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

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Date:   Fri, 12 May 2017 13:19:50 -0700
From:   Rohit Jain <rohit.k.jain@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        Morten Rasmussen <morten.rasmussen@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 1/2] sched: Interrupt Aware Scheduler

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?

>
> 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.