linux-kernel - Re: [PATCH] sched: Introduce scaled capacity awareness in enqueue

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Message-ID: <6a613277-8e2c-4c05-1420-b288d15a036b@oracle.com>
Date:   Mon, 5 Jun 2017 11:08:38 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        vincent.guittot@...aro.org, morten.rasmussen@....com,
        dietmar.eggemann@....com, Dhaval Giani <dhaval.giani@...cle.com>
Subject: Re: [PATCH] sched: Introduce scaled capacity awareness in enqueue

On 06/02/2017 11:26 AM, Peter Zijlstra wrote:
> On Fri, Jun 02, 2017 at 11:20:20AM -0700, Rohit Jain wrote:
>> On 06/01/2017 05:37 AM, Peter Zijlstra wrote:
>>> On Thu, Jun 01, 2017 at 02:28:27PM +0200, Peter Zijlstra wrote:
>>>> On Wed, May 31, 2017 at 03:19:46PM -0700, Rohit Jain wrote:
>>>>
>>>>>>> 2) This scaled capacity is normalized and mapped into buckets.
>>>>>> Why?
>>> And its not at all clear why you'd need
>>> that to begin with.
>> Here is the problem I am trying to solve:
>>
>> The benchmark(s) have a high degree of variance when run multiple
>> times.
>>
>> We believe it is because of the scheduler not being aware of the scaled
>> down capacity of the CPUs because of IRQ/RT activity.
>>
>> This patch helps in solving the above problem. Do you have any thoughts
>> on solving this problem in any other way?
> Why does determining if a CPU's capacity is scaled down need to involve
> global data? AFAICT its a purely CPU local affair.

The global array is used to determine the threshold capacity, so
that any CPU which lies below decides that a CPU is 'running low' on
available capacity. This threshold can also be statically defined to
be a fixed fraction, but having dynamic calculation to determine the
threshold works for all benchmarks.

Did you mean we should use a static cutoff and decide whether a CPU
should be treated low on capacity and skip it during idle CPU search?