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu, + struct perf_domain *pd) +{ + unsigned long prev_energy = ULONG_MAX, best_energy = ULONG_MAX; + int cpu, best_energy_cpu = prev_cpu; + struct perf_domain *head = pd; + unsigned long cpu_cap, util; + struct sched_domain *sd; + + sync_entity_load_avg(&p->se); + + if (!task_util_est(p)) + return prev_cpu; + + /* + * Energy-aware wake-up happens on the lowest sched_domain starting + * from sd_asym_cpucapacity spanning over this_cpu and prev_cpu. + */ + sd = rcu_dereference(*this_cpu_ptr(&sd_asym_cpucapacity)); + while (sd && !cpumask_test_cpu(prev_cpu, sched_domain_span(sd))) + sd = sd->parent; + if (!sd) + return prev_cpu; + + while (pd) { + unsigned long cur_energy, spare_cap, max_spare_cap = 0; + int max_spare_cap_cpu = -1; + + for_each_cpu_and(cpu, perf_domain_span(pd), sched_domain_span(sd)) { + if (!cpumask_test_cpu(cpu, &p->cpus_allowed)) + continue; + + /* Skip CPUs that will be overutilized. */ + util = cpu_util_next(cpu, p, cpu); + cpu_cap = capacity_of(cpu); + if (cpu_cap * 1024 < util * capacity_margin) + continue; + + /* Always use prev_cpu as a candidate. */ + if (cpu == prev_cpu) { + prev_energy = compute_energy(p, prev_cpu, head); + if (prev_energy < best_energy) + best_energy = prev_energy; + continue; + } + + /* + * Find the CPU with the maximum spare capacity in + * the performance domain + */ + spare_cap = cpu_cap - util; + if (spare_cap > max_spare_cap) { + max_spare_cap = spare_cap; + max_spare_cap_cpu = cpu; + } + } + + /* Evaluate the energy impact of using this CPU. */ + if (max_spare_cap_cpu >= 0) { + cur_energy = compute_energy(p, max_spare_cap_cpu, head); + if (cur_energy < best_energy) { + best_energy = cur_energy; + best_energy_cpu = max_spare_cap_cpu; + } + } + pd = pd->next; + } + + /* + * Pick the best CPU if prev_cpu cannot be used, or if it saves at + * least 6% of the energy used by prev_cpu. + */ + if (prev_energy == ULONG_MAX || + (prev_energy - best_energy) > (prev_energy >> 4)) + return best_energy_cpu; + + return prev_cpu; +} + /* * 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, @@ -6360,13 +6468,37 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f int want_affine = 0; int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING); + rcu_read_lock(); if (sd_flag & SD_BALANCE_WAKE) { record_wakee(p); - want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) - && cpumask_test_cpu(cpu, &p->cpus_allowed); + + /* + * Forkees are not accepted in the energy-aware wake-up path + * because they don't have any useful utilization data yet and + * it's not possible to forecast their impact on energy + * consumption. Consequently, they will be placed by + * find_idlest_cpu() on the least loaded CPU, which might turn + * out to be energy-inefficient in some use-cases. The + * alternative would be to bias new tasks towards specific types + * of CPUs first, or to try to infer their util_avg from the + * parent task, but those heuristics could hurt other use-cases + * too. So, until someone finds a better way to solve this, + * let's keep things simple by re-using the existing slow path. + */ + if (sched_feat(ENERGY_AWARE)) { + struct root_domain *rd = cpu_rq(cpu)->rd; + struct perf_domain *pd = rcu_dereference(rd->pd); + + if (pd && !READ_ONCE(rd->overutilized)) { + new_cpu = find_energy_efficient_cpu(p, prev_cpu, pd); + goto unlock; + } + } + + want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu) && + cpumask_test_cpu(cpu, &p->cpus_allowed); } - rcu_read_lock(); for_each_domain(cpu, tmp) { if (!(tmp->flags & SD_LOAD_BALANCE)) break; @@ -6401,6 +6533,7 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f if (want_affine) current->recent_used_cpu = cpu; } +unlock: rcu_read_unlock(); return new_cpu;
If an Energy Model (EM) is available and if the system isn't overutilized, re-route waking tasks into an energy-aware placement algorithm. The selection of an energy-efficient CPU for a task is achieved by estimating the impact on system-level active energy resulting from the placement of the task on the CPU with the highest spare capacity in each performance domain. This strategy spreads tasks in a performance domain and avoids overly aggressive task packing. The best CPU energy-wise is then selected if it saves a large enough amount of energy with respect to prev_cpu. Although it has already shown significant benefits on some existing targets, this approach cannot scale to platforms with numerous CPUs. This is an attempt to do something useful as writing a fast heuristic that performs reasonably well on a broad spectrum of architectures isn't an easy task. As such, the scope of usability of the energy-aware wake-up path is restricted to systems with the SD_ASYM_CPUCAPACITY flag set, and where the EM isn't too complex. Cc: Ingo Molnar <mingo@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Signed-off-by: Quentin Perret <quentin.perret@arm.com> --- kernel/sched/fair.c | 139 +++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 136 insertions(+), 3 deletions(-)