Messages in this thread | | | Date | Sat, 26 Aug 2023 21:41:56 +0100 | From | Qais Yousef <> | Subject | Re: [PATCH v4 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0 |
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On 08/23/23 12:30, Dietmar Eggemann wrote: > On 22/08/2023 00:45, Qais Yousef wrote: > > When uclamp_max is being used, the util of the task could be higher than > > the spare capacity of the CPU, but due to uclamp_max value we force fit > > it there. > > > > The way the condition for checking for max_spare_cap in > > find_energy_efficient_cpu() was constructed; it ignored any CPU that has > > its spare_cap less than or _equal_ to max_spare_cap. Since we initialize > > max_spare_cap to 0; this lead to never setting max_spare_cap_cpu and > > hence ending up never performing compute_energy() for this cluster and > > missing an opportunity for a better energy efficient placement to honour > > uclamp_max setting. > > > > max_spare_cap = 0; > > cpu_cap = capacity_of(cpu) - task_util(p); // 0 if task_util(p) is high > > Nitpick: > > s/task_util(p)/cpu_util(cpu, p, cpu, ...) which is > > max(cpu_util + task_util, cpu_util_est + task_util_est) > > > > > ... > > > > util_fits_cpu(...); // will return true if uclamp_max forces it to fit > > > > ... > > > > // this logic will fail to update max_spare_cap_cpu if cpu_cap is 0 > > if (cpu_cap > max_spare_cap) { > > max_spare_cap = cpu_cap; > > max_spare_cap_cpu = cpu; > > } > > > > prev_spare_cap suffers from a similar problem. > > > > Fix the logic by converting the variables into long and treating -1 > > value as 'not populated' instead of 0 which is a viable and correct > > spare capacity value. We need to be careful signed comparison is used > > when comparing with cpu_cap in one of the conditions. > > > > Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions") > > Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org> > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> > > --- > > kernel/sched/fair.c | 11 +++++------ > > 1 file changed, 5 insertions(+), 6 deletions(-) > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > index 0b7445cd5af9..5da6538ed220 100644 > > --- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7707,11 +7707,10 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > > for (; pd; pd = pd->next) { > > unsigned long util_min = p_util_min, util_max = p_util_max; > > unsigned long cpu_cap, cpu_thermal_cap, util; > > - unsigned long cur_delta, max_spare_cap = 0; > > + long prev_spare_cap = -1, max_spare_cap = -1; > > unsigned long rq_util_min, rq_util_max; > > - unsigned long prev_spare_cap = 0; > > + unsigned long cur_delta, base_energy; > > int max_spare_cap_cpu = -1; > > - unsigned long base_energy; > > int fits, max_fits = -1; > > > > cpumask_and(cpus, perf_domain_span(pd), cpu_online_mask); > > @@ -7774,7 +7773,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > > prev_spare_cap = cpu_cap; > > prev_fits = fits; > > } else if ((fits > max_fits) || > > - ((fits == max_fits) && (cpu_cap > max_spare_cap))) { > > + ((fits == max_fits) && ((long)cpu_cap > max_spare_cap))) { > > /* > > * Find the CPU with the maximum spare capacity > > * among the remaining CPUs in the performance > > @@ -7786,7 +7785,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > > } > > } > > > > - if (max_spare_cap_cpu < 0 && prev_spare_cap == 0) > > + if (max_spare_cap_cpu < 0 && prev_spare_cap < 0) > > continue; > > > > eenv_pd_busy_time(&eenv, cpus, p); > > @@ -7794,7 +7793,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu) > > base_energy = compute_energy(&eenv, pd, cpus, p, -1); > > > > /* Evaluate the energy impact of using prev_cpu. */ > > - if (prev_spare_cap > 0) { > > + if (prev_spare_cap > -1) { > > prev_delta = compute_energy(&eenv, pd, cpus, p, > > prev_cpu); > > /* CPU utilization has changed */ > > We still need a solution to deal with situations in which `pd + task > contribution` > `pd_capacity`: > > compute_energy() > > if (dst_cpu >= 0) > busy_time = min(pd_capacity, pd_busy_time + task_busy_time); > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > pd + task contribution > > busy_time is based on util (ENERGY_UTIL), not on the uclamp values > (FREQUENCY_UTIL) we try to fit into a PD (and finally onto a CPU). > > With that as a reminder for us and the change in the cover letter:
This is not being ignored, but I don't see this as an urgent problem too. There are more pressing issues that make uclamp_max not effective in practice, and this ain't a bottleneck yet. Actually it might be doing a good thing as there's a desire to keep those tasks away on smallest CPU. But we shall visit this later for sure, don't worry :-) Ultimately we want EAS algorithm to be the judge of best placement for sure.
I hope to send patches to address load balancer and max aggregation issues in the coming weeks.
> > Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Thanks for the review!
I will wait for the maintainers to see if they would like a v5 to address the nitpicks or it's actually good enough and happy to pick this up. I think the commit messages explain the problem clear enough and doesn't warrant sending a new version. But happy to do so if there's insistence :-)
Thanks!
-- Qais Yousef
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