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Thu, 18 Dec 2025 21:24:20 -0800 (PST) Received: from localhost.localdomain ([114.231.217.195]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-7ff7a939ea2sm1029972b3a.4.2025.12.18.21.24.17 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 18 Dec 2025 21:24:19 -0800 (PST) Date: Fri, 19 Dec 2025 13:24:14 +0800 From: Vernon Yang To: "David Hildenbrand (Red Hat)" Cc: akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang Subject: Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been collapsed Message-ID: References: <20251215090419.174418-1-yanglincheng@kylinos.cn> <20251215090419.174418-3-yanglincheng@kylinos.cn> <26e65878-f214-4890-8bcb-24a45122bfd6@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <26e65878-f214-4890-8bcb-24a45122bfd6@kernel.org> X-Rspamd-Queue-Id: 0269820005 X-Stat-Signature: fjz5szufpuqq3xqxqiezz1xkxs5obi3h X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1766121861-612186 X-HE-Meta: U2FsdGVkX19KgK82rVEVmbf1huJSlOXdXkPTkeR8IbrX9HG66Ry/43bAkMXfZr3KrZIVEFSd9Rlcn5gARebs6ltaoWeVSMZdBl+ivE7G57qevsFo5fpv5S++42VAWXsxRkFbOxKHqcGdBDQljIF+zYgTWG4d7vcDesSsUsXOJW8qeZ3bxzWmOvweFhx0EfzKrZI8BqBFgk8SNqOzoH+SkaPEaVVxpN6NY3VPs0/DA1Mjh/LbvY/Re1HHtCegviLuM9QjvdV7/ToYzD31jpB2Mw9hzy7rgHBC1YTTapFhXLchQVrczgHmXJjHJem95mQOtrbM59eHsypKhIeuVbeE9yau+jqei1RceZAR3mh4Nq6EHZMJ16Rrp2CmOiIQNl0PTBZppDlXwV8s4YEZ+3kxs0Sxl2TsgF4Z6CP1w1oGRwqxAVY4Cx+cxD+hkTJ1RiojNFc0XZOxphn9QXgdEl4vu4BrPK3gpinYfXmBVlxBjdDjwhEZR+i+OD4ajyHerXVij1se9ted2Fuh3JIGHnFxfHzsdXzomdff6Z/hOZI9sZvWcef7M0rhUHsnHQcAcQqXKuwiyTwHyupY82abYmbntx1E1tN19g4oVckILFTvFgulH+X8IA2tgV5Oj+g1Cz+7k0HhSyTseEkh8dL+9wfd7LwpbHvMf+dggpOS/YrJmNRMJs7nDUWgRog/wDRZAM3bOM+/40EKQiU2QjoCSX+HRXCTtVjstd5p0ZKcT3TZNu2u/C2UYEz56ZVmYl3rJaRPrs3mubW9B9CCkB64S8DPKEaiS+a0xEL4n+6FVZHVEDZ2MAinlMrA8q46xJTedpD3fEV4jJ5ztia/Y4pw5UmoMHZfapVKnks7daurDXKk9zSzwZH6CemY6mV2RkwPSTRPDCHRFnZkdIur7Q4vKrz3FyqYyFvV7I9klE86jl22LkitaShbs+IoIVVecDewSv6xWPgx4LiivClHP+HrgQs wOnxgRXz 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 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 Thu, Dec 18, 2025 at 10:29:18AM +0100, David Hildenbrand (Red Hat) wrote: > On 12/15/25 10:04, Vernon Yang wrote: > > The following data is traced by bpftrace on a desktop system. After > > the system has been left idle for 10 minutes upon booting, a lot of > > SCAN_PMD_MAPPED or SCAN_PMD_NONE are observed during a full scan by > > khugepaged. > > > > @scan_pmd_status[1]: 1 ## SCAN_SUCCEED > > @scan_pmd_status[4]: 158 ## SCAN_PMD_MAPPED > > @scan_pmd_status[3]: 174 ## SCAN_PMD_NONE > > total progress size: 701 MB > > Total time : 440 seconds ## include khugepaged_scan_sleep_millisecs > > > > The khugepaged_scan list save all task that support collapse into hugepage, > > as long as the take is not destroyed, khugepaged will not remove it from > > the khugepaged_scan list. This exist a phenomenon where task has already > > collapsed all memory regions into hugepage, but khugepaged continues to > > scan it, which wastes CPU time and invalid, and due to > > khugepaged_scan_sleep_millisecs (default 10s) causes a long wait for > > scanning a large number of invalid task, so scanning really valid task > > is later. > > > > After applying this patch, when all memory is either SCAN_PMD_MAPPED or > > SCAN_PMD_NONE, the mm is automatically removed from khugepaged's scan > > list. If the page fault or MADV_HUGEPAGE again, it is added back to > > khugepaged. > > I don't like that, as it assumes that memory within such a process would be > rather static, which is easily not the case (e.g., allocators just doing > MADV_DONTNEED to free memory). > > If most stuff is collapsed to PMDs already, can't we just skip over these > regions a bit faster? /* default scan 8*HPAGE_PMD_NR ptes (or vmas) every 10 second */ static unsigned int khugepaged_pages_to_scan __read_mostly; The observed phenomenon is that when scanning these regions, the loop is broken upon reaching the number of khugepaged_pages_to_scan, thereforce the khugepaged enters 10s sleep. So if we just skip over these regions, will break the semantics of khugepaged_pages_to_scan. I also think this approach is great because it is simple sufficiently. If we can skip over these regions directly, that's excellent. > -- > Cheers > > David -- Thanks, Vernon