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bh=UzWU4Me4IIb1YZ7xC+da7RUfQPj8e+Q+gI/W5LlD530=; b=aK5fwrL/a3WY/3FmW8PMiqoQLel/HkW3139XOwd9x9kHLoqz8HUOXme1PnV0N4EgDTI4wZ XO7LvBlyRruCR/1LCa7FdDe5JPXrOCZzfid6Wn1pZeNoypAinIen6LJisKJxsZo03xuMaJ TxcAy05VcD7DZH9iZexy9jvmOc0xI/0= ARC-Authentication-Results: i=1; imf17.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b="eU/k2Zti"; spf=pass (imf17.hostedemail.com: domain of sj@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=sj@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id DCEC841744; Mon, 29 Jun 2026 14:56:38 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 666941F00A3F; Mon, 29 Jun 2026 14:56:38 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782744998; bh=UzWU4Me4IIb1YZ7xC+da7RUfQPj8e+Q+gI/W5LlD530=; h=From:To:Cc:Subject:Date; b=eU/k2ZtiggJzLKB6N/h3ekSHuXEFvvxTuHsNUCvt3BMkKk34KwIqJyXFjA2cxdjFM 08yP2tYQM7ByK4lzFUmUQzVEUpSoqHh+Mttcr5hxyk1899yibNC7OHdq9oHJ/7aXcS 065uWXUqfgwULYoaXHSPuMF08GEV83YlaR9uF3SvrHb2zKv8U27CiYiTpFtY7vsLum 8yo6sJAj92rzTYuTjWlCjDSzQS/BDsjhKLe0GXz5yFqOsoMYLhYLizQZXFzw0+0a3i dNRaHs+ZyR+D7PwOzmqIhD8TFly4EL49TkqW0vQSJOx3/0l4IOea16f9fyDJC6nNzb EAkspDiPbx5Rw== From: SJ Park To: Andrew Morton Cc: SJ Park , Brendan Higgins , David Gow , Jiayuan Chen , Shu Anzai , damon@lists.linux.dev, kunit-dev@googlegroups.com, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v3 0/2] mm/damon/core: detect internal variation above max_nr_regions/2 Date: Mon, 29 Jun 2026 07:56:27 -0700 Message-ID: <20260629145630.134891-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: C4D5940007 X-Stat-Signature: utyqm8b6s6r13snyicy1ys46f9aiwmn9 X-HE-Tag: 1782744999-337680 X-HE-Meta: U2FsdGVkX19XGfZ7JHgrFIiwuh0cnC+8K14PELXzfXcM3rXzwNLsloy2x4KW1Zybo35NMOV4usrSBhgeGoLHWW5asBSGR1GDc7lHE5QPLPe+3ewb0oVqY+XFxy1EsGbj1irbLI8VjmgWoJk81BzAbFyfAc2DsFkCebiVUaHQRqT40a3L2ire6yEmF7QhvTtd1R+bFlH3pDJN45TeMJDLrw/IZGRP7CWv+t6Um5rHRIQcCvEPZOtUH4HKOfqiNHuaX9TNd/bO8oXNQs0aXsl0ykuN6KfFBLOMmM+h0H1kXAfYfIMtWGn99k8/b8OuO7RhZ9PJHvlQL/VBlIMGHNjsxep4THce6bF+5gZLt1pUZB2JqYbPhCeZe5QzPAYO9oVPN7EvjQVrpLS2aZ2O6JY3EsusV6s+GwTfv2S47wHnE2O7v+CoGyR0/t/fiXIfim8bLruhyJ5M+6h6TqO81a1NkPYoKpbZNXpNYK1sTXx12P3Fg7PKS0mNer1E9y0ayTmM7Ngs3kGCAnjaZxOBrYjqW2bWazDi1PitXZJgaVN1bhZgIVZq6W2oJP0fzANk/Hi01sN1ehjc2TVPgpUy72UCsNu/onD6twMkLQdjDDwThXQ1mHEYjywutAxlSDx1ARGuOJsaVo/qcXUiHiHz6IypkVCh1G0DNps/Ea4897j9tjxOY4Y0r3kVCbrbiIm/9S1XAkzRukklDmHfdrKlcMM74uQdxBuL7z7KIIF9kJ9AHhW5FwTi7L5Uu5Ynp3MOxi1VWCMPh6TFMbKooOZ3nGoyJMGEfMV8/vlLTJvzsaWKz3qQuwwr2UkDylBnWE13U3rLIMqhL810sEM0baTu6fX+pCHqTx5LGJM6WDH3kWBDtPsrqqiFJ9zloYpdSjRUh5b0XFAZJetMeBFW7cLZl/Y4gN88Wv0JcZDWwzeGBDzvKmgf8u9+Fn40DLVzbH62gHjMNdt47wI/13wXsJKxsf7 ndxrPip8 YwUFZbnlpkYwXm2XTewcLiagXk+gDtddq/OSMqQ7cEntcJbgxbZTJ7fRVv2pU7DbmGyu2ameet0Um3OnwLHIOAMLVBSwf7slDo7K6yPheUJ2si6kVcc5zIy5xIbGICB0rf534q5NwjiNSgk6ge9U9e19aEmzPHLx94ggKJoj9HX/eLluhQWk909AOpAEQY3iEda7uJEpUrn63eyz4DIlt6iui/Ph/esYINwWhCwLOHrKFCnzNq3cadZyNnv1rV8kH4CPmFacKNHdb5k05AjFdkgtJZs5++7cv39ZV Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: kdamond_split_regions() bails out early when nr_regions is already above max_nr_regions / 2. A large region that picks up new internal variation after that point never gets split, so we lose visibility into its hot/cold structure. We hit this with damon-paddr on hugepage workloads and damon-vaddr on processes that mmap a large anonymous range. Example with max_nr_regions == 1500. A target ends up with 799 small hot/cold regions plus one big region (an earlier merge collapsed a uniformly-accessed range into a single piece): H:hot C:cold r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................| nr_regions = 800 > max_nr_regions / 2 = 750 Now a cold subarea shows up inside r800: r1 r2 r3 r800 HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............| The small regions can't merge with each other (their access counts differ), so budget never frees up. r800 can't be split because nr_regions > max_nr_regions / 2 returns early. The cold subarea stays invisible. Patch 1 keeps refining on this path: when nr_regions is above max_nr_regions / 2 but still under the maximum, it splits a fraction of the regions instead of returning. The fraction shrinks as the remaining budget shrinks, so the count approaches max_nr_regions smoothly. A useless split is undone by the next merge cycle. Patch 2 adds a KUnit test for the case where nr_regions is already above max_nr_regions / 2. Thanks to SJ for the suggestion to drive the split fraction from the remaining budget rather than an age-based filter. Changes from v2 - v2: https://lore.kernel.org/20260626085851.70754-1-jiayuan.chen@linux.dev - Collect R-b: from SJ. - Rebase to latest mm-new. Changes from v1 - v1: https://lore.kernel.org/damon/20260521045236.115749-1-jiayuan.chen@linux.dev/ - Some feedback from SJ. Jiayuan Chen (2): mm/damon/core: split a fraction of regions when nr_regions exceeds max/2 mm/damon/tests/core-kunit: test split above max_nr_regions/2 mm/damon/core.c | 49 +++++++++++++++++++++++++--- mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++ 2 files changed, 108 insertions(+), 5 deletions(-) base-commit: 86100fb7e27ebb5da22fc8a2810eebcf8cc897e8 -- 2.47.3