On 20/04/2021 14:56, pierre.gond...@arm.com wrote: > From: Pierre Gondois <pierre.gond...@arm.com> > > find_energy_efficient_cpu() (feec()) searches the best energy CPU > to place a task on. To do so, compute_energy() estimates the energy > impact of placing the task on a CPU, based on CPU and task utilization > signals. > > Utilization signals can be concurrently updated while evaluating a > perf_domain. In some cases, this leads to having a 'negative delta', > i.e. placing the task in the perf_domain is seen as an energy gain. > Thus, any further energy comparison is biased. > > In case of a 'negative delta', return prev_cpu since: > 1. a 'negative delta' happens in less than 0.5% of feec() calls, > on a Juno with 6 CPUs (4 little, 2 big) > 2. it is unlikely to have two consecutive 'negative delta' for > a task, so if the first call fails, feec() will correctly > place the task in the next feec() call > 3. EAS current behavior tends to select prev_cpu if the task > doesn't raise the OPP of its current perf_domain. prev_cpu > is EAS's generic decision > 4. prev_cpu should be preferred to returning an error code. > In the latter case, select_idle_sibling() would do the placement, > selecting a big (and not energy efficient) CPU. As 3., the task > would potentially reside on the big CPU for a long time > > The patch also: > a. groups the compute_energy() calls to lower the chances of having > concurrent updates in between the calls > b. skips the base_energy_pd computation if no CPU is available in a > perf_domain
Did you run some tests to make sure you didn't regress on energy consumption? You could run EAS' Energy tests w/ and w/o the patch depicted in: https://lkml.kernel.org/r/20181203095628.11858-1-quentin.per...@arm.com > Fixes: eb92692b2544d sched/fair: Speed-up energy-aware wake-up > Reported-by: Xuewen Yan <xuewen....@unisoc.com> > Suggested-by: Xuewen Yan <xuewen....@unisoc.com> > Signed-off-by: Pierre Gondois <pierre.gond...@arm.com> > --- > kernel/sched/fair.c | 69 +++++++++++++++++++++++++-------------------- > 1 file changed, 39 insertions(+), 30 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 0dba0ebc3657..577482aa8919 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6594,8 +6594,8 @@ static int find_energy_efficient_cpu(struct task_struct > *p, int prev_cpu) > { > unsigned long prev_delta = ULONG_MAX, best_delta = ULONG_MAX; > struct root_domain *rd = cpu_rq(smp_processor_id())->rd; > + int cpu, best_energy_cpu = prev_cpu, target = -1; > unsigned long cpu_cap, util, base_energy = 0; > - int cpu, best_energy_cpu = prev_cpu; > struct sched_domain *sd; > struct perf_domain *pd; > > @@ -6614,19 +6614,18 @@ static int find_energy_efficient_cpu(struct > task_struct *p, int prev_cpu) > if (!sd) > goto fail; > > + target = prev_cpu; > + > sync_entity_load_avg(&p->se); > if (!task_util_est(p)) > - goto unlock; > + goto fail; > > for (; pd; pd = pd->next) { > unsigned long cur_delta, spare_cap, max_spare_cap = 0; > + bool compute_prev_delta = false; > unsigned long base_energy_pd; > int max_spare_cap_cpu = -1; > > - /* Compute the 'base' energy of the pd, without @p */ > - base_energy_pd = compute_energy(p, -1, pd); > - base_energy += base_energy_pd; > - > for_each_cpu_and(cpu, perf_domain_span(pd), > sched_domain_span(sd)) { > if (!cpumask_test_cpu(cpu, p->cpus_ptr)) > continue; > @@ -6647,26 +6646,41 @@ static int find_energy_efficient_cpu(struct > task_struct *p, int prev_cpu) > if (!fits_capacity(util, cpu_cap)) > continue; > > - /* Always use prev_cpu as a candidate. */ > if (cpu == prev_cpu) { > - prev_delta = compute_energy(p, prev_cpu, pd); > - prev_delta -= base_energy_pd; > - best_delta = min(best_delta, prev_delta); > - } > - > - /* > - * Find the CPU with the maximum spare capacity in > - * the performance domain > - */ > - if (spare_cap > max_spare_cap) { > + /* Always use prev_cpu as a candidate. */ > + compute_prev_delta = true; > + } else if (spare_cap > max_spare_cap) { > + /* > + * Find the CPU with the maximum spare capacity > + * in the performance domain. > + */ > max_spare_cap = spare_cap; > max_spare_cap_cpu = cpu; > } > } > > + if (max_spare_cap_cpu < 0 && !compute_prev_delta) > + continue; > + > + /* Compute the 'base' energy of the pd, without @p */ > + base_energy_pd = compute_energy(p, -1, pd); > + base_energy += base_energy_pd; Maybe add a comment /* Evaluate the energy impact of using prev_cpu. */ To be in sync with the if condition further below. > + if (compute_prev_delta) { > + prev_delta = compute_energy(p, prev_cpu, pd); > + /* Prevent negative deltas and select prev_cpu */ Not sure if this comment helps in understanding the code. We don't comment that we return prev_cpu if !task_util_est(p) or we're not entering the `Pick the best CPU ...` condition. > + if (prev_delta < base_energy_pd) > + goto fail; > + prev_delta -= base_energy_pd; > + best_delta = min(best_delta, prev_delta); > + } > + > /* Evaluate the energy impact of using this CPU. */ better /* Evaluate the energy impact of using max_spare_cap_cpu. */ since `this` has lost its context. > - if (max_spare_cap_cpu >= 0 && max_spare_cap_cpu != prev_cpu) { > + if (max_spare_cap_cpu >= 0) { > cur_delta = compute_energy(p, max_spare_cap_cpu, pd); > + /* Prevent negative deltas and select prev_cpu */ Not sure if this comment helps in understanding the code. > + if (cur_delta < base_energy_pd) > + goto fail; > cur_delta -= base_energy_pd; > if (cur_delta < best_delta) { > best_delta = cur_delta; > @@ -6674,25 +6688,20 @@ static int find_energy_efficient_cpu(struct > task_struct *p, int prev_cpu) > } > } > } > -unlock: > - rcu_read_unlock(); You don't close the RCU read-side critical section here anymore but include the following if condition as well. Don't we always want to close them as quick as possible? We could still return target (prev_cpu) after the if condition below ... > > /* > - * Pick the best CPU if prev_cpu cannot be used, or if it saves at > - * least 6% of the energy used by prev_cpu. > + * Pick the best CPU if: > + * - prev_cpu cannot be used, or > + * - it saves at least 6% of the energy used by prev_cpu > */ Why changing the layout of this comment? > - if (prev_delta == ULONG_MAX) > - return best_energy_cpu; > - > - if ((prev_delta - best_delta) > ((prev_delta + base_energy) >> 4)) > - return best_energy_cpu; > - > - return prev_cpu; > + if ((prev_delta == ULONG_MAX) || > + (prev_delta - best_delta) > ((prev_delta + base_energy) >> 4)) > + target = best_energy_cpu; > > fail: > rcu_read_unlock(); > > - return -1; > + return target; > } > > /* >
