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30 Oct 2022 18:57:10 -0700 X-IronPort-AV: E=McAfee;i="6500,9779,10516"; a="878563686" X-IronPort-AV: E=Sophos;i="5.95,227,1661842800"; d="scan'208";a="878563686" 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; 30 Oct 2022 18:57:07 -0700 From: "Huang, Ying" To: Yang Shi Cc: Feng Tang , "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> <87y1t0ijbk.fsf@yhuang6-desk2.ccr.corp.intel.com> Date: Mon, 31 Oct 2022 09:56:28 +0800 In-Reply-To: (Yang Shi's message of "Fri, 28 Oct 2022 10:23:53 -0700") Message-ID: <878rkweoxf.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; charset=ascii ARC-Authentication-Results: i=1; imf30.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=cS6TlElf; spf=pass (imf30.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.115 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1667181433; a=rsa-sha256; cv=none; b=2rNbCjOrkSdIIRC7lrLgosrkYegcYeDR3R0BsxD65VcPdXkEz0J3to7Q5uOQQGWCRKaZVR 7Qy+Rw/BGEMZ4/GXo56x7dYXQ8+yvWzASU+uGXR1+BndA958C3Yr6F69KDX7Fkcbi+URVn RmprjFCmXwKjOo9d4yHcXZEW3C2IYKo= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1667181433; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=72zlXGup0kmMCbJ5Mk2ouZUDy9Zg9dKD0/P0EcuRrEU=; b=ourMphKjuxeEizhKxb9O+5EDdqL+AxjJTWulVmThDR0xwAMbfj0QHmyVMYDVLme/MJMsTB 0bgeTrirZzXJ2P/j1YLWB31mUUNUkZ+B8vxFWtW1GvTRoEsUqUsxwZmIeDmQMRZgZHsiOn NZYx8QxCmRYMV9j+v8V7lvQcB/NbMKA= X-Rspam-User: Authentication-Results: imf30.hostedemail.com; dkim=none ("invalid DKIM record") header.d=intel.com header.s=Intel header.b=cS6TlElf; spf=pass (imf30.hostedemail.com: domain of ying.huang@intel.com designates 192.55.52.115 as permitted sender) smtp.mailfrom=ying.huang@intel.com; dmarc=pass (policy=none) header.from=intel.com X-Rspamd-Server: rspam07 X-Stat-Signature: c9gihc61as9gphmyicwjt53h1bqd3sbk X-Rspamd-Queue-Id: 617F28000A X-HE-Tag: 1667181432-927511 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: Yang Shi writes: > On Thu, Oct 27, 2022 at 10:55 PM Huang, Ying wrote: >> >> 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`. > > Not only multi threads, but also may be broken for shared pages. When > you do rmap walk, you may get multiple contradict mempolicy, which one > would you like to obey? > > TBH I'm not sure whether such half-baked solution is worth it or not, > at least at this moment. The cost is not cheap, but the gain may not > be worth it IMHO. Per my understanding, this can cover most cases. For example, run workload with `numactl`, or control the page placement of some memory areas via mbind(). Although there are some issue in the corner cases. Best Regards, Huang, Ying