From mboxrd@z Thu Jan 1 00:00:00 1970 From: Michal Hocko Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Date: Thu, 27 Oct 2022 10:01:33 +0200 Message-ID: References: <20221026074343.6517-1-feng.tang@intel.com> <87wn8lkbk5.fsf@yhuang6-desk2.ccr.corp.intel.com> <87o7txk963.fsf@yhuang6-desk2.ccr.corp.intel.com> Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.com; s=susede1; t=1666857694; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=qa6sD3OJX290NYnGc0ulUW01Mac/lvuPb2vaepZWVak=; b=GOfRQdi/m0WYSTD5CYAhkOmN4NswRTwudSaHgtIJ1oHw16QVwyrpau8AnA+4eYPePOcGhe 4VPRB2hWHIuofx9bzVfEvuQ+PfHUE5qM3DhDGpewPt3/bELf+bC3WB+x4DEvQu67yIfq1O IKDmab7lHY5SApO2WVHdJr/goa+3CBw= Content-Disposition: inline In-Reply-To: <87o7txk963.fsf-fFUE1NP8JkzwuUmzmnQr+vooFf0ArEBIu+b9c/7xato@public.gmane.org> List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: "Huang, Ying" Cc: Feng Tang , 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 Thu 27-10-22 15:39:00, Huang, Ying wrote: > Michal Hocko writes: > > > On Thu 27-10-22 14:47:22, Huang, Ying wrote: > >> Michal Hocko writes: > > [...] > >> > 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? > >> > >> If my understanding were correct, prctl() configures the process or > >> thread. > > > > Not necessarily. There are properties which are per adddress space like > > PR_[GS]ET_THP_DISABLE. This could be very similar. > > > >> How can we get process/thread configuration at demotion time? > > > > As already pointed out in previous emails. You could hook into > > folio_check_references path, more specifically folio_referenced_one > > where you have all that you need already - all vmas mapping the page and > > then it is trivial to get the corresponding vm_mm. If at least one of > > them has the flag set then the demotion is not allowed (essentially the > > same model as VM_LOCKED). > > Got it! Thanks for detailed explanation. > > One bit may be not sufficient. For example, if we want to avoid or > control cross-socket demotion and still allow demoting to slow memory > nodes in local socket, we need to specify a node mask to exclude some > NUMA nodes from demotion targets. Isn't this something to be configured on the demotion topology side? Or do you expect there will be per process/address space usecases? I mean different processes running on the same topology, one requesting local demotion while other ok with the whole demotion topology? > >From overhead point of view, this appears similar as that of VMA/task > memory policy? We can make mm->owner available for memory tiers > (CONFIG_NUMA && CONFIG_MIGRATION). The advantage is that we don't need > to introduce new ABI. I guess users may prefer to use `numactl` than a > new ABI? mm->owner is a wrong direction. It doesn't have a strong meaning because there is no one task explicitly responsible for the mm so there is no real owner (our clone() semantic is just to permissive for that). The memcg::owner is a crude and ugly hack and it should go away over time rather than build new uses. Besides that, and as I have already tried to explain, per task demotion policy is what makes this whole thing expensive. So this better be a per mm or per vma property. Whether it is a on/off knob like PR_[GS]ET_THP_DISABLE or there are explicit requirements for fine grain control on the vma level I dunno. I haven't seen those usecases yet and it is really easy to overengineer this. To be completely honest I would much rather wait for those usecases before adding a more complex APIs. PR_[GS]_DEMOTION_DISABLED sounds like a reasonable first step. Should we have more fine grained requirements wrt address space I would follow the MADV_{NO}HUGEPAGE lead. If we really need/want to give a fine grained control over demotion nodemask then we would have to go with vma->mempolicy interface. In any case a per process on/off knob sounds like a reasonable first step before we learn more about real usecases. -- Michal Hocko SUSE Labs