From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2EB9322A4FC; Sun, 26 Apr 2026 23:16:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777245389; cv=none; b=HM5UBJfSs1lhy4oYbKqoQL9GnV8RuSrYw5qKJPuuGsGHteXqbDBoS3xsxTHgycJn46S0plK3SEGK3NoST2LV8ik6NXp3n3piH4rxU/gBPc1QGLZ/5k9kI/5pa7Q0yooNF3D4YeKipBVXu4sDV0lhoqh1P7FQPorKI38YqG2zDAA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777245389; c=relaxed/simple; bh=T/rtKaW+eKFk9u31EesKB4EAeHDfVjDpSaLKG/xuO0w=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=QqRBQwABEHWWynD4/AeXsvqL0eGyZyWGzy3Xah/Jmyi0YERORWuWPoTujuiqWE7bB1nn55wTksdhXGMzk5A1SAPfcVMl3wx2xrwl578TLmn47+yGlutoSKAJE240rG+FCzLsFJg/9OSKL9eIWRNo53QiQC18NXLS5Am4/F/JQOI= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=skzjHFp9; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="skzjHFp9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D2938C2BCB4; Sun, 26 Apr 2026 23:16:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777245389; bh=T/rtKaW+eKFk9u31EesKB4EAeHDfVjDpSaLKG/xuO0w=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=skzjHFp9vHqFSaoK85/avT28EYhWOljnnUy0mw4KWXm0pAy1IFAlmiTH/tst2LYf2 ZgW4dNHiP0uozx2R6Ip5VE+kJZB6yhe0eH3FO7kadd9KvK8d9rzOvV/N+PlQWgGZ7y ca/l+jDNveyLvwNt53F7LuXtGPwPfqXEigWOVDB3pcsT+Yo+ugSAO7L9ls3y2BcysN yMpBv89mFHBdW0uxHR5hVQ0HJAb+BPkt9JRYFOd5tTKinXShubbdJkNxk76WaburaE gdqmjrONgPp7ytMBA2Tu+cCh6W1axX39Pe9CRJ/XGUaHwnowhIxinXSCZy7W2VpuX6 Qa1SFSjGAv2ng== From: SeongJae Park To: Andrew Morton Cc: Liew Rui Yan , SeongJae Park , damon@lists.linux.dev, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v2 1/4] mm/damon/ops-common: optimize damon_hot_score() using ilog2() Date: Sun, 26 Apr 2026 16:16:14 -0700 Message-ID: <20260426231619.107231-2-sj@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260426231619.107231-1-sj@kernel.org> References: <20260426231619.107231-1-sj@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Liew Rui Yan 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 Signed-off-by: SeongJae Park --- Changes from v2 (https://lore.kernel.org/20260320192020.33004-1-aethernet65535@gmail.com) - Rebase to latest mm-new. Changes from v1 (actually it was RFC) (https://lore.kernel.org/20260320072431.248235-1-aethernet65535@gmail.com) - 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. 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 8c6d613425c13..3a0ddc3ac7196 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) -- 2.47.3