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27 Oct 2022 22:55:39 -0700 X-IronPort-AV: E=McAfee;i="6500,9779,10513"; a="877848613" X-IronPort-AV: E=Sophos;i="5.95,220,1661842800"; d="scan'208";a="877848613" Received: from yhuang6-desk2.sh.intel.com (HELO yhuang6-desk2.ccr.corp.intel.com) ([10.238.208.55]) by fmsmga006-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 27 Oct 2022 22:55:35 -0700 From: "Huang, Ying" To: Feng Tang Cc: Yang Shi , "Hocko, Michal" , Aneesh Kumar K V , Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "linux-mm@kvack.org" , "cgroups@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" , "Chen, Tim C" , "Yin, Fengwei" Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion References: <20221026074343.6517-1-feng.tang@intel.com> Date: Fri, 28 Oct 2022 13:54:55 +0800 In-Reply-To: (Feng Tang's message of "Fri, 28 Oct 2022 11:37:46 +0800") Message-ID: <87y1t0ijbk.fsf@yhuang6-desk2.ccr.corp.intel.com> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/27.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain; 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b=OzDV/vCkpjvrYQZPenq32wAU4j9Wo+p5YgQ97vDk5pqYquLmqqOnW6KqEYB/tb4siGCxbk IByOGaPWKJhHQiG47ZB76BkI9/9iATcZtpbOXohzU99WsAcpPKcCkvOXz/C1USATdZOG05 +IYL1c5W3ZpdYf43nwd8gGAefWB6JC4= Authentication-Results: imf09.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=QWfUaRdL; spf=pass (imf09.hostedemail.com: domain of ying.huang@intel.com designates 134.134.136.31 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com X-Rspam-User: X-Rspamd-Queue-Id: 77D3A140018 X-Rspamd-Server: rspam03 X-Stat-Signature: 1e1ps6d4ytuqnw7rcnson36pattckdjx X-HE-Tag: 1666936540-348272 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: Feng Tang writes: > On Thu, Oct 27, 2022 at 10:55:58AM -0700, 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. > > Yes, this is a valid concern! We don't have good solution for this. > For memory policy, we may only handle the per-vma policy for now whose > cost is relatively low, as a best-effort try. Yes. The solution isn't perfect, especially for multiple-thread processes with thread specific memory policy. But the proposed code above can support the most common cases at least, that is, run workload with `numactl`. Best Regards, Huang, Ying