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

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Message-ID: <09e234a2-ea65-4d09-5215-9b0fe4ec09fe@arm.com>
Date:   Thu, 21 Nov 2019 14:51:06 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Quentin Perret <qperret@...gle.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 21/11/2019 13:30, Quentin Perret wrote:
> 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.
> 
 
Hm right, had to let that spin in my head for a while but I think I got it.

I was only really thinking of:

  (h960: LITTLE = 462 cap, big = 1024)
  p.uclamp.min = 512 -> skip LITTLEs regardless of the actual util_est

but your point is we also want stuff like:

  p.uclamp.max = 300 -> accept LITTLEs regardless of the actual util_est

I'll keep the feec() change as-is and add something like the above in the
changelog for v2.

> 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 ?
> 

uclamp_util_with() (or uclamp_rq_util_with() ;)) picks the max between the
rq-aggregated clamps and the task clamps, which isn't what we want. If the
task has a low-ish uclamp.max (e.g. the 300 example from above) but the
rq-wide max-aggregated uclamp.max is ~800, we'd clamp using that 800. It
makes sense for frequency selection, but not for task placement IMO.

> Thanks,
> Quentin
>