Date:   Fri, 21 Feb 2020 11:51:07 -0500
From:   chris hyser <chris.hyser@...cle.com>
To:     Parth Shah <parth@...ux.ibm.com>, Qais Yousef <qais.yousef@....com>
Cc:     vincent.guittot@...aro.org, patrick.bellasi@...bug.net,
        valentin.schneider@....com, dhaval.giani@...cle.com,
        dietmar.eggemann@....com, linux-kernel@...r.kernel.org,
        peterz@...radead.org, mingo@...hat.com, pavel@....cz,
        qperret@...rret.net, David.Laight@...LAB.COM, pjt@...gle.com,
        tj@...nel.org
Subject: Re: [PATCH v3 0/3] Introduce per-task latency_nice for scheduler
 hints

On 2/21/20 5:01 AM, Parth Shah wrote:
> 
> 
> On 2/21/20 2:59 PM, Qais Yousef wrote:
>> On 02/20/20 11:34, chris hyser wrote:
>>>>> Whether called a hint or not, it is a trade-off to reduce latency of select
>>>>> tasks at the expense of the throughput of the other tasks in the the system.
>>>>
>>>> Does it actually affect the throughput of the other tasks? I thought this will
>>>> allow the scheduler to reduce latencies, for instance, when selecting which cpu
>>>> it should land on. I can't see how this could hurt other tasks.
>>>
>>> This is why it is hard to argue about pure abstractions. The primary idea
>>> mentioned so far for how these latencies are reduced is by short cutting the
>>> brute-force search for something idle. If you don't find an idle cpu because
>>> you didn't spend the time to look, then you pre-empted a task, possibly with
>>> a large nice warm cache footprint that was cranking away on throughput. It
>>> is ultimately going to be the usual latency vs throughput trade off. If
>>> latency reduction were "free" we wouldn't need a per task attribute. We
>>> would just do the reduction for all tasks, everywhere, all the time.
>>
>> This could still happen without the latency nice bias. I'm not sure if this
>> falls under DoS; maybe if you end up spawning a lot of task with high latency
>> nice value, then you might end up cramming a lot of tasks on a small subset of
>> CPUs. But then, shouldn't the logic that uses latency_nice try to handle this
>> case anyway since it could be legit?
>>
>> Not sure if this can be used by someone to trigger timing based attacks on
>> another process.
>>
>> I can't fully see the whole security implications, but regardless. I do agree
>> it is prudent to not allow tasks to set their own latency_nice. Mainly because
>> the meaning of this flag will be system dependent and I think Admins are the
>> better ones to decide how to use this flag for the system they're running on.
>> I don't think application writers should be able to tweak their tasks
>> latency_nice value. Not if they can't get the right privilege at least.
>>
> 
> AFAICT if latency_nice is really used for scheduler hints to provide
> latency requirements, then the worst that can happen is the user can
> request to have all the created tasks get least latency (which might not
> even be possible). Hence, unless we bias priority or vruntime, I don't see
> the DoS possibility with latency_nice.

A latency nice that does not allow reducing the latency of select tasks at the expense of the throughput of other tasks 
(ie the classic trade-off) doesn't seem to be all that useful and would fail to meet the requirements. Latency nice in 
this view is a directive on how to make that trade-off much like nice is a directive on how to trade-off what should get 
to run next and we don't accept the system optionally treating +19 and -20 tasks the same. Even when we don't get 
exactly what we want we expect "best effort" and that is not the same as optional.

-chrish

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