linux-kernel - Re: [PATCH 3/3] sched/fair: Consider uclamp for "task fits capacity" checks

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Date:   Thu, 21 Nov 2019 13:30:43 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, patrick.bellasi@...bug.net,
        qais.yousef@....com, morten.rasmussen@....com
Subject: Re: [PATCH 3/3] sched/fair: Consider uclamp for "task fits capacity"
 checks

On Thursday 21 Nov 2019 at 12:56:39 (+0000), Valentin Schneider wrote:
> > @@ -6274,6 +6274,15 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >  			if (!fits_capacity(util, cpu_cap))
> >  				continue;
> >  
> > +			/*
> > +			 * Skip CPUs that don't satisfy uclamp requests. Note
> > +			 * that the above already ensures the CPU has enough
> > +			 * spare capacity for the task; this is only really for
> > +			 * uclamp restrictions.
> > +			 */
> > +			if (!task_fits_capacity(p, capacity_orig_of(cpu)))
> > +				continue;
> 
> This is partly redundant with the above, I think. What we really want here
> is just
> 
> fits_capacity(uclamp_eff_value(p, UCLAMP_MIN), capacity_orig_of(cpu))
> 
> but this would require some inline #ifdeffery.

This suggested change lacks the UCLAMP_MAX part, which is a shame
because this is precisely in the EAS path that we should try and
down-migrate tasks if they have an appropriate max_clamp. So, your first
proposal made sense, IMO.

Another option to avoid the redundancy would be to do something along
the lines of the totally untested diff below.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 69a81a5709ff..38cb5fe7ba65 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6372,9 +6372,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                        if (!cpumask_test_cpu(cpu, p->cpus_ptr))
                                continue;
 
-                       /* Skip CPUs that will be overutilized. */
                        util = cpu_util_next(cpu, p, cpu);
                        cpu_cap = capacity_of(cpu);
+                       spare_cap = cpu_cap - util;
+                       util = uclamp_util_with(cpu_rq(cpu), util, p);
+
+                       /* Skip CPUs that will be overutilized. */
                        if (!fits_capacity(util, cpu_cap))
                                continue;
 
@@ -6389,7 +6392,6 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
                         * 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;

Thoughts ?

Thanks,
Quentin