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Sun, 23 Aug 2026 15:17:41 -0400 (EDT) Date: Sun, 23 Aug 2026 20:17:40 +0100 From: Kiryl Shutsemau To: "Lorenzo Stoakes (ARM)" Cc: akpm@linux-foundation.org, david@kernel.org, nico.pache@linux.dev, baolin.wang@linux.alibaba.com, baohua@kernel.org, dev.jain@arm.com, hughd@google.com, lance.yang@linux.dev, liam@infradead.org, mhocko@suse.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, surenb@google.com, usama.arif@linux.dev, vbabka@kernel.org, ziy@nvidia.com, usama.anjum@arm.com, agordeev@linux.ibm.com, linux-mm@kvack.org, linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v4 03/19] selftests/mm: scale khugepaged's collapse wait with the PMD size Message-ID: References: <20260815015901.1236937-1-kirill@shutemov.name> <20260815015901.1236937-4-kirill@shutemov.name> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspamd-Queue-Id: 8E6821A0005 X-Rspamd-Server: rspam10 X-Rspam-User: X-Stat-Signature: d75tweq6ch6nymw796wca75sb89jfpdh X-HE-Tag: 1787512665-925787 X-HE-Meta: U2FsdGVkX194hlWDhoDvV3Hl1PxHSkrlLl5Q4bQ+znUzdlkOyC+HbUQf1I9p7WXXEcpqn84UGjAMMqDHm7Vb3D5IKig+vHhjafNcGqHNdmaSHtTCfnrrwzu0+hfxSvRny5pQ3CRE8a84tQ7mXhGlGtCdGroubDOu1C4S0xIv8ZYUvZwGMD48/vFixRUHmapWbIHkTV0V8ISlVl0BO57z+XApc2HD1fbnO3emhZIS3MpNemp4OuJASSvAjqa19R+vj6tRDJ8DVgs5P3ZHZS6VsJnO0KC98cRPfhb3m+QmbyaL71kfbhPQbC9zhQxBpF3i7uiAYg0ztlc8RQOIFJXRXZPNcMvt3FfBEONg498yOzlQIsHOd0Jbi0HkcpwUySZGP7v4hRF4oOQOmr19awVf+XQ9Zyc1qwwWyuSgRP2RC4hAXDv9aaJL+JiwDTNmIiuFjiqdBlvH4IpK9GJL/oEYfysZteMkRoF+Ds7d3UCLnhErtYx4bZGkDFKm1AuzFC0ZSvy8R7fkDr7ADmIktM7bpp8GxeTLGSxmm2hK/156dFeOyoxeE4JfADWojLuCEeCRfvrI4TUi9yhqYtw3HYY0Cqnbk+Gd2JTouXNO6KlOh5NUeZly4pRikpsn5+/nzOVzx/T97fR3hi4nJOOM4cid3IJ/YbXsxXL/jiJjDxdHVhgmA2D1L5GsnPyoaIR/GXXhROYwVg9d+DZbEa+h9Is26sVgljxTH3qMnma2UxhWM972YTR1oBTk/DvlpuZSZuUuGk9iOTOzHUwIyk/0c/T8wSUVqQT9btRciPHokylNPXd4GehYV0WwDUScjBR3wnNP//65uPEmdVOUo8alMFMqruAEH6E687PX03Nl+sx5/h3Hem7hLtpblSLZkL4opbdMRUqVdQOHQ3zSpRxMA2TIcLh2j/AqPYYvy4eVto+UMje5lJ5K+mXrwAEi74LePpp5A8eH1uZEIkuTASb/1p2 C9PtLl48 n18pVRvdmMLv4TEP4VdZmEpj9EJqsIeTiLqsKogZG+9hk1RZkvshXPxy4ibAy+BKuamvXb7+etfCwsJonFya4OCi1pZsKGTSk5Us2WbzEFSWMKVw0qtLuvA994opV1T+j5rza2Hsa1bzO6ZWZiDXbixzbc5Tu5T0OytTM8dQ5Tewzf9peeBHm9lqrPrSGMl4fhUaoUu/Z6ghzx7W2yyq+xdofrMyi04VLRVCL1ojakUAqq32WT3BeaYbKCgnrFJ0tPPHf5oGSHWZpOrfBR4YQ7EOL9mNUDbGsT2FCGbqWqOf2w7Cuh71xLmUbs7J4QuQNtlzjU9Em8pS55WamzBSkeCveR7jXnHdOHJzwNxThfgczdNXJedJ8tGpPBcGVKZUloT5EIWMYEGS7Jvud3gLEvxFCAWHO6ZZsfkSVHdujIX4uPx0XbQMccmay4vjXtE8tORUopa3odc1sckBf9TSQ7Y7n4A== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Tue, Aug 18, 2026 at 11:04:57AM +0100, Lorenzo Stoakes (ARM) wrote: > On Sat, Aug 15, 2026 at 02:58:45AM +0100, Kiryl Shutsemau wrote: > > From: "Kiryl Shutsemau (Meta)" > > > > wait_for_scan() gives every case the same three seconds, whatever the huge > > page costs to build. collapse_full() asks for four of them: 8M at a 2M > > PMD, but 2G at a 512M PMD -- arm64 with 64K base pages. > > > > Three seconds is thin at that size rather than generous. Across 80 runs of > > collapse_full() on arm64 with 64K pages the wait was half a second in 73 of > > them, with a tail to two seconds. The case has also timed out in a full > > matrix run, reporting a failure for a collapse that was still going. > > > > Keep three seconds as the floor and add a second per 128M collapsed. A 2M > > PMD is unchanged. A 512M PMD gets 19 seconds. > > Yikes that does make this run really slow for 64 KiB page size machines and a > key aim in the selftests is that they don't take too egregiously long to run. > > Can this somehow be trimmed some other way? It does not change how long the run takes. The timeout is a ceiling on a poll loop, not a sleep: wait_for_scan() breaks as soon as check_huge() sees the collapse, or as soon as `full_scans` has advanced by two. On arm64 with 64K pages the wait was half a second in 73 of 80 runs of `collapse_full()`. The 19 seconds is spent only when two khugepaged passes have not completed in that time, which is the case this patch exists to stop reporting as a failure. This can happen on very big machines with high memory usage or in an emulator. I use QEMU TCG for testing on ARM. I will put that in the changelog for v5, with the before and after runtime of the whole `./khugepaged` run. -- Kiryl Shutsemau / Kirill A. Shutemov