linux-kernel - Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding actual_cpu

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Message-ID: <64115627-c6c7-416b-99f9-0df22cbdca6b@arm.com>
Date: Fri, 7 Jun 2024 12:30:12 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, vincent.guittot@...aro.org,
 mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
 rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
 bristot@...hat.com, vschneid@...hat.com, vincent.donnefort@....com,
 qyousef@...alina.io, ke.wang@...soc.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Prevent cpu_busy_time from exceeding
 actual_cpu_capacity

On 07/06/2024 10:20, Xuewen Yan wrote:
> Hi Dietmar
> 
> On Fri, Jun 7, 2024 at 3:19 PM Dietmar Eggemann
> <dietmar.eggemann@....com> wrote:
>>
>> On 06/06/2024 09:06, Xuewen Yan wrote:
>>> Because the effective_cpu_util() would return a util which
>>> maybe bigger than the actual_cpu_capacity, this could cause
>>> the pd_busy_time calculation errors.
>>
>> Doesn't return effective_cpu_util() either scale or min(scale, util)
>> with scale = arch_scale_cpu_capacity(cpu)? So the util sum over the PD
>> cannot exceed eenv->cpu_cap?
> 
> In effective_cpu_util, the scale = arch_scale_cpu_capacity(cpu);
>  Although there is the clamp of eenv->pd_cap, but let us consider the
> following simple scenario:
> The pd cpus are 4-7, and the arch_scale_capacity is 1024, and because
> of cpufreq-limit,

Ah, this is due to:

find_energy_efficient_cpu()

   ...
   for (; pd; pd = pd->next)
       ...
       cpu_actual_cap = get_actual_cpu_capacity(cpu)

       for_each_cpu(cpu, cpus)
           ...
           eenv.pd_cap += cpu_actual_cap

and:

get_actual_cpu_capacity()

   ...
   capacity = arch_scale_cpu_capacity(cpu)

   capacity -= max(hw_load_avg(cpu_rq(cpu)), cpufreq_get_pressure(cpu))

which got introduced by f1f8d0a22422 ("sched/cpufreq: Take cpufreq
feedback into account").

[...]