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[2003:d8:2f19:6500:e1c1:8216:4c25:efe4]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-447f23bfe80sm227992765e9.20.2025.05.26.02.29.09 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 26 May 2025 02:29:11 -0700 (PDT) Message-ID: Date: Mon, 26 May 2025 11:29:09 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v0 2/2] mm: sched: Batch-migrate misplaced pages To: Zi Yan Cc: Bharata B Rao , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Jonathan.Cameron@huawei.com, 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 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 purposes (CMA, compaction, long-term pinning, whatever) until that list was drained. >>>>>> >>>>>> Bad. >>>>> >>>>> Probably we can mark these pages and when others want to migrate the 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 exact page to a different location? >>>> >>>> It cannot isolate it for migration because ... it's already isolated ... 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 the 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 someone >> 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 another flag for that. I guess what you mean, indicating that the migration destination should be on a different node than the current one. 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 immediately, could that information ("migrate this page") actually now be wrong? I guess a way to obtain the destination node would suffice: if the destination node matches, no need to migrate from that NUMA scanner. 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 factors :) -- Cheers, David / dhildenb