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b=X0TH5GsVRRkHvMW8e8oMJvYSg1DV6BW3dUIUqWUR9hqW3FPkPM+e1U83V3DKmvrfW sNHgNC52nVAqULjrYJuxWmbsOyHlzio3kPJ5KcGKdQ9yk9ac5tzdu6BTFPGujTT2Kw STT/1GuoKq7lQdhAeziARjfzRQpsvKKjscmsGKu2XDHH1OrHt2wnpoyS0lnZaDYtO7 1hGsF9BnqpxPMgtYsQB9YbJD8/RE/7DjYtNVqVNbzh39rmK5QU5IztEpa1ypNrnhbx wVlvvEfOWw9OW6bTLfd0TVkpt0BaAKXtayqpoqhuePorKtV2eNYf63nZGNb6PgBYBl njwwMP8e4TYXA== Message-ID: Date: Fri, 19 Dec 2025 10:00:53 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/4] mm: khugepaged: remove mm when all memory has been collapsed To: Vernon Yang 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 References: <20251215090419.174418-1-yanglincheng@kylinos.cn> <20251215090419.174418-3-yanglincheng@kylinos.cn> <26e65878-f214-4890-8bcb-24a45122bfd6@kernel.org> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 6E093140018 X-Stat-Signature: my4b3gg7tw84d88451fet6fm49fswwii X-Rspam-User: X-HE-Tag: 1766134859-821727 X-HE-Meta: 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 iPVZZ+id AZSnAn+FaquckxAcHJzGi5C4OJ0aLcahjjDNuhu3bz4w8PDZN1CO6gjEH7urrVBuxFVz78Sv2kzDMZIcsxzYxJaOI/YZKthyRax0t0k3hOD4DcIFb6S5NmTRiAnCpMgfHzqQIa0en6gNmmP6bPVyvW8AbS587UORO84S72kx6EC4L6Q2ZtWmeNvtu7m3Ve5eqaqo8LU8MjIDU18lyPGp9CZpgkX0UD1dHTfllKWMqef0fciOFO60w6z/orXg8G4YUCwdQ 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 12/19/25 06:24, Vernon Yang wrote: > 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. BTW, the 10s sleep is ridiculous :) I wonder whether we were more careful in the past regarding canning overhead due to the mmap read lock. Nowadays page faults typicaly use per-vma locks, so I wonder whether the scanning overhead is still a problem. (I assume there is more to optimize long-term) -- Cheers David