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dkim=pass header.d=gmail.com header.s=20210112 header.b=Lmp4haqg; spf=pass (imf11.hostedemail.com: domain of shy828301@gmail.com designates 209.85.210.181 as permitted sender) smtp.mailfrom=shy828301@gmail.com; dmarc=pass (policy=none) header.from=gmail.com X-Rspamd-Server: rspam04 X-HE-Tag: 1666893373-528891 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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. > > if (task) { > mpol = get_task_policy(task); > if (mpol) > mpol_get(mpol); > } > } > > if (!mpol) > return ret; > > if (mpol->mode != MPOL_BIND) > goto put_exit; > > nid = folio_nid(folio); > dnid = next_demotion_node(nid); > if (!node_isset(dnid, mpol->nodes)) { > *skip_demotion = true; > ret = false; > } > > put_exit: > mpol_put(mpol); > return ret; > } > > static unsigned int shrink_page_list(struct list_head *page_list,..) > { > ... > > bool skip_demotion = false; > struct rmap_walk_control rwc = { > .arg = &skip_demotion, > .rmap_one = __check_mpol_demotion, > }; > > /* memory policy check */ > rmap_walk(folio, &rwc); > if (skip_demotion) > goto keep_locked; > } > > And there seems to be no simple solution for getting the memory > policy from a page. > > Thanks, > Feng > > > > > > > -- > > > Michal Hocko > > > SUSE Labs > > > > >