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b=NzYkT75bJMXUIzr1h1EQdvkNi+7yiwjbuzRLg7JIonZ4kq7iN2GkaOPOarRVTV8Vz A6wqun+XI0Xa+GJdF0SIvRxl3zNPUgbu0AWh96UN/6X7ti/AKtRSIrmQUPk0UBbM43 aZhkFIuXQMNjORk/9Ag+soufMSU+0P6Odgap6egDcTtws/1vaNsEmPG+o8Ow+O9ODc D5KZVSmef0biiV5olr6UzsY5dfTz+r52zBFSUcK9mm5R9ZiLvcUWJKHLlC7mFmQNM+ dOg6mxI2rZkIsSn+ez3I+7D+ZRmTup+c+kQGQBvM6Lb1yFJKVFg3BWBdAPe3TkcU0F Aq7AnUrU4W0Tg== Message-ID: <26e65878-f214-4890-8bcb-24a45122bfd6@kernel.org> Date: Thu, 18 Dec 2025 10:29:18 +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 , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com Cc: ziy@nvidia.com, npache@redhat.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> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <20251215090419.174418-3-yanglincheng@kylinos.cn> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Stat-Signature: 31yf3d375bjf68zf3hho1q1tx7oy7ytf X-Rspam-User: X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 3B44240004 X-HE-Tag: 1766050162-201889 X-HE-Meta: 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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 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? -- Cheers David