From mboxrd@z Thu Jan 1 00:00:00 1970 From: Aneesh Kumar K V Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Date: Fri, 28 Oct 2022 10:39:31 +0530 Message-ID: <7b5f533d-4b2e-b45b-ee42-5e1cc3e8a279@linux.ibm.com> References: <20221026074343.6517-1-feng.tang@intel.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ibm.com; h=message-id : date : mime-version : subject : to : cc : references : from : in-reply-to : content-type : content-transfer-encoding; s=pp1; bh=mC2HFC3gAoqIozZrCZ1GBd6I46486FF/GbrfZbHoDJs=; b=sORyH0u7NNmXJ2xhjH2ho9CGfQC2uXTTsV6CCo/wUNjh93gWjsOOergxQFNQ0dGJ+brs Sk6ThQx3eCPe0YtR0s/fvl2YLEW4Pb095jSEbrVjXEG5wJ0NVIwkCI7GCLEz/QetJtYF q9Yn6cvxHNMTsFgojdXw7jBRBgY89a3aXoRMsdeAoQFrmP0FyI+/ndrpcT/6Fg8k6+Az FNLGrmbdQSSW+qhAmSWP9E2Foqa5+MAz3T4g2qPzCD5t+9iL0sZBmVZugSwsMOrwL/NZ tvbr6nv4A2EAS7p1Nq8EIvRDMOF4TR+Fmq40JGckJnJGoGaMGYdweYMIM6DRk0pJrF8O vQ== Content-Language: en-US In-Reply-To: List-ID: Content-Type: text/plain; charset="us-ascii" To: Yang Shi , Feng Tang Cc: "Hocko, Michal" , Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "Huang, Ying" , "linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org" , "cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , "Hansen, Dave" , "Chen, Tim C" , "Yin, Fengwei" 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