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b=VK2jk6qMdVNH/RTkBn4PiZI8l5MImtqd3I3/rcyPP3Z0H9vIq89Aqw22/KwApHpvq sq8EsJleJfgC6Cjn62uzJ0PV5sSqtxM5ZHTry/w8PU8mlXfWxmH9jGrONj71JsufQn BdVquPDJF368SCGof+kurkib3QSmOjvG/gWWEwXssUECfiN/Re+GlMjz62a1bxPFDV XgCdx/GqOrPGwHc8J29qkAlYhs9rU6NH0I4eWzlJw1qNGsNbubg51pUq0zBCzySD1j R16wu5PI+4GtNFfiN9cHyLrRR1T1D7o65TYAd88FFUWkm5QxeduSsN9z66q/PyV3jm 32hAyDmOWkq1w== From: SJ Park To: Aveline Noir Cc: SJ Park , Andrew Morton , damon@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [BUG] DAMON_STAT: idle percentiles out of order on 32-bit Date: Thu, 15 Oct 2026 07:13:19 -0700 Message-ID: <20261015141319.88393-1-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: References: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Stat-Signature: ba95cnqfx8cqoypszgprbuop6o3ugg4f X-Rspam-User: X-Rspamd-Queue-Id: 755E940014 X-Rspamd-Server: rspam08 X-HE-Tag: 1792073607-249660 X-HE-Meta: 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 IyeGdOns K72Mznmz9RS9XXD5IW6ag7geO51xYraHiOwpokgRVQzzy2dpRQFZzotgVUSv4aH6GAyqLjXPj54uxoAHrJHZwtxsUNZdXZhd2FAHG0e/t01nhet+kKQhx2uUCj8YjLqA1zF6OE0Z8tiNQDNv4c/5yfIIx0giFfj7fQqIBM3jtPNwuPkLfPRuS2C/CxX+blG/Ni/6+1alJ6rBUzqKQeybcmUnFBHa7wTCFmPUyO1iKXNW5HxjWvLmvl9zumWn7dyZsEMYZs/qtY0LtnsNQ4k+gMM/dZlEajWnMtM7YJj6n/dDbC+WUBQUhhJwFyw== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Avelin, On Thu, 15 Oct 2026 04:24:12 -0700 Aveline Noir wrote: > Hi, > > In an i386 QEMU guest built from mm-new commit ad9f2d463666, > DAMON_STAT reported these memory_idle_ms_percentiles values after a > statistics update: > > guest uptime: 22.51 s > aggr_interval_us: 3200000 > percentile 0: 19200 ms > percentile 50: 0 ms > percentile 100: 0 ms > > The 0th percentile is greater than the 50th and 100th. The guest had > 256 MiB of RAM (MemTotal: 254216 kB), a running kdamond, > CONFIG_X86_32=y, CONFIG_DAMON_STAT=y, > CONFIG_DAMON_STAT_ENABLED_DEFAULT=y, and CONFIG_COMPAT_32BIT_TIME=y. > aggr_interval_us changed from 0 to 1600000 and then 3200000, showing > that statistics updates ran before the final read. > > In damon_stat_set_idletime_percentiles(), the loop compares > > next_percentile <= accounted_bytes * 100 / total_sz > > where accounted_bytes and total_sz are unsigned long. On 32-bit, when > total_sz exceeds 42,949,672 bytes, the wrapped product cannot yield > 100%, even after all bytes are accounted for. Upper entries in > memory_idle_ms_percentiles then retain an earlier value or remain zero > on first use. DAMON_STAT monitors the system RAM address range, which > exceeds that threshold in this guest. I did not capture a per-region > trace in this run. > > The guest init read the module parameters after 22 seconds of uptime; > it did not write to them. The i386 config, guest init, serial log, and > build/run steps: Could you share more details about your real use case that require the setup and how this becomes a real problem? Thanks, SJ [...]