From mboxrd@z Thu Jan 1 00:00:00 1970 From: Feng Tang Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Date: Mon, 31 Oct 2022 10:19:02 +0800 Message-ID: References: <87y1t0ijbk.fsf@yhuang6-desk2.ccr.corp.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1667182755; x=1698718755; h=date:from:to:cc:subject:message-id:references: in-reply-to:mime-version; bh=MwdmFBghoqobBapJMV8vdjEtYXx44iK8DBj4tay9J4U=; b=B1r6/4sEOk3JQfhoETgxvfR1vN4lbLtX645Dwko/4906EtyqZ/mNLyGT cXhv0rQw076mvFh4lw8IqSkdCetYJZvxy+7aEGxpuxSLP566ZH7jcbRYq i5tecX7clEE6e/0Y2yhQE57MQwlAkrnIJi6ntzQUmf6FsR21tgy3MAT9x lLx09fUVCnkHC7wWFl9hKZzDdhe/Bn74UYXtuhByTfzEtwavUteoImtEU /l8U1nqSOuN9LpiHnBN/hSE7c+6+J9fXOABo0w766kTH9O8E+PZpXjhj/ QovFGfnh3a6fSZMR1XL24HVbX/ASD8MIb3a7vTFRm/FpXBnf3MzX5T/hE g==; Content-Disposition: inline In-Reply-To: List-ID: Content-Transfer-Encoding: 7bit To: Yang Shi Cc: "Huang, Ying" , "Hocko, Michal" , Aneesh Kumar K V , Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "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 Sat, Oct 29, 2022 at 01:23:53AM +0800, Yang Shi wrote: > 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? In our test code, it follows the stricter policy, that if the rmap walk meets a mempolicy disallowing the demotion, it will stop the walk and return with 'skip_demotion' flag set. Thanks, Feng