linux-kernel - Re: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small background tasks on fewer cores

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Message-Id: <86dc25e4-9f19-627f-9581-d74608b7f20c@linux.ibm.com>
Date:   Wed, 9 Oct 2019 14:27:14 +0530
From:   Parth Shah <parth@...ux.ibm.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Valentin Schneider <valentin.schneider@....com>,
        Pavel Machek <pavel@....cz>,
        Doug Smythies <dsmythies@...us.net>,
        Quentin Perret <qperret@...rret.net>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: [RFC v5 4/6] sched/fair: Tune task wake-up logic to pack small
 background tasks on fewer cores



On 10/8/19 10:22 PM, Dietmar Eggemann wrote:
> [- Quentin Perret <quentin.perret@....com>]
> [+ Quentin Perret <qperret@...rret.net>]
> 
> See commit c193a3ffc282 ("mailmap: Update email address for Quentin Perret")
> 

noted. thanks for notifying me.

> On 07/10/2019 18:53, Parth Shah wrote:
>>
>>
>> On 10/7/19 5:49 PM, Vincent Guittot wrote:
>>> On Mon, 7 Oct 2019 at 10:31, Parth Shah <parth@...ux.ibm.com> wrote:
>>>>
>>>> The algorithm finds the first non idle core in the system and tries to
>>>> place a task in the idle CPU in the chosen core. To maintain
>>>> cache hotness, work of finding non idle core starts from the prev_cpu,
>>>> which also reduces task ping-pong behaviour inside of the core.
>>>>
>>>> Define a new method to select_non_idle_core which keep tracks of the idle
>>>> and non-idle CPUs in the core and based on the heuristics determines if the
>>>> core is sufficiently busy to place the incoming backgroung task. The
>>>> heuristic further defines the non-idle CPU into either busy (>12.5% util)
>>>> CPU and overutilized (>80% util) CPU.
>>>> - The core containing more idle CPUs and no busy CPUs is not selected for
>>>>   packing
>>>> - The core if contains more than 1 overutilized CPUs are exempted from
>>>>   task packing
>>>> - Pack if there is atleast one busy CPU and overutilized CPUs count is <2
>>>>
>>>> Value of 12.5% utilization for busy CPU gives sufficient heuristics for CPU
>>>> doing enough work an
> 
> [...]
> 
>>>> @@ -6483,7 +6572,11 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>>>>         } else if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
>>>>                 /* Fast path */
>>>>
>>>> -               new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
>>>> +               if (is_turbosched_enabled() && unlikely(is_background_task(p)))
>>>> +                       new_cpu = turbosched_select_non_idle_core(p, prev_cpu,
>>>> +                                                                 new_cpu);
>>>
>>> Could you add turbosched_select_non_idle_core() similarly to
>>> find_energy_efficient_cpu() ?
>>> Add it at the beg select_task_rq_fair()
>>> Return immediately with theCPU if you have found one
>>> Or let the normal path select a CPU if the
>>> turbosched_select_non_idle_core() has not been able to find a suitable
>>> CPU for packing
>>>
>>
>> of course. I can do that.
>> I was just not aware about the effect of wake_affine and so was waiting for
>> such comments to be sure of. Thanks for this.
>> Maybe I can add just below the sched_energy_present(){...} construct giving
>> precedence to EAS? I'm asking this because I remember Patrick telling me to
>> leverage task packing for android as well?
> 
> I have a hard time imaging that Turbosched will be used in Android next
> to EAS in the foreseeable future.
> 
> First of all, EAS provides task packing already on Performance Domain
> (PD) level (a.k.a. as cluster on traditional 2-cluster Arm/Arm64
> big.LITTLE or DynamIQ (with Phantom domains (out of tree solution)).
> This is where we can safe energy without harming latency.
> 
> See the tests results under '2.1 Energy test case' in
> 
> https://lore.kernel.org/r/20181203095628.11858-1-quentin.perret@arm.com
> 
> There are 10 to 50 small (classified solely by task utilization) tasks
> per test case and EAS shows an effect on energy consumption by packing
> them onto the PD (cluster) of the small CPUs.
> 
> And second, the CPU supported topology is different to the one you're
> testing on.
> 

cool. I was just keeping in mind the following quote
" defining a generic spread-vs-pack wakeup policy which is something
Android also could benefit from " (https://lkml.org/lkml/2019/6/28/628)

BTW, IIUC that does task consolidation only on single CPU unless
rd->overload is set, right?

> [...]
>