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dave.hansen@intel.com, gourry@gourry.net, hannes@cmpxchg.org, mgorman@techsingularity.net, mingo@redhat.com, peterz@infradead.org, raghavendra.kt@amd.com, riel@surriel.com, rientjes@google.com, sj@kernel.org, weixugc@google.com, willy@infradead.org, ying.huang@linux.alibaba.com, dave@stgolabs.net, nifan.cxl@gmail.com, joshua.hahnjy@gmail.com, xuezhengchu@huawei.com, yiannis@zptcorp.com, akpm@linux-foundation.org Subject: Re: [RFC PATCH v0 2/2] mm: sched: Batch-migrate misplaced pages Date: Mon, 26 May 2025 10:20:39 -0400 X-Mailer: MailMate (2.0r6255) Message-ID: <94BF4806-ABCD-4D01-8577-9E138A634815@nvidia.com> In-Reply-To: References: <20250521080238.209678-1-bharata@amd.com> <20250521080238.209678-3-bharata@amd.com> <62cef618-123c-4ffa-b45a-c38b65d2a5a3@redhat.com> <5d6b92d8-251f-463b-adde-724ea25b2d89@redhat.com> <996B013E-4143-4182-959F-356241BE609A@nvidia.com> <382839fc-ea63-421a-8397-72cb35dd8052@redhat.com> Content-Type: text/plain Content-Transfer-Encoding: quoted-printable 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IImPrvEIRzWn/N+IZQrDK8G5guN+YyH4Nli15vdBjxojOjaBgGU3ahIVHeS2HJd8rq8isckpHSAyX25BxG5P1691fLjXU0PmQ+uZBLguAjWIPeWSA943ybvG50wfUcaJxUzNSFY3pRF13Wb14pcOHTiOc5xRTJEv+6SzuGdgzf5VkrVCH8A2w7IWEZVHomT/cYnw08UakSSsP1jP8fbNME7duymT98Lce746EeU/t5npk8lGG6G4oaDZ/b508xxdPiFSJ74aZlHEArwr+HS80ghboOhXNowiNxGouf5MO9ksPXJpvySuGfJQZkVrwUUBBcBvAsvXyDF7kfvZ5s6UpjJIbMRemQT9BMyuE0t+/5OT8VWplD1kWJUchh7WjBSKI25YlUdn9oebunLTsK6F6pILXdSXljIOTKACOIfAs2pONmdhS9Y1OpvZrnE3tVmF0jfHyrscytxQxfDww1jBW7bUzzT4f7UgFIOIE 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: List-Subscribe: List-Unsubscribe: On 26 May 2025, at 5:29, David Hildenbrand wrote: > On 22.05.25 19:30, Zi Yan wrote: >> On 22 May 2025, at 13:21, David Hildenbrand wrote: >> >>> On 22.05.25 18:38, Zi Yan wrote: >>>> On 22 May 2025, at 12:26, David Hildenbrand wrote: >>>> >>>>> On 22.05.25 18:24, Zi Yan wrote: >>>>>> On 22 May 2025, at 12:11, David Hildenbrand wrote: >>>>>> >>>>>>> On 21.05.25 10:02, Bharata B Rao wrote: >>>>>>>> Currently the folios identified as misplaced by the NUMA >>>>>>>> balancing sub-system are migrated one by one from the NUMA >>>>>>>> hint fault handler as and when they are identified as >>>>>>>> misplaced. >>>>>>>> >>>>>>>> Instead of such singe folio migrations, batch them and >>>>>>>> migrate them at once. >>>>>>>> >>>>>>>> Identified misplaced folios are isolated and stored in >>>>>>>> a per-task list. A new task_work is queued from task tick >>>>>>>> handler to migrate them in batches. Migration is done >>>>>>>> periodically or if pending number of isolated foios exceeds >>>>>>>> a threshold. >>>>>>> >>>>>>> That means that these pages are effectively unmovable for other p= urposes (CMA, compaction, long-term pinning, whatever) until that list wa= s drained. >>>>>>> >>>>>>> Bad. >>>>>> >>>>>> Probably we can mark these pages and when others want to migrate t= he page, >>>>>> get_new_page() just looks at the page's target node and get a new = page from >>>>>> the target node. >>>>> >>>>> How do you envision that working when CMA needs to migrate this exa= ct page to a different location? >>>>> >>>>> It cannot isolate it for migration because ... it's already isolate= d ... so it will give up. >>>>> >>>>> Marking might not be easy I assume ... >>>> >>>> I guess you mean we do not have any extra bit to indicate this page = is isolated, >>>> but it can be migrated. My point is that if this page is going to be= migrated >>>> due to other reasons, like CMA, compaction, why not migrate it to th= e target >>>> node instead of moving it around within the same node. >>> >>> I think we'd have to identify that >>> >>> a) This page is isolate for migration (could be isolated for other >>> reasons) >>> >>> b) The one responsible for the isolation is numa code (could be someo= ne >>> else) >>> >>> c) We're allowed to grab that page from that list (IOW sync against >>> others, and especially also against), to essentially "steal" the >>> isolated page. >> >> Right. c) sounds like adding more contention to the candidate list. >> I wonder if we can just mark the page as migration candidate (using >> a page flag or something else), then migrate it whenever CMA, >> compaction, long-term pinning and more look at the page. > > I mean, all these will migrate the page either way, no need to add anot= her flag for that. > > I guess what you mean, indicating that the migration destination should= be on a different node than the current one. Yes. > > Well, and for the NUMA scanner (below) to find which pages to migrate. > > ... to be this raises some questions: like, if we don't migrate immedia= tely, could that information ("migrate this page") actually now be wrong?= I guess a way to Could be. So it is better to evaluate the page before the actual migratio= n, in case the page is no longer needed in a remote node. > obtain the destination node would suffice: if the destination node matc= hes, no need to migrate from that NUMA scanner. Right. The destination node could be calculated by certain metric like mo= st recent accesses or last remote node access time. If most recent accesses are sti= ll coming from a remote node and/or last remote node access time is within a short = time frame, the page should be migrated. Since it is possible that the page is freque= ntly accessed by a remote node but when it comes to migration, it is no longer needed b= y a remote node and the access pattern would look like 1) a lot of remote node acces= ses, but 2) the last remote node access is long time ago. > > In addition, >> periodically, the migration task would do a PFN scanning and migrate >> any migration candidate. I remember Willy did some experiments showing= >> that PFN scanning is very fast. > > PFN scanning can be faster than walking lists, but I suspect it depends= on how many pages there really are to be migrated ... and some other fac= tors :) Yes. LRU list is good since it restricts the scanning range, but PFN scan= ning itself does not have it. PFN scanning with some filter mechanism might wo= rk and that filter mechanism is a way of marking to-be-migrated pages. Of co= urse, a quick re-evaluation of the to-be-migrated pages right before a migratio= n would avoid unnecessary work like we discussed above. -- Best Regards, Yan, Zi