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Fri, 21 Feb 2020 16:51:18 GMT Received: from aserv0121.oracle.com (aserv0121.oracle.com [141.146.126.235]) by aserp3020.oracle.com with ESMTP id 2y8udnw61c-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 21 Feb 2020 16:51:18 +0000 Received: from abhmp0007.oracle.com (abhmp0007.oracle.com [141.146.116.13]) by aserv0121.oracle.com (8.14.4/8.13.8) with ESMTP id 01LGpBo7017970; Fri, 21 Feb 2020 16:51:12 GMT Received: from [192.168.0.195] (/69.207.174.138) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 21 Feb 2020 08:51:11 -0800 Subject: Re: [PATCH v3 0/3] Introduce per-task latency_nice for scheduler hints To: Parth Shah , Qais Yousef Cc: 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 References: <8ed0f40c-eeb4-c487-5420-a8eb185b5cdd@linux.ibm.com> <971909ed-d4e0-6afa-d20b-365ede5a195e@linux.ibm.com> <8e984496-e89b-d96c-d84e-2be7f0958ea4@oracle.com> <1e216d18-7ec0-4a0d-e124-b730d6e03e6f@oracle.com> <7429e0ae-41ff-e9c4-dd65-3ef1919f5f50@linux.ibm.com> <20200220150343.dvweamfnk257pg7z@e107158-lin.cambridge.arm.com> <9bb1437b-3de0-b0ca-6319-6be903b0758d@oracle.com> <20200221092956.jpsfps2dgmhiu5vg@e107158-lin.cambridge.arm.com> From: chris hyser Message-ID: <2983937b-d2de-d1ab-e387-271c7caf2a81@oracle.com> Date: Fri, 21 Feb 2020 11:51:07 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.2 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9538 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 spamscore=0 mlxlogscore=999 phishscore=0 suspectscore=2 mlxscore=0 malwarescore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2002210126 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9538 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 phishscore=0 impostorscore=0 mlxlogscore=999 malwarescore=0 mlxscore=0 suspectscore=2 priorityscore=1501 bulkscore=0 adultscore=0 spamscore=0 lowpriorityscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2001150001 definitions=main-2002210126 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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