Messages in this thread | | | From | Vincent Guittot <> | Date | Mon, 20 Feb 2023 18:02:02 +0100 | Subject | Re: [PATCH v2 1/3] sched/uclamp: Set max_spare_cap_cpu even if max_spare_cap is 0 |
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On Sat, 11 Feb 2023 at 18:28, Qais Yousef <qyousef@layalina.io> wrote: > > On 02/07/23 10:45, Vincent Guittot wrote: > > On Sun, 5 Feb 2023 at 23:43, Qais Yousef <qyousef@layalina.io> 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 > > > > > > ... > > > > > > 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. > > > > > > Fixes: 1d42509e475c ("sched/fair: Make EAS wakeup placement consider uclamp restrictions") > > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io> > > > --- > > > kernel/sched/fair.c | 9 ++++----- > > > 1 file changed, 4 insertions(+), 5 deletions(-) > > > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > > > index c6c8e7f52935..7a21ee74139f 100644 > > > --- a/kernel/sched/fair.c > > > +++ b/kernel/sched/fair.c > > > @@ -7382,11 +7382,10 @@ 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); > > > @@ -7461,7 +7460,7 @@ 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); > > > @@ -7469,7 +7468,7 @@ 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 */ > > > > I think that you also need the change below to make sure that the > > signed comparison will be used. I have quickly checked the assembly > > code for aarch64 and your patch keeps using unsigned comparison (b.ls) > > ((fits == max_fits) && (cpu_cap > max_spare_cap))) { > > ffff8000080e4c94: f94067e0 ldr x0, [sp, #200] > > ffff8000080e4c98: eb00003f cmp x1, x0 > > ffff8000080e4c9c: 54ff98a9 b.ls ffff8000080e3fb0 > > <select_task_rq_fair+0x570> // b.plast > > > > Whereas the change below make it to use the signed version (b.le) > > ((fits == max_fits) && ((long)cpu_cap > max_spare_cap))) { > > ffff8000080e4c94: f94067e0 ldr x0, [sp, #200] > > ffff8000080e4c98: eb00003f cmp x1, x0 > > ffff8000080e4c9c: 54ff98ad b.le ffff8000080e3fb0 <select_task_rq_fair+0x570> > > > > -- a/kernel/sched/fair.c > > +++ b/kernel/sched/fair.c > > @@ -7522,7 +7522,7 @@ 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 > > Isn't it better to go back to v1 form then? The inconsistent type paired with > the cast isn't getting too ugly for me :(
the cast into a long of the cpu capacity in the condition was a good way to fix this unsigned/signed comparison and make is consistent with the use of -1 as default value IMO ((long)cpu_cap > max_spare_cap) > > I don't think we can convert cpu_cap to long without having to do more work as > it is used with 'util'. > > > Cheers > > -- > Qais Yousef
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