Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 21 Feb 2020 09:29:57 +0000
From:   Qais Yousef <qais.yousef@arm.com>
To:     chris hyser <chris.hyser@oracle.com>
Cc:     Parth Shah <parth@linux.ibm.com>, vincent.guittot@linaro.org,
        patrick.bellasi@matbug.net, valentin.schneider@arm.com,
        dhaval.giani@oracle.com, dietmar.eggemann@arm.com,
        linux-kernel@vger.kernel.org, peterz@infradead.org,
        mingo@redhat.com, pavel@ucw.cz, qperret@qperret.net,
        David.Laight@ACULAB.COM, pjt@google.com, tj@kernel.org
Subject: Re: [PATCH v3 0/3] Introduce per-task latency_nice for scheduler
 hints
Message-ID: <20200221092956.jpsfps2dgmhiu5vg@e107158-lin.cambridge.arm.com>
References: <8ed0f40c-eeb4-c487-5420-a8eb185b5cdd@linux.ibm.com>
 <c7e5b9da-66a3-3d69-d7aa-0319de3aa736@oracle.com>
 <971909ed-d4e0-6afa-d20b-365ede5a195e@linux.ibm.com>
 <8e984496-e89b-d96c-d84e-2be7f0958ea4@oracle.com>
 <1e216d18-7ec0-4a0d-e124-b730d6e03e6f@oracle.com>
 <de5d8886-6f70-a3fa-8061-5877cd1d98f5@linux.ibm.com>
 <7429e0ae-41ff-e9c4-dd65-3ef1919f5f50@linux.ibm.com>
 <a332d633-7826-b85d-5d9f-5e34f9de084a@oracle.com>
 <20200220150343.dvweamfnk257pg7z@e107158-lin.cambridge.arm.com>
 <9bb1437b-3de0-b0ca-6319-6be903b0758d@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <9bb1437b-3de0-b0ca-6319-6be903b0758d@oracle.com>
User-Agent: NeoMutt/20171215
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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.

> 
> > 
> > Can you expand on the scenario you have in mind please?
> 
> Hopefully, the above helps. It was my original plan to introduce this with a
> data laden RFC on the topic, but I felt the need to respond to Parth
> immediately. I'm not currently pushing any particular change.

Thanks!

--
Qais Yousef