linux-kernel - Re: [PATCH 1/2] sched: Interrupt Aware Scheduler

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Date:   Thu, 18 May 2017 10:24:36 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH 1/2] sched: Interrupt Aware Scheduler

On 05/17/2017 12:52 AM, Vincent Guittot wrote:
> On 12 May 2017 at 22:19, Rohit Jain wrote:
>> On 05/12/2017 12:46 PM, Peter Zijlstra wrote:
>>> On Fri, May 12, 2017 at 11:04:26AM -0700, Rohit Jain wrote:
>>>> The patch avoids CPUs which might be considered interrupt-heavy when
>>>> trying to schedule threads (on the push side) in the system. Interrupt
>>>> Awareness has only been added into the fair scheduling class.
>>>>
>>>> It does so by, using the following algorithm:
>>>>
>>>> --------------------------------------------------------------------------
>>>> 1) When the interrupt is getting processed, the start and the end times
>>>> are noted for the interrupt on a per-cpu basis.
>>> IRQ_TIME_ACCOUNTING you mean?
>>
>> Yes. Exactly
>>
>>>> 2) On a periodic basis the interrupt load is processed for each run
>>>> queue and this is mapped in terms of percentage in a global array. The
>>>> interrupt load for a given CPU is also decayed over time, so that the
>>>> most recent interrupt load has the biggest contribution in the interrupt
>>>> load calculations. This would mean the scheduler will try to avoid CPUs
>>>> (if it can) when scheduling threads which have been recently busy with
>>>> handling hardware interrupts.
>>> You mean like like how its already added to rt_avg? Which is then used
>>> to lower a CPU's capacity.
>>
>> Right. The only difference I see is that it is not being used on the
>> enqueue side as of now.
>>
>>>> 3) Any CPU which lies above the 80th percentile in terms of percentage
>>>> interrupt load is considered interrupt-heavy.
>>>>
>>>> 4) During idle CPU search from the scheduler perspective this
>>>> information is used to skip CPUs if better are available.
>>>>
>>>> 5) If none of the CPUs are better in terms of idleness and interrupt
>>>> load, then the interrupt-heavy CPU is considered to be the best
>>>> available CPU.
>>> I would much rather you work with the EAS people and extend the capacity
>>> awareness of those code paths. Then, per the existing logic, things
>>> should just work out.
>>
>> Did you mean we should use the capacity as a metric on the enqueue side
>> and not introduce a new metric?
> If fact, the capacity is already taken into account in the wake up
> path. you can look at wake_affine(), wake_cap() and
> capacity_spare_wake()
> The current implementations takes care of original capacity but it
> might be extended to take into account capacity stolen by irq/rt as
> well

Thanks, I have a new prototype to account for the stolen capacity, I
will send it out once I have more test results.

>>> It doesn't matter how the capacity is lowered, at some point you just
>>> don't want to put tasks on. It really doesn't matter if that's because
>>> IRQs, SoftIRQs, (higher priority) Real-Time tasks, thermal throttling or
>>> anything else.