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Fri, 28 Oct 2022 05:09:34 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 50367A4040; Fri, 28 Oct 2022 05:09:34 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C8ABBA404D; Fri, 28 Oct 2022 05:09:31 +0000 (GMT) Received: from [9.109.205.170] (unknown [9.109.205.170]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Fri, 28 Oct 2022 05:09:31 +0000 (GMT) Message-ID: <7b5f533d-4b2e-b45b-ee42-5e1cc3e8a279@linux.ibm.com> Date: Fri, 28 Oct 2022 10:39:31 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.3.1 Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Content-Language: en-US To: Yang Shi , Feng Tang Cc: "Hocko, Michal" , Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "Huang, Ying" , "linux-mm@kvack.org" , "cgroups@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" , "Chen, Tim C" , "Yin, Fengwei" References: <20221026074343.6517-1-feng.tang@intel.com> From: Aneesh Kumar K V In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: ASFUWSHe5MjllGjLmOisvHNof7sieJp4 X-Proofpoint-GUID: BJChHJSB3LO7Isg854w_XwBD54gS9x6z X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.545,FMLib:17.11.122.1 definitions=2022-10-28_02,2022-10-27_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 malwarescore=0 impostorscore=0 bulkscore=0 mlxlogscore=999 mlxscore=0 suspectscore=0 phishscore=0 priorityscore=1501 lowpriorityscore=0 clxscore=1015 spamscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2210170000 definitions=main-2210280031 X-Spam-Status: No, score=-2.0 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_EF,NICE_REPLY_A,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE, SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/27/22 11:25 PM, Yang Shi wrote: > On Thu, Oct 27, 2022 at 12:12 AM Feng Tang wrote: >> >> On Thu, Oct 27, 2022 at 01:57:52AM +0800, Yang Shi wrote: >>> On Wed, Oct 26, 2022 at 8:59 AM Michal Hocko wrote: >> [...] >>>>>> This all can get quite expensive so the primary question is, does the >>>>>> existing behavior generates any real issues or is this more of an >>>>>> correctness exercise? I mean it certainly is not great to demote to an >>>>>> incompatible numa node but are there any reasonable configurations when >>>>>> the demotion target node is explicitly excluded from memory >>>>>> policy/cpuset? >>>>> >>>>> We haven't got customer report on this, but there are quite some customers >>>>> use cpuset to bind some specific memory nodes to a docker (You've helped >>>>> us solve a OOM issue in such cases), so I think it's practical to respect >>>>> the cpuset semantics as much as we can. >>>> >>>> Yes, it is definitely better to respect cpusets and all local memory >>>> policies. There is no dispute there. The thing is whether this is really >>>> worth it. How often would cpusets (or policies in general) go actively >>>> against demotion nodes (i.e. exclude those nodes from their allowes node >>>> mask)? >>>> >>>> I can imagine workloads which wouldn't like to get their memory demoted >>>> for some reason but wouldn't it be more practical to tell that >>>> explicitly (e.g. via prctl) rather than configuring cpusets/memory >>>> policies explicitly? >>>> >>>>> Your concern about the expensive cost makes sense! Some raw ideas are: >>>>> * if the shrink_folio_list is called by kswapd, the folios come from >>>>> the same per-memcg lruvec, so only one check is enough >>>>> * if not from kswapd, like called form madvise or DAMON code, we can >>>>> save a memcg cache, and if the next folio's memcg is same as the >>>>> cache, we reuse its result. And due to the locality, the real >>>>> check is rarely performed. >>>> >>>> memcg is not the expensive part of the thing. You need to get from page >>>> -> all vmas::vm_policy -> mm -> task::mempolicy >>> >>> Yeah, on the same page with Michal. Figuring out mempolicy from page >>> seems quite expensive and the correctness can't be guranteed since the >>> mempolicy could be set per-thread and the mm->task depends on >>> CONFIG_MEMCG so it doesn't work for !CONFIG_MEMCG. >> >> Yes, you are right. Our "working" psudo code for mem policy looks like >> what Michal mentioned, and it can't work for all cases, but try to >> enforce it whenever possible: >> >> static bool __check_mpol_demotion(struct folio *folio, struct vm_area_struct *vma, >> unsigned long addr, void *arg) >> { >> bool *skip_demotion = arg; >> struct mempolicy *mpol; >> int nid, dnid; >> bool ret = true; >> >> mpol = __get_vma_policy(vma, addr); >> if (!mpol) { >> struct task_struct *task; >> if (vma->vm_mm) >> task = vma->vm_mm->owner; > > But this task may not be the task you want IIUC. For example, the > process has two threads, A and B. They have different mempolicy. The > vmscan is trying to demote a page belonging to thread A, but the task > may point to thread B, so you actually get the wrong mempolicy IIUC. > But if we swap out this page and fault back in via thread B the page would get allocated as per thread B mempolicy. So if we demote based on thread B policy are we breaking anything? -aneesh