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Fri, 20 Mar 2026 17:52:53 -0700 (PDT) From: Liew Rui Yan To: sj@kernel.org Cc: damon@lists.linux.dev, linux-mm@kvack.org, Liew Rui Yan Subject: [PATCH v3] mm/damon/ops-common: optimize damon_hot_score() using ilog2() Date: Sat, 21 Mar 2026 08:52:48 +0800 Message-ID: <20260321005248.6114-1-aethernet65535@gmail.com> X-Mailer: git-send-email 2.53.0 MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 48844180003 X-Stat-Signature: 1xb6i5qee5aeswjxfofmr5kntc1chjur X-Rspam-User: X-HE-Tag: 1774054375-831628 X-HE-Meta: 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 3E8hQMmV asN8Ayv/lbrlnNuoxky3WgSLsY49aYzSntuSdFjij6Pdob1fynDtgmJMq/Vqaw6S8n4AMkG4XI7eKXYjqwL7At9KufxbSqU+8UhIh Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: The current implementation of damon_hot_score() uses a manual for-loop to calculate the value of 'age_in_log'. This can be efficiently replaced by ilog2(), which is semantically more appropriate for calculating the logarithmic value of age. In a simulated-kernel-module performance test with 10,000,000 iterations, this optimization showed a significant reduction in latency (average latency reduced from ~12ns to ~1ns). Test results from the simulated-kernel-module: - ilog2: DAMON Perf Test: Starting 10000000 iterations ============================================= Total Iterations : 10000000 Average Latency : 1 ns P95 Latency : 41 ns P99 Latency : 41 ns --------------------------------------------- Range (ns) | Count | Percent --------------------------------------------- 0-19 | 0 | 0% 20-39 | 2625000 | 26% 40-59 | 7374000 | 73% 60-79 | 0 | 0% 80-99 | 0 | 0% 100+ | 1000 | 0% ============================================= - for-loop: DAMON Perf Test: Starting 10000000 iterations ============================================= Total Iterations : 10000000 Average Latency : 12 ns P95 Latency : 51 ns P99 Latency : 60 ns --------------------------------------------- Range (ns) | Count | Percent --------------------------------------------- 0-19 | 0 | 0% 20-39 | 0 | 0% 40-59 | 9862000 | 98% 60-79 | 135000 | 1% 80-99 | 1000 | 0% 100+ | 2000 | 0% ============================================= Full raw benchmark results can be found at [1]. [1] https://github.com/aethernet65535/damon-hot-score-fls-optimize/tree/master/result-raw Signed-off-by: Liew Rui Yan Reviewed-by: SeongJae Park --- Changes from v2: - Send as a new thread instead of a reply to previous version. - Add SeongJae Park's Reviewed-by tag. - Link to v2: https://lore.kernel.org/20260320192020.33004-1-aethernet65535@gmail.com Changes from v1: - Replace fls() with ilog2() per SeongJae Park's suggestion for better semantic clarity. - Move performance benchmark results into the commit message and add comparison between for-loop and ilog2. - Link to v1: https://lore.kernel.org/20260320072431.248235-1-aethernet65535@gmail.com/ mm/damon/ops-common.c | 9 ++++++--- 1 file changed, 6 insertions(+), 3 deletions(-) diff --git a/mm/damon/ops-common.c b/mm/damon/ops-common.c index a218d9922234..ac604fb7b409 100644 --- a/mm/damon/ops-common.c +++ b/mm/damon/ops-common.c @@ -117,9 +117,12 @@ int damon_hot_score(struct damon_ctx *c, struct damon_region *r, damon_max_nr_accesses(&c->attrs); age_in_sec = (unsigned long)r->age * c->attrs.aggr_interval / 1000000; - for (age_in_log = 0; age_in_log < DAMON_MAX_AGE_IN_LOG && age_in_sec; - age_in_log++, age_in_sec >>= 1) - ; + if (age_in_sec) + age_in_log = min_t(int, ilog2(age_in_sec) + 1, + DAMON_MAX_AGE_IN_LOG); + else + age_in_log = 0; + /* If frequency is 0, higher age means it's colder */ if (freq_subscore == 0) -- 2.53.0